Hey guys, tomorrow I'm doing the purge valve mod to help out with some A/C issues and I noticed the wiring harness is different. ADMIN EDIT: REMOVED OLD IMAGE I was talking to Joe and he said that 2X is actually 4T on my unit but were unsure where 1Q would be on my mspnp. Best guess is that it would be the green/black wire on 1K. Before I place the jumper I wanted to see if anyone has done this on the 96-97 unit and can confirm 1K. Thanks

listen to Joe. that diagram is old and has been replaced.



and the fans are incorrect on the above. lol

Thanks man, I pulled that from an old thread. Appreciate the help guys.

This morning I went outside and did the purge falve mod. This is the one that activates the unused charcoal canister's purge valve to give a little idle bump when the a/c turns on. So I placed my jumper between 1K and 4T and it worked! Everytime the a/c compressor turned on the valve would open and voila, no stall.

Until it started stalling. It stalled with the a/c on, it stalled when I turned the a/c off, it just refused to run. I turned the car off and pulled the jumper out but no dice.

What was happening is that the a/c compressor was not shutting off, at all. I finally noticed green a/c oil dripping out so I shut the car off and disconnected the two large a/c relays at the front on the passenger side.

So anyone got any guesses as to why the a/c refuses to shut off? I'm assuming the MSPNP is no longer closing the connection. When I press the switch in the cabin the little light will turn on and off but it doesn't seem to make any difference whatsoever. I checked the MS itself and no smells of ozone or apparent damage on any connectors or solder points. I also checked the harness but all seems to be well. The wires I jumped both appear to be firmly seated in their connectors. However if there were a wire that would cause this which would it be?

At the moment the car runs, just no A/C, if I plug the relays back up the whole problem comes back. However with the GA summer looming I'd really like to find a solution.

I suppose it's possible that this problem isn't MS related although Occam would say it would be odd for it to happen as I was modifying something to do with the a/c. I just don't see how a jumper could cause this.

Ok so I did more diagnosis but I'm still stuck. It's my understanding that the ECU sends a signal out through the Lt/G wire I jumpered. That signal goes through the pressure switch, the temperature switch, the button on the dash, and then out to the relay that turns on the compressor clutch.

I checked the relay and it seemed to be operating as normal. Also, the button on the HVAC does nothing, so it should be safe to assume that's fried. However with the a/c constantly on the pressure switch never seems to kill the signal either. So it would seem that, at least, the pressure switch and HVAC switch are dead and stuck open.

Does that seem odd to anyone else? Usually when a switch breaks it breaks in the closed position. I understand the pressure switch is normally closed but the dash button is mechanical, it wouldn't seem like one or two extra volts would somehow just make it a solid connection.

I'm really stumped on this one. Is there another way the compressor clutch can activate? Can the ECU override the switches? From the wiring diagram it doesn't seem possible but it's certainly happening.

When the LG/B wire is pulled to ground, the ECU sends a ground out on the BLU/BLK wire to energize the A/C relay and A/C clutch. What you did was jumper another potential ground path into the LG/B wire. The function of the YEL/RED wire is to provide a ground to close the purge valve solenoid. To be safe, a diode should have been used so that a ground on the LG/B wire would be seen by the YEL/RED wire, but not vice versa.

In addition, whenever you add an inductor (such as a coil) to a silicon circuit, you need a protection diode to dissipate flyback voltages that happen when the inductor discharges. Your stock wiring for the LG/B wire only has switches . . . no coils. Therefore, I would presume it is an unprotected circuit. By jumpering into the YEL/RED wire, you've added a coil to the circuit (the coil in the purge valve solenoid).

Time to get out the multi-meter. Is the LG/B wire at ground? If so, where is the ground coming from? I suspect internal frying. You've likely also done some mechanical damage if you're leaking R-134 oil.

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90 1.6L Greddy + MSPNP, WI, Cowl Induction, FCM
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The mechanical damage I expected. That doesn't worry me too much as a compressor can always be replaced, it's the ECU stuff that bothers me.

So what you're saying, just to make sure I understand correctly. Is that the LG/B wire is likely grounded which causes the BLU/BLK wire to send the signal for the compressor to activate?

Would it most likely be grounding within the unit itself or somewhere on the car? If it's somewhere on the car than it would likely be the A/C switch in the HVAC right as the other two switches don't connect directly to a ground? If it's somewhere inside the ECU would you expect it to be something that could be fixed or just a fried unit (like perhaps internally to the chip)?

Thanks for your response btw

I wouldn't have expected the jumper he added would kill anything, but it sounds like the a/c relay is stuck open. pull the connector out of it and replace.

