95 Mustang - A/C not working, electrical issue

Not knowing where else to turn...

What voltage should I have at pin 22 of the CCRM on my 1995 Mustang GT when throttle is closed?

Reference diagram;

With the key on, engine off, and a/c controls set to A/C with lowest fan speed I have no power to the a/c compressor clutch. I have good continuity through the low pressure and high pressure switches. I have no power exiting pin 23 (or 16, which based on this drawing I wouldn't expect to unless I'm reading this wrong). I have about 93 ohms of resistance between battery negative post and pin 22 on the CCRM.

I have a PCM that already seems to have a fault with the OBD-1 "grounding pin", I cannot get the ECU into diagnostic mode anymore, this is an old issue already discussed here. I have not checked PCM pin 54 for power. All fuses good so I'm fairly certain that there's no power to be found at pin 54 but I know what assumptions get me. Still, this feels like I've got an ECU that is dying, is that a reasonable assumption? Car has no other real malfunctions. Back when I could get diag data out of the PCM it had a stored code for fan driver circuit error but the fan operated just fine as far as I could tell and has never overheated.

When I got this car a year ago I was told the A/C worked great, was refilled a couple years prior. Surprise, attempt to use a month later, nothing. So I left that off until a month ago when things got really hot, hooked up a set of gauges expecting to need to refill & add dye, and the system worked just fine. Fantastic! Then it stopped working a week later. The compressor clutch doesn't click in and out, it just stays off, which made me wonder if it wasn't an electrical control component.

Without knowing what that A/C Clutch Control is supposed to do I am a little lost. Referencing old WOT relay information, I want to think there should be 12VDC+ at pin 22 at all times save for WOT but I saw now change going from closed throttle through movement to WOT at the TPS.

Or my testing methodology is screwed. What say you, mighty GRM smarty people?

Just out of curiosity, you've hot wired the compressor to make sure it is working with the engine running right? If your compressor checks out then try replacing the clutch cycle switch(very common failure on sn95's, its on the passenger side in the corner.

In reply to Caleb:

The low pressure switch had good continuity with the car off, I have doubts that it is a problem, I have read a lot of forum posts pointing towards that but then the CCRM seems to play up and the PCM too.

Just hope you don't have to take the dash out to fix it. It is a boat load of fun. If you do replace the Heater Core while you are in there. One of the best while I was there things I have done in a long time.

I would think it would be a signal return, aka Ford's wording for a "sensor" ground, or 5v. But if you don't have the 12v out of the relay output, I would swap the relay with another nearby. If that doesn't work, I would back probe for 12v on the input of the relay.

put in a switch to turn on the compressor whenever you want cold air..

Key on, engine off (KOEO) you probably won't have power. My Trooper won't energize the compressor clutch until the engine has been running for about 5 seconds.

In reply to Ranger50:

Ford decided to put this relay along with the fuel pump and the cooling fan relay into a common body called the Constant Current Relay Module, or CCRM. What used to be a trio of $10 relays because a $100 POS they hang behind the radiator on the overflow bottle bracket. You know, a nice cool place with no vibration added, and with the serp belt running a few inches away very easy to troubleshoot safely with the engine running.

In reply to Curmudgeon:

I've wondered about that and will see what I can do this evening, thanks for the idea.

In reply to pres589:

I know all about the CCRM. Reason most people throw them away and rewire a whole car.

In reply to Ranger50:

If I were to do anything like that I would try and remove the WOT function inre: the A/C compressor clutch circuit. Which actually shouldn't be that hard... wonder how hard it would be to de-pin the CCRM connector and work with the stock pins / wiring to mod the system (corrected from previous bad idea that wasn't really what I meant).

Is your 95 a manual or auto, because i have an extra auto ecu out of our 95gt from were we converted to a manual trans. Let me know if your interested in it/

In reply to Caleb:

What are the differences in the ECU's? I would be if I can use it, I have a manual trans, are you sure you had to swap?

EEC-V has transmission control added for the AODE/4R70W when they went to the 104 pin connector. The manual EEC's don't have it.

In reply to Ranger50:

Mine is EEC-IV. Would having the pins running out to non-existent cabling throw a check engine light?

In reply to pres589:

Sorry, I meant 4, given the hybrid nature it shared with the 5.

To answer the question, yes.

lol Yes I most defiantly had to swap ecu's when i did the trans swap, but the car was drivable with the auto ecu. The issue with using the auto ecu is that it pulls timing like crazy every time it wants to shift to help protect the drivetrain. Basically your driving and all of a sudden your ecu decideds it needs to shift and pulls the timing back to like 5-10 degrees,feels like you hit the rev limiter and pretty much have to shift,Also the auto ecu has a much lower redline.

In the garage; engine running, controls set to normal a/c, less than 0.10 dc volts at pin 22 on ccrm. Thoughts on feeding batt + to that pin to see what develops, or am I risking ecu damage?

Edit: forget all that, for ha has I tried jumping around the low pressure switch and the comp clutch engaged! Pull the connector off the switch, check resistance betweNen the two pins on the switch, and it's about 2 ohms. Plug back in, pull jumper wire, and through the switch itself the system is inop. Back with the jumper and it works again. Is 2 ohms too high?? What the Berkeley...

Hm. I'd say 2 ohms is within the 'noise' of the DVOM. It may be that the switch can't carry the correct amperage, I have seen circuits which had continuity etc but had maybe only 1 strand of wire meaning they could not conduct the proper amount of power to perform a given task. I'd dig up a new or known good low pressure switch and try again.

Closure: The car seems fixed with a new Chinese low side pressure switch. Will monitor of course. I guess the take away from this is "do some easy testing anyway".