95 Grand AM SE A/C Trouble

[teletekman]

I am trying to narrow down a problem with my mom's 95 Grand Am SE. When you turn on the A/C, neither the compressor not the elctric fan kicks on. This system worked fine last year and this is the first time this year that we have tried to use it.

I assuming that there is freon in the system so I started to check the wiring. It appears that neither the fan relay nor the compressor relay is getting engaged when the A/C is turned on. I have 12VDC going to the PCM but it does not appear that the PCM is sending a ground signal to activate both relays. When I manually ground the relay's, both the fan and the compressor start working.

Does this sound like my PCM needs to be replaced or is there somthing else that I am not looking that might contribute to my problem.

Thanks for the help!

[Matt95GT]

The PCM will refuse the A/C request if the refrigerant pressure sensor is reading pressure that's too low or too high. Sounds like it might be time for a refrigerant recharge. Don't jump the A/C relay to engage the compressor for any length of time... if the refrigerant/lube leaked out it will damage the compressor.

As for the fan, it barely ever runs on the 94-95 Quad 4... the t-stat is 180 degrees and the fan isn't set to start until 220.

[teletekman]

Thanks for the reply Matt95GT. So just so I'm understanding what your saying, if the system's pressure is too low or too high, then the PCM does not send a ground signal to activate the relay's?

Unless this car acts differently, does the electric fan not run all the time during operation of the A/C? If that is the case, this is the first I have seen like this.

Thanks!

[Matt95GT]

Originally posted by teletekman
Thanks for the reply Matt95GT. So just so I'm understanding what your saying, if the system's pressure is too low or too high, then the PCM does not send a ground signal to activate the relay's?

Unless this car acts differently, does the electric fan not run all the time during operation of the A/C? If that is the case, this is the first I have seen like this.

Thanks!

No prob.

Correct... if the PCM determines bad conditions for the A/C, it will not activate the relay. In addition to pressure too low/high, it will also deny the A/C request for a couple other reasons... such as coolant/IAT temp too high, engine load too high (wide-open throttle, parasitic loss from power steering turns while idling, etc), or certain codes that will cause an SES light.

The electric radiator fan will run during A/C operation, but since the PCM denies the request, it does not activate the fan either. In other words, if your refrigerant is low, when you turn on the A/C nothing should happen at all other than the vent fan blowing warm air.

I've confirmed all of this from the GM factory service manuals (The manuals explained how the PCM handles A/C requests in detail) and experience with my 94 with bad A/C system.

[rixGAphx]

Matt-

Same problem with my '96:
No A/C in the GA for the first time ever.

I've never messed with a vehicle's AC, owing to the need for special tools and the possibility of really f'ing something.

Can I just throw a bottle of 134a in it with a $10 set of K-mart hoses?
Or should I go the route that I've always gone previously:
Throw $$ at a shop and pray I'm not being screwed too badly?

-Rick

[teletekman]

Thanks again Matt! I'll look into getting it recharged.

Just one last question, is there a way that I can bypass a switch (let's say the pressure switch or some other) that can fool the system into thinking everything is okay, just to make sure the compressor and fan are actually getting the signal properly from the PCM. I'll just need to trigger the A/C for a quick second to test it.

Thanks!

[Matt95GT]

Originally posted by rixGAphx
Can I just throw a bottle of 134a in it with a $10 set of K-mart hoses?
Or should I go the route that I've always gone previously:
Throw $$ at a shop and pray I'm not being screwed too badly?


I tried that on my 94, didn't work. After some research, it appears the pressure can escape but leaves some refrigerant behind that needs to be evacuated before the system is refilled. Since the car came that way and I had no history on it, I didn't bother having it refilled... I barely use A/C anyway but am thankful the 95 has never needed any service. It's still ice-cold after 6 years of use.

Those "refill" kits are supposed to work good if the system is just a tad low and needs to be topped off. I've seen it work on another car that just needed a top-off.

Originally posted by teletekman
Just one last question, is there a way that I can bypass a switch (let's say the pressure switch or some other) that can fool the system into thinking everything is okay, just to make sure the compressor and fan are actually getting the signal properly from the PCM. I'll just need to trigger the A/C for a quick second to test it.


It's a pressure sensor that varies in resistence with pressure... I'm not sure what the acceptable resistence values would be in order to fool the PCM with a resistor.

If the car not overheating, idles normally, and no SES/codes it shouldn't give you any problems if the A/C system is properly charged.

[rixGAphx]

Originally posted by Matt95GT
Those "refill" kits are supposed to work good if the system is just a tad low and needs to be topped off. I've seen it work on another car that just needed a top-off.
Thanks, Matt.

I'll give that a try this weekend.

It's only 95-98*F here this week, but it's going to get HOT soon and I want that AC.

It's a pressure sensor that varies in resistence with pressure.
I'm not sure what the acceptable resistance values would be in order to fool the PCM with a resistor.
I looked at mine this morning, pulled the plug to make sure the problem wasn't just some crud inside the plug (wasn't, :dammit: ).

Three wires (don't know which is which):
* Hot
* Lo pressure
* Hi pressure
This *prolly* is a microvoltage circuit rather than 12v neg ground.
Normally, I would determine 'hot' by continuity check to ground, but that has potential to short the 'puter in this instance.

Jumper from the hot to BOTH 'sensor' wires, with a resistor in-line at each jumper.
I don't know what resistance is needed, either.
If you use a pair of small rheostats as the resistance, at some point you'll be able to dial-in enough resistance to fool the 'box'.

5-watt incandescent auto bulbs make good test resistors for 12v DC circuits, since you can 'see' the current flowing, but 5 watts is undoubtably MUCH more power than this circuit ever uses normally.

All-in-all, I think we'd be better going to a pro.
I HATE having to say that :roll: :D :D

Good luck.
-Rick