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Harrison compressor superheat switch Pages: 12

alexb on Wed February 02, 2005 5:54 AM User is offline

Year: 1984
Make: Jaguar
Model: XJS
Engine Size: 5343
Refrigerant Type: R143a
Country of Origin: United Kingdom

Can anyone tell me how the superheat switch works on these compressors. I think it senses both temperature and pressure, but no one seems to know much about them. This is the early type with the probe that protrudes from the back (I think the later ones without the probe were pressure switches). It is supposed to blow the fuse if it senses too low a pressure, or compressor overheats, but is it plumbed into low or high side, and what pressure/temp does it switch at? I have replaced the proper Jaguar fuse with an in-line 5a fuse as the proper Jaguar ones are rather expensive. My car trips this switch for a second or so (only occasionally) as the compressor clutch engages, but pressures are approx 160/25 in ambient 5 to 10 degrees. Is this normal? I have now overcharged the system to over 200psi high, but this has not made any difference. I think though, that there is a resistor wired inside the Jaguar fuse and I wonder if this is to reduce the current flow through the fuse to allow for the switch to flicker on for short periods without blowing the fuse. I have replaced the switch twice, but still no change. I am now running the system with the switch disconnected and it works fine. I am reluctant to refit the proper fuse until I know that it will not blow again. Any advice would be appreciated.

Thanks,
Alex

Karl Hofmann on Wed February 02, 2005 5:13 PM User is offlineView users profile

I have seen a few of those systems, but my memories of them are hazy. If I remember the sensor senses temperature of the high pressure side and as you correctly state when tripped will cause the fuse to blow. Although it is tricky to diagnose much when ambient temps are so low, I have found that jags of this age do suffer from flakes of corrosion clogging up the system, especially the expansion valve. I am curious to know what refrigerant you are using in this aplication

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Never knock on deaths door... Ring the doorbell and run away, death really hates that!

alexb on Wed February 02, 2005 7:45 PM User is offline

Retro-fitted R134a (used to be R12). Prior to retrofitting, various drop-in gasses were used when R12 was not available. Used an oil additive from my air con suppliers. All o-rings replaced following a long standing slow leak which started after the compressor pulley bearing collapsed during a 130mph run (oops!!! I meant to say 70mph). I can't see how the problem is related though as this can be replaced without having to recover any refrigerant. Leak now appears to be cured, but during this time has developed this problem. It has had new compressor (reconditioned) and new condenser. Complete system has been blown through with compressed air - I know it's not the best way - and vacuumed for over one hour before regassing. There was no evidence of any contamination.

Can you confirm if the superheat switch is connected to the high side. I'm fairly sure the later pressure switch is, but I suspect the earlier superheat switch may not. I am told they are not interchangeable even though the later switch will physically fit in an earlier compressor - (not vice versa for obvious reasons).

Expansion valve appears to work ok as the compressor cycles on and off correctly. The little cone shaped filter inside it has been removed and cleaned before refitting.

If you can't help, these switches are made by AC Delco and the two I have purchased were sourced from the US (I live in UK). Is there a contact - maybe at AC Delco factory - who may be able to provide any more info - after all, someone must have designed the system!

Thanks again,
Alex

Karl Hofmann on Fri February 04, 2005 8:04 AM User is offlineView users profile

You have a harrison A6,

An '84 model should have a high temperature switch inserted in to the high pressure side, so your switch may be dodgy.

This switch is available from your local Hella ac supplier, as would be the fuse.

When retrofitting the drier should have been changed and the lubricant in the system flushed out and compatible lubricant used for the R134a, if there was crud in the mesh filter , then there will be more in the system. I would clean it out.

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

alexb on Fri February 04, 2005 3:53 PM User is offline

Thanks for that Karl, however I have replaced the switch twice, thinking as you said I had a faulty switch. I forgot to say that I have also replaced the drier as I do every time. I think I will now refit the proper fuse that has the resistor in it and see if it stands up to the brief tripping of the switch without blowing. I can't think of any other reason why it should have a resistor in it. I'll keep you posted - I should know in a few days. Thanks, Alex

Karl Hofmann on Fri February 04, 2005 5:37 PM User is offlineView users profile

Good luck with that fuse.

