I had my AC converted to R134a. It gets nice and cold!

I have a small electrical bug: The AC clutch would not pick up when using the controls, but worked when jumpered. I chased it down to this:



Where the red circle is - this line should go to ground when the AC is selected on and the pressure cycling switch is made. This +12V signal makes it to the ECM (and to the relay coil + side) but I don't get the ground out of the ECM to turn on the AC pressure control relay. I believe the ECM is processing the signal because my idle changes when the compressor turns on.

My temporary fix was to lift this wire from the ECM and ground the 'C' pin of of the relay coil. AC now works but I don't know why the ECM is not allowing the relay to turn on. Anyvbody know anything about the ECM program and what is uses to make the decision to allow the clutch to come on?

Is this the stock engine? Im thinking bad ac relay...
Allso sounds like a ground issue...

[This message has been edited by James Bond 007 (edited 07-05-2010).]

quote Originally posted by James Bond 007: Is this the stock engine? Im thinking bad ac relay... Allso sounds like a ground issue...

Thanks for the reply. Not a stock motor, but all wiring and AC components are stock.

The relay works OK, just not getting a ground that it needs to turn on. I'm sure the problem lies outside of the relay itself.

Since the ECM is providing the ground to the relay I don't think its a ground wire problem. If the ECM had a ground issue I'd have multiple ECM-type problems.

Year? Engine? It matters. Some year/engine has 3rd wire...

Scan the ECM and/or test lt blu wire with a meter. (left of red circle in image above.)

ECM will not switch to ground unless it read all other switches is turn on.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

I'd check the continuity of the lt blu and dk blu wires, end to end, when unplugged. Then I'd check to be certain that the lt blu wire isn't chaffed through somewhere by ensuring it isn't continuous to ground. Finally, check the blk/wht wire (once unplugged from the ECM) to be certain that it is continuous to ground. If you don't find a problem in one of these wires, then the problem seems to be narrowing down to the ECM itself.

I ran in to this when I installed factory AC into my non AC car. The ECM would not play by the rules. I think the ECM would have worked if I had used an 'AC' car ECM. I just grounded it to the chassis. Still working.

Thanks, ogre...

quote Originally posted by theogre: Year? Engine? It matters. Some year/engine has 3rd wire...

1986 GT with 3.4 PR swap and manual trans. Still has the orginal 2.8 4spd ECM. Car came to me with AC parts on it - seems to be factory-equipped.

quote Originally posted by theogre: Scan the ECM and/or test lt blu wire with a meter. (left of red circle in image above.)

Don't know how to scan the ECM - can you describe?. I haven't checked continuity of the control wire. Will do so.

quote Originally posted by theogre: ECM will not switch to ground unless it read all other switches is turn on.

I don't know what other inputs the ECM might be sampling. The ones for the AC itself (coming from the pressure switch) are working or I wouldn't have +12V at the top side of the relay coil.

I'll let you know what I find out in ringing out the wire. Do you know of any other sensor inputs the ECM might be looking for?

quote Originally posted by Bloozberry: I'd check the continuity of the lt blu and dk blu wires, end to end, when unplugged. Then I'd check to be certain that the lt blu wire isn't chaffed through somewhere by ensuring it isn't continuous to ground. Finally, check the blk/wht wire (once unplugged from the ECM) to be certain that it is continuous to ground. If you don't find a problem in one of these wires, then the problem seems to be narrowing down to the ECM itself.

This would be a really unusual failure for an ECM, but I guess its not out of the question. If it turns out to be that, I think I would consider leaving my fix in place rather than replacing the ECM. Josef644's car seems to be tolerating it. I can't think of a reason why it wouldn't be OK, can you?

[This message has been edited by fyrebird68 (edited 07-05-2010).]

quote Originally posted by fyrebird68: I don't know what other inputs the ECM might be sampling. The ones for the AC itself (coming from the pressure switch) are working or I wouldn't have +12V at the top side of the relay coil.

To be more specific, are you talking about a lt blue wire or a black wire at the compressor control relay? The compressor cycling switch if closed will enable +12v on the lt blue wire and will close the compressor control relay when the ecm provides the ground signal. The black wire is the power waiting at the compressor control relay to energize the compressor clutch through the dk green wire.

The only two pressure switches I can see are the compressor cycling switch (ends up being the lt blue wire at the compressor control relay) and the A/C cutout switch for over pressure at the compressor which will open the ground circuit for the clutch coil. So as you state, it could only be the ECM failing to complete the circuit on the CCS side of the compressor control relay unless you are somehow getting confused... (possible but not likely)

Charlie

Just for kicks and giggles you might want to pull out the control head ( the controls for the heater ) and look at the circuit board. I found mine had one trace burned off on it that would not let the relay engage on it's own. May not be the problem for you but might be worth looking at.

See cave, scan tool help under ECM section.

Relay and clutch is fine.... Red circle.... Ground at wire mean relay and clutch works...

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 07-06-2010).]

Got sidetracked with chasing and repairing freon leaks. I haven't forgotten - I'll post my findings when I get back to this problem.

Pull the control head...remove the cover...you'll likely find one or more of the printed circuits is fried.

You can bridge the burned circuits with wire if you're good at soldering.

A failing clutch can cause excessive current flow through this circuit which will fry the printed circuits on the board.

Also don't forget to check the fuses.

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