My guess is that something inside the ECU has failed. Likely a diode or transistor (i.e., a silicon PN junction somewhere). Quick test, pull the connector and see if the pin for the LG/B wire is at ground. If everything is OK, it shouldn't be.

the compressor is activated through the a/c relay...something is holding it open. the ECU provides a ground to that circuit.


pull the wiper fuse and replace and see what happens.

Hornet: Which connector do you mean? Does the car need to be running for this test? I was just wondering because I know I can't unplug that section of the harness while the car is running. Excuse my ignorance but what is a PN Junction? I've been trying to learn this stuff recently but I havent gotten to that yet.

Hopefully it's just a diode or transistor as I have a ton of those laying around from other projects. I'm still trying to figure out exactly what I need to do to locate the faulty piece. Do I check for ground and then just work my way from the pin until I find a point that is no longer grounded?

Brain: I assume you mean the relay under the hood next to the passenger side headlight? Right now I have it unplugged so I can drive the car. I checked it with my multimeter and it was closed although I have no solid way to test with the car running. I'm currently searching around to see if anyone has one locally I can try. Does the car need to be running for the ECU to hold the ground to the relay open or can it just be on?

I drove the car for about 20 minutes today with the relay unplugged and it ran fine. But is there any danger of the unit possibly overheating if it is holding that pin open like that?

BTW, I was talking to Joe about this earlier and I think, at least till a proper fix is discovered, there might need to be a warning in the "Great A/C FAQ" thread. I know people still reference that thread regularly.

The wiring diagrams I have show that the AC Relay is normally open. It is pulled closed by a ground on the ECU BLU/BLK wire. So, what you suspect is that the relay is stuck closed (melted contacts)?

yeah that.

I meant the ECU connector. Car not running. If you have a failed component, it won't matter whether the ECU is powered or not. However, Brain knows the guts of these things better than just about everyone else, and he seems convinced that this jumper should not have caused damage.

You just stated that "you checked [the AC relay] with my multimeter and it was closed . . . ." That's a failure. The AC relay should be normally open. Heck of a coincidence but a nice easy fix if that is all it is.

You should get a wiring diagram for your car BTW.

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90 1.6L Greddy + MSPNP, WI, Cowl Induction, FCM
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The only wiring diagram I have is the Hayne's manual.

I actually I just pulled the relay and I misspoke. Completely disconnected from the car the switch inside the relay is open. So the MS must be powering the relay.

So next step is to open the MS on Thursday and find the faulty connector. Any recommendations on how I could narrow down the search and isolate the fried part? Diagnosing circuit boards is still foreign to me.

Bummer.

So, start at the AC relay socket and, with power on and AC controls off (car does not need to be running), confirm that a voltage and a ground are present to operate the coil. If the ground is present, move on to the ECU. ECU tests will be done with power off since we have to pull the connector.

On the ECU, check if the ECU pin for the LG/B wire is at ground. If it is, there is an issue inside the ECU. If not, then do the same thing on the wiring side. Ohm out the normal LG/B wiring path and your jumper separately. Start writing the stuff down so you know where you are.

If it is internal to the ECU, then it will get a bit tricky. I don't believe that schematics for the MSPNP daughter board are available. Anyway, don't open the MS until you know that is where the problem is. Hopefully you'll find something in the wiring.

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90 1.6L Greddy + MSPNP, WI, Cowl Induction, FCM
Build Thread

follow the output you jumped back into the MS. They use a fairly simple relay circuit to activate the A/C. Most likely a red zener diode, a p2n2222 transitor, and a 1K resistor.

Matt at DIY seems to think it might be the switch on the dash, which would be ideal. I have tomorrow off so I'm going to dive into this and at least find out whether it's in the MS or in the switches. I'll let you guys know what I find.

I assume to test a diode I simply see if it's allowing current through in both directions?

yes. or none in any.

Well it's confirmed, it's something in the ECU. The pin for the LG/B wire was groused. So I opened the ECU to start looking for the faulty component but the connections where the pin connects to the circuit board are not visible.

Not sure what to do next I remembered Brain saying that a 1K resistor would likely be used on the switch so I checked and both 1k's on the daughterboard and they were fine. So then I figured I would just check for continuity between the pin and the resistors to see what it a part of the circuit. Well, almost every resistor on the daughterboard is connected to the pin, most are between 600 and 800 ohms. So that didn't really narrow anything down for me. Brain also mentioned a 2N2222 transistor, there are 4 of them on the daughterboard next to the harness but they were the same as the resistors, all connected to the pin with varying amounts of resistance.


I really need a bloody diagram for this thing but I know DIY won't provide it.