If you were so minded you could work out the resistor value and make your own fuses for the purposes of testing, Im afraid that I do not have the values, as these things are pretty rare these days, and most of the XJSs that I see now are the later six cylinder cars with sanden or denso compressors, even the Rollers that I see have the sanden compressors fitted

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

acpeter on Fri February 04, 2005 8:38 PM User is offlineView users profile

Don't get confused between the superheat switch and the pressue switch. They both are located in the same spot on the rear head of the compressor, but there are some big differences in how they operate.

If you pop the switch out (of course you must discharge the systes) and look into the port, the superheat switch will have a larger hole in the CENTER of the port. The HIGH PRESSURE switch port will have a smaller hole drilled on the SIDE of the port. Very noticable. That is the port on the compressor.

The superheat switch itself has an extended tip that will fit into (or go through ) the CENTER hole for the superheat switch port on the compressor. IT WILL NOT fit into the port if it is made for PRESSURE switch. On the other hand, the pressure switches had a flat bottom.

Sounds a little confusing, but just know that the pressure switches are flat... and the superheat switch will have a long prong on the back end.

Have you considered that you have a pressure switch that is mounted in a compressor that is made for a superheat switch. That could be causeing the switch to be sensing LOW pressure (hole in the center of the port) vs. sensing pressure (hole on the side of the port hole). Additionally the superheat switch ONLY will use the 3 prong Thermal Limiter fuse that should not be replaced with any other fuse configuration.

For the high pressure switches, they were identified by the color of plastic around the pin.

Sorry to be so long. Hope this helps.

NickD on Sat February 05, 2005 6:22 AM User is offline

Would be interesting to see a circuit diagram of this AC system and exactly what that resistor fuse is for, you have my curiosity raised. Is this in series with the clutch coil?

TRB on Sat February 05, 2005 11:03 AM User is offlineView users profile

It's on its way!

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When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

NickD on Sat February 05, 2005 3:58 PM User is offline

The only mention of the Super Heater switch I can seem to find on the web is at:

Super heater

What is does is a good question, Tim sent me the circuit diagram, it's too fadded out for me to read the compressor clutch circuit and no word in the text as to exactly what this switch does. This appears to be a very low res copy as expanding it just shows rather large pixels.

That loose wire I think is labeled NEA must go somewhere or maybe it doesn't, seems like the servo applies the activating voltage to the clutch coil that would be through the fuse portion of the thermal plug, and assuming the super heater switch is some sort of pressure switch activation of this switch would ground the high side of the clutch coil through what may be a small resistor that seems an awkward way to protect the system.

From the different compressors offered for this vehicle, it appears maybe a modification has been made in the aftermarket to avoid using this switch. Afraid without the actual components, can't really make any measurements to guess exactly what they are doing. Typically in any MVAC system there are two concerns, too low of pressure to safely lubricate the compressor or too high of pressure that would blow the system up and this is certainly accomplished in many a system without blowing up an expensive part.

All in all, an interesting electromechanical type of system with a servo motor activating several microswitches couple with vacuum control, I would feel the AC amplifier just contains a simple comparator circuit whose output positions the servo motor.

TRB on Sat February 05, 2005 4:06 PM User is offlineView users profile

If you blow up the image you can read it somewhat. It is a poor image of the wiring diagram. I'll email our friends at BAT Auto and see if Alldata's images is any better.

-------------------------

When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

TRB on Sat February 05, 2005 4:59 PM User is offlineView users profile

Nick the second set of images are on the way thanks to our friends @ BAT Auto

-------------------------

When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

Karl Hofmann on Sat February 05, 2005 6:25 PM User is offlineView users profile

Nick, that temperature switch is normally open unless excessive temperatures cause it to close causing the fuse that runs in parallell with the resistor to blow, the resistor is there to delay the blowing of the fuse for small glitches. Indeed an awkward means of control

Last XJS that I worked on the owner had his own Jaguar workshop manual, there were fuses under the dash that weren't even shown in jags own book!

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Never knock on deaths door... Ring the doorbell and run away, death really hates that!

Edited: Sat February 05, 2005 at 6:29 PM by Karl Hofmann

NickD on Sat February 05, 2005 9:57 PM User is offline

Sounds like a crazy scheme to me, you can't have excessive temperatures without excessive pressures at that point and an HPCO with hysteresis would serve the same exact function.

