Intermittent AC

[J. M. van Swaay]

94 E500

AC intermittent. Seems to always work on cold start but intermittent when car is hot. When AC not working, compressor is not engaged. Verified function of low speed cooling fans. Suspect faulty relay in base module. With hot car, engine running and AC commanded on but compressor not engaged, opened module box to “tap” on base module. No effect on compressor engagement but noticed electronic modules very very hot to the touch.

Is it possible that high temp in module box is causing faults in compressor relay function?

I noticed cooling air plumbing entering box. Not sure but I can see what looks like a blower attached to end of air plumbing. Anyone know air flow path for blower system and how it can be tested?

Thanks.

J. M. van Swaay

 

[gsxr]

JM, you can check codes and see if there are any on the BM indicating either issues with the AC, or with excessive module box temperature (yes, there's a code for that).

The cooling blower for the module box pulls air from under the dash, behind the passenger airbag, and returns air to the same location. See the 500E Introduction to Service manual, group 54. The small blower is located directly behind the CAN box.

Edit - also check if the high/low pressure switch isn't acting up, this also would cause the compressor to not engage. I'm assuming refrigerant pressures are normal when the compressor does run, and cooling performance is also normal.

BTW - shoot me an email if you need help!

 

[samiam44]

I had a hot water issue w a 034. It would randomly quite in 100+f weather. Replacedo the base module.

 

[J. M. van Swaay]

Codes:

Base Module: max temp module box exceeded
AC: no faults

AC cooling is normal when compressor engaged. I’m not familiar with location and function/test of pressure switch-will do a little research and post results...

Still suspect relay in base module, will take a chance on an eBay replacement....

 

[J. M. van Swaay]

OK, found two sensors on receiver dryer. One on top has two individual spade terminals, one lower and on side has two wire red colored connector. Assuming one is pressure and one is temperature..... Which is which and can one or both be jumped/interrupted as check when compressor commanded on but not engaged?

Thanks.

J. M. van Swaay

 

[gsxr]

OK, found two sensors on receiver dryer. One on top has two individual spade terminals, one lower and on side has two wire red colored connector. Assuming one is pressure and one is temperature..... Which is which and can one or both be jumped/interrupted as check when compressor commanded on but not engaged?

JM, the red one is a pressure switch, that triggers the twin electric fans on low speed (you can test by shorting the leads from the red switch). This red switch has no effect on the compressor engagement, it's only for the fans, and the circuits are totally separate.

The other one is the pressure safety switch that will open if pressure is too high, or too low (i.e., empty system). You can test this by removing the 2 wires and shorting them together, temporarily bypassing the safety switch. This switch would shut off the compressor if pressures are "wrong" or if the switch is bad.

I honestly don't think the safety switch is the cause of your problem, but it's relatively easy to test and rule it out, so you can focus on other possibilities.

 

[J. M. van Swaay]

Thanks Dave!

Just to confirm, with normal operating refrigerant pressures, the switch is closed. If pressures not normal, switch opens thereby disabling compressor. So, defective switch could be suspected/confirmed if, when compressor commanded on but not engaged, compressor reengages and system cooling normal with jumpered leads.

By the way, can this switch be replaced without loss of refrigerant? (ie, opening up the system)

 

[S70/2]

Correct, and you can easily jump it with a normal blade fuse or check for voltage with a DMM. If the switch is defective you will need to have the system evacuated to replace it, I would also recommend replacing the drier at the same time.

Have you checked High and Low side pressures with a gauge set when the compressor is engaged?

 

[gsxr]

Just to confirm, with normal operating refrigerant pressures, the switch is closed. If pressures not normal, switch opens thereby disabling compressor. So, defective switch could be suspected/confirmed if, when compressor commanded on but not engaged, compressor reengages and system cooling normal with jumpered leads.

By the way, can this switch be replaced without loss of refrigerant? (ie, opening up the system)

S70/2 is correct on both counts... you can test with jumpered leads, and the switch cannot be replaced without discharging / evacuating the refrigerant.

Again, I don't think this is likely to be the cause of your problem, but it's easy to check. If this does seem to be the issue, I'd connect manifold gauges next and verify the high pressure is not excessive. (If pressure were too low, you wouldn't get cold air out the vents.)

 

[J. M. van Swaay]

Thanks again for all the help!

I’ll keep a jumper lead handy—next time the AC fails, I’ll by bypass the switch and see what happens.

In the meantime, I have a used base module coming in a few days. Don’t usually believe in diagnosis by part replacement but it is cheap and easy...

I have had three Klima relays fail on my other cars, same symptoms. Makes me think the replacement base module is worth a try.

J. M. van Swaay

 

[J. M. van Swaay]

Update:

AC now consistently not working, when not working compressor not engaged. Jumpered pressure switch, compressor immediately engaged and system functions with normal cooling. Will hook up the gauges in the next few days and check pressures. That should confirm defective switch.

Thanks again for the help.

J. M. van Swaay

 

[J. M. van Swaay]

Just curious, if the switch is doing it’s job correctly and there is indeed an incorrect (most likely to high) pressure condition, what could be damaged if the system is allowed to operate with the switch jumpered? Reason I ask is it may be after the new year before I can get to this, just want to know if what my risk is if I operate the AC with the switch jumpered....

 

[nocfn]

I suspect if the pressure is out of whack and the switch is perfect you could have a problem as the switch is a prevention for bad pressures thus shutting off the system to protect it.

Id drive without the ac if you must use the car, perhaps old school 55 mph ac would work.

 

[gsxr]

Just curious, if the switch is doing it’s job correctly and there is indeed an incorrect (most likely to high) pressure condition, what could be damaged if the system is allowed to operate with the switch jumpered? Reason I ask is it may be after the new year before I can get to this, just want to know if what my risk is if I operate the AC with the switch jumpered....

If the switch is working normally, there may be an overpressure condition. You should be able to confirm this with manifold gauges connected, to see if the pressures are excessive.

Worst case, if operating the system with abnormally high pressures, some part may fail and result in either expensive repairs, and/or sudden discharge of the system. Best case, the switch is wonky and the rest of the system is fine!

 

[J. M. van Swaay]

Update: After compressor ran for less than 20 minutes with switch jumpered, cooling failed completely.

Evac/recharge with dye revealed pressure switch leaking. Replaced pressure switch, evac and recharge again, now all is normal.

Suspect pressure switch failed electrically as a result of refrigerant where it should not be. In other words, “internal” leak in switch caused electrical issues, then shortly after, leak became “external” and allowed refrigerant to escape...

Thanks for all the help!

 

[gsxr]

Excellent! Thanks for the follow-up report, and glad to hear everything is cool now. *slaps knee*