Apparently someone at Jaguar thought the same thing as this one time blow away thermal fuse with the super heater switch was only used in the early models according to the Alldata.com circuits Tim sent. Guess what they are using in the later models, a HPCO switch simply in series with the clutch coil, looks like a simple change to dump that heat switch. Feel Tim can send this circuit to alexb.

Karl Hofmann on Sun February 06, 2005 4:31 AM User is offlineView users profile

My guess is that Jag felt that if the pressures got to high, then there is a fault that must be investigated, popping a fuse would cause the driver to take the car to be looked at, a switch that reset itself would be ignored.

I see these temperature switches on Saab compressors too, but they are surface mounted on the side of the compressor. When we first saw these fuses on Jags we assumed that they were some daft American idea, seems that they are some daft British idea

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

NickD on Sun February 06, 2005 5:52 AM User is offline

Well the daft American idea at the time was to blow off R-12 with that infamous high pressure relief valve that was even incorporated in FWD vehicles with the electric condenser fans that could go out at any time. Kind of like adding insult to injury, if the fans go or the condenser gets loaded with swamp flies, you have two things to repair instead of one. One of the things that led me to start driving Japanese cars with the dual function switch that offered both high and low pressure protection without blowing an expensive fuse nor half of your R-12.

At the era, Klixon was making a nice thermal cut off switch, wouldn't open until the device cooled down and was self resetting, could be surface mounted and not in the refrigerant circuit as a means to provide another leak path. So there were better options at the time. It's just like they don't care in such things as mounting the fuel pump in the gas tank. I liked my 84 Honda, the fuel pump was external or my Supra with an access plate, makes an all day job cut down to about ten minutes.

Kaps on Sun February 06, 2005 7:59 AM User is offlineView users profile

Dear Alex,
Does this Jag have separate Thermostatic eXpansion Valve (TXV) and Suction Throttling Valve (STV), or a combination Valve In Receiver (VIR) system?

A superheat switch works like this-normally open, closing at approx minus 1-5 PSI, which causes a short to a slow acting (the resistor) fuse, normally with three terminals. The slow acting feature allows the fuse to withstand momentary surges of current(caused by momentary closing of the switch) without opening the circuit. A standard fuse, which opens immediately when shorted, will not allow for temporary fluctuations which can be normal under certain conditions.

The superheat switch senses compressor return (suction, low side) pressure through the large hole in the center of the switch port. The switch/fuse circuit is designed to prevent compressor failure when the refrigerant and oil return is low. This less than 0 pressure can be caused by low refrigerant, or by a stuck closed Suction Throttling Valve(STV). The STV, depending on manufacturer, is often called a Pilot Operated Absolute valve (POA) because the actuator is controlled by a pilot mechanism unaffected by barometric pressure.

The superheat switch/fuse system originated on GM vehicles in the early 1970s, to prevent compressor failure when
1)the thermostatic expansion valve or POA valve sticks closed, starving the compressor's moving parts of lubrication
2)low refrigerant due to a leak causes oil starvation.
The addition of this safety circuit substantially reduced the frequency of compressor replacement.

It is necessary to remove the 3 prong thermal limiter switch and jumper the circuit during system charging, till lower than normal suction side pressures are raised by a full charge.

To test the x valve and POA valve operation:
If the system has 2 ports on the low side, one will sense evaporator pressure, the other senses pressure after the suction throttling POA valve. Connect gauges to both.

remove Thermal limiter switch, jumper circuit, run engine and AC system, system pressures should be normal.
disconnect blower motor, raise engine RPM to 1000-1500, watch evaporator pressure gauge, it should cycle between 25-32 PSI. Compressor return pressure will cycle between 0-32 PSI, it should not go below 0 PSI. You will see the STV cycling when the compressor return pressure goes up and down. If return pressure goes below the range of minus 1 to minus 5, TXV or POA is defective. Don't do this for more than 5 minutes, to prevent compressor damage.
I hope this helps.
Kaps



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A. C. Doyle said "Eliminate all other factors, and the one which remains must be the truth."

George Bray on Sun February 06, 2005 2:17 PM User is offline

The XJ-S / A-6 compressor superheat switch is normally open. It closes when suction pressure drops to an unacceptable level (negative pressure I believe), or it could be when temperature increases to an unacceptable level. I’m not certain. Anyway, closure of the switch completes a circuit to earth, causing the thermal fuse to blow after a delay of around 2-5 minutes. The delay is enabled by a resistor acting as a heater in the 3-terminal fuse box assembly.

The objective when the thermal fuse blows is to stop the compressor, semi-permanently, until a service engineer rectifies the underlying problem, as Jaguar would envisage. No allowing for DIY!

Low pressure may occur for reasons such as a low charge of refrigerant or possibly a blockage, e.g. of the expansion valve. Yes, the XJ-S has an expansion valve. I’m 99% certain there is no suction throttling valve (STV), or combination valve in receiver (VIR).

My suggestion is to leave the superheat switch disconnected, by-pass any fuse and fit a HPCO switch instead.

Jaguar recommend the following checks “if the refrigerant level is satisfactory and there is not a blockage in the air conditioning system but the thermal fuse persists in melting:”
“TEST PROCEDURE A
For use with a cold engine and at ambient temps below 30C.
Connect a test lamp in series with the superheat switch. Note: with the test lamp connected in the circuit, it will prevent the thermal fuse from operating as a safety device, therefore great care must be taken when carrying out the test.
Connect a manifold gauge set to the air con system.
Ensure a serviceable thermal fuse is fitted.
With the ignition and air con switched on.
Depressurize and evacuate the air con system, then close the manifold hand valves.
The test lamp should not light during this operation.

It the test lamp lamp does not light then follow test procedure B.

TEST PROCEDURE B
With test lamp not illuminated, connect the centre hose of the manifold gauge set to a refrigerant supply container.
Start and run the engine at about 2000 rpm. After a few minutes the lamp should light. As soon as the test lamp lights, carefully operate the low pressure hand valve to allow refrigerant to charge the system. As soon as the system becomes charged the lamp should go out.
If the above lamp functions do not occur, renew the superheat switch.
After checking, remove the test lamp from the circuit and reconnect the switch lead onto the terminal.”

The Jaguar manual also says:
“On ‘early’ vehicles (and this is written in a manual published c. 1989/90), a superheat switch is included in the compressor clutch circuit to provide a compressor protection system. The superheat switch and thermal fuse guards against a low refrigerant charge or blockages causing extreme superheated refrigerant vapour conditions resulting in compressor damage. The thermal fuse is a sealed unit containing a heater and a meltable fuse. The superheat switch is located in the rear of the compressor in contact with the suction side refrigerant vapour. With a low refrigerant charge or a blockage, the pressure drops and the temperature rises. This condition closes the superheat switch contacts which complete the circuit, melts the fuse, and disconnects the battery supply to the compressor clutch winding and the thermal fuse heater. The compressor ceases to operate and damage from insufficient lubrication will be avoided.
The thermal fuse melts at 157 to 182C
Time taken: 2 mins - 14v system voltage
Or 5.5 mins - 11.5 system voltage
The heater resistance, cold 8 to 10 ohms.
CAUTION: After a thermal fuse melt, establish and rectify the cause before replacing the thermal fuse unit complete.

High Side Low Pressure Switch (HSLP)
On later vehicles the thermal fuse and superheat switch has been replaced by a high side low pressure switch....etc, etc.”

I hope this information is useful.

Regards
George

NickD on Sun February 06, 2005 3:59 PM User is offline

I also see an ambient sensor in the circuit and assume it should prevent energizing the clutch relay in ambient temps below 0*C. The thermal limiter can't blow unless the coil is engaged, seems like the Japanese style dual function switch is a far superior option providing both high and lo pressure protection and doesn't need replacing if these pressure limits are exceeded.

Appreciate the explanation, though I really doubt if I will ever get a Jaguar.

alexb on Mon February 07, 2005 6:09 AM User is offline

Hi Chaps again,

I've been away for a few days and havn't been online, so please excuse my absence.

I've read through all your replies which are very useful - thank you. I'll try to go through them all best possible. I have measured the resistance in the fuse and it is around 8 ohms, which would indicate an acceptable current flow of 1.5 amps. Without getting more involved, I couldn't say what current the clutch windings draw, or how long it would take for the fuse to blow when the superheat switch closes - nor do I know what the rating of the fuse is. The expansion valve is the TX type located next to the evaporator (high side) with a temperature capillary sensing the outlet temp, and a pressure bleed capillary.

In reply to acpeter, I am aware of the differences of the superheat/pressure switches - this is definitely a superheat switch with a probe about an inch long protruding from the centre - as you said this will not fit in the wrong compressor. Jags changed to pressure switches I think around 1985/6.

What I have learnt is this:- It appears that the resistor is there to allow for momentary tripping of the superheat switch - as I suspected, but also confirmed by yourselves. (If it takes a few minutes to blow the fuse according to George Bray, this will explain why I have never tripped one before during charging as I have never disconnected it!). Also, it seems this switch is fitted on the low pressure side (which I was not sure about) as opposed to the pressure switch which is high side. This explains why, although they look the same are not interchangeable.

Reading through George Bray's instructions for testing the switch, is it correct to run the system discharged for seveal minutes? Sounds like a recipe for disaster! I suppose it was Jags who thought of this one!!

Finally, I don't think the ambient sensor controls the operation of the compressor - it controls the amplifier/servo operation to maintain constant in-car temperatures. Climate control. The compressor cycles on/off via a ranco sensor in the evaoprator.

Still haven't done it but I will reconnect the thermal fuse as I said before and keep my fingers crossed. I have been monitoring the tripping of the switch and it only trips for a second or less, so it sounds that there may be nothing wrong with it now.

Thanks for all your help guys - I'll let you know what happens.

Alex

NickD on Mon February 07, 2005 6:48 AM User is offline

Typically, the key function of the ambient sensor is to prevent compressor operation below 0*C with many systems using that same sensor to display the outside temperature as well. If you disconnect the ambient sensor in automatic systems, the compressor won't even engage as an open sensor emulates some very cold ambient temperatures.

I view use of the high pressure cutoff switch as an improvement, more so than just depending on the high pressure relief valve, although in theory, the high pressure relief valve should never kick in. This is in reference to our domestic vehicles where your vehicle depends upon that superheat switch for the same function. Why not change to the Jaguar later configuration? Your vehicle was intended for R-12 operation and I assume you will have to go to R-134a with the higher operating pressures would be a strike against you.

George Bray on Mon February 07, 2005 12:10 PM User is offline

Quote
Originally posted by: alexb
...Reading through George Bray's instructions for testing the switch, is it correct to run the system discharged for several minutes? Sounds like a recipe for disaster! I suppose it was Jags who thought of this one!!

Just to re-confirm. I was careful to use quotes. I typed out those instructions almost word for word from the Jaguar XJ-S workshop manual. It's not my suggested procedure, but Jaguar's. I don't fully understand or agree with their approach, but that's what's written.

Regards
George

alexb on Tue February 08, 2005 9:46 AM User is offline

I also thought of changing to the later pressure switch but this is not so easy as the compressor would need changing as well. The superheat switch senses low pressure and the pressure switch senses high. They are different compressors. I also suspect some changes may need to be done to the wiring. Depending on whether the pressure switch is normally open or closed, I guess it could be used to operate a relay, so that wouldn't be a problem.

I have now refitted the original jag fuse - we'll have to see in the next few days/weeks whether it is ok.

I've been working with jags for around 24 years now and never had this type of problem with these switches. I suppose there is a first time for everythimg!! Typically, this is my own car that gives me the biggest headache.

Regards,
Alex

George Bray on Fri February 11, 2005 9:39 AM User is offline

Alex

I previously wrote: "My suggestion is to leave the superheat switch disconnected, by-pass any fuse and fit a HPCO switch instead.

Then you said: " I also thought of changing to the later pressure switch but this is not so easy as the compressor would need changing as well."

I will be 'cutting into' the relevant refrigerant pipe to insert a a HPCO switch there, on my XJ-S, rather than messing around with the compressor. I will simply leave the compressor switch disconnected, out of use.

Regards
George

NickD on Fri February 11, 2005 9:47 AM User is offline

I did note different compressors were used depending on the superheat switch or the HPCO switch. R-134a adapters are available with a built in HPCO switch that would make the switch very simple. Blowing off R-12 or burning up an expensive fuse just doesn't make good sense if the pressure gets a tad bit too high. Why not just switch off the compressor?

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