I have a 1994 Chevrolet Silverado 2500 7.4 with NV4500.
My air-conditioning has always worked great. A few weeks ago I started it in the morning, no ac. It worked fine the evening before.
I first thought low pressure switch or connecter has gone bad again. I did the paperclip trick and no cigar. So I paper clipped the high side also no cigar.
I checked the voltage at both plugs. Getting 12 volts.
Next I checked the voltage at the plug to compressor. Got 1.4 volts with ac turned on. Someone said, I bet your diode is bad. I checked the compressor clutch with 12 volts and it engages that way.
So I ordered new diode and installed. The volt reading was now 1.9 volts.
So I pulled the glove box out to check the relay. I could hear it clicking when turning the ac on. I went ahead checked voltage at relay it was 12v. I checked resistance got 1.9-2.0 reading. I switched relays around, didn't do any good.
So I read the wiring schematic and see it travels through the ecm. I thought maybe the computer went bad. Had an extra and installed it. Still no cigar.
I'm at a loss. I don't see any breaks in wires. I checked grounds with test light and it glows bright.
Any ideas or thoughts???
Obviously the 12V clutch coil is not going to work on 1.9 volts. Try testing the new diode with an ohm meter, it should have high resistance in one direction and low in the other. Make sure it's installed in the correct direction. Also check the voltage before the diode to make sure it's being fed 12V.
I'll check the voltage before the diode. It's hooked up correctly. I was very careful and had a good diagram to follow. If the voltage is low before the diode, what would be the next thing to check?
Thanks
You would have to trace the electrical fault preventing 12V from reaching that point. If there is 12V at the relay contacts that would suggest the problem is between the relay and the AC clutch coil.
Agree. I'm thinking there has to be a break somewhere.
I went ahead and put on a new clutch relay. When I start the truck and turn on ac all the wires come in at 13.8 Volts. Except 87 green wire which is -0.10 reading. I'm testing right on the backside where connectors plug into relay. I should be getting same voltage there right??? Also I'm not getting voltage at either pressure switch now...
I'm not familiar with your truck's electrical system. All I can really say is generically that the clutch coil needs to be getting in the vicinity of 12 volts to engage and if it's not receiving that voltage when the AC is turned on you have a fault somewhere. You can test the clutch coil by hot-wiring it to the battery and if working it will engage. (In fact Scotty has talked about just hot-wiring the compressor clutch to a toggle switch in the car to run it manually if electronics have failed.)
Ok. Maybe someone else has an idea. Thanks for your help
Sometime in the 1990s Chevy switched control of the AC clutch relay from the Heater/AC Control Head (when the PCM just monitored the load side of the circuit)[Old Style] to the PCM exclusively [New Style]. (The PCM still monitored the load side of the circuit on the new style).
I think you keep seeing that low voltage on pin 87 (and the compressor connector) because you're seeing the PCM bias voltage when the Load Side relay contact isn't closed.
You want to determine if this is a problem on the Load Side of the circuit or on the Control Side of the circuit.
Assuming yours is PCM controlled it looks like this. (I highlighted the Power in RED and the Ground in GREEN of the relay socket wires)
OK. First, I can't agree with those pin assignments on the Load Side of the relay in the diagram.
On the Load Side, Power (almost) always comes in on Pin 30 and Goes to the Device on Pin 87.
Like this:
But that doesn't matter because you just want to determine if the Load Side of the circuit is working.
The diagram shows Power to the Load Side fuse is "Hot At All Times" so you shouldn't even have to turn the Key ON for this.
Remove the relay and take a hefty paper clip or a piece of wire and "jump" relay sockets 30 and 87. The compressor clutch should engage.
If it doesn't, grab a test light and make sure there's Power on socket 30 (or at least on socket 87 if the diagram isn't wrong about that ((which it is))). Use a test light. It isn't much of a load on the circuit but it's better than a multimeter (no load) for this. (If you don't have Power at socket 30 check the fuse). Also check the Ground at the compressor clutch connector
The old type provided Power to the Control side and the Load side through only 1 fuse with Key in RUN.
The new type used 2 fuses and 2 separate circuits for Power to the Load Side and Power to the Control Side of the relay. You can see that in the wiring diagram.
So if you aren't sure if you have the old style or not, turn the key to RUN and check again.
If you have power going into the Load Side of the relay but the compressor clutch doesn't engage, and you don't see Battery Voltage on the compressor clutch connector either when you jump relay sockets 30 and 87 then there's a problem in the Load Side wiring.
Use your multimeter to check for continuity between relay socket 87 and the power side of the AC clutch wiring connector to see if there's a broken wire.
However, if you jump relay sockets 30 and 87 and the compressor clutch engages then this is an issue on the Control Side of the relay. (sockets 85 & 86).
Here I agree with the wiring diagram. So on the "new style" (unlike relay socket 30 which always has Power) relay socket 86 only gets Power with the Key in Start or Run.
Use your test light to see if that's happening. If it isn't, check that fuse too.
But if you can jump sockets 30 & 87 and the compressor clutch engages, AND you also have Power on relay socket 86 with the Key ON then this is an issue with the Control Side Ground (relay socket 85) which energizes the relay coil (and closes the Load Side contacts between sockets 30 & 87).
Let us know what you find out and we can go from there.
... I'll post the "Old Style" wiring diagram too just in case you need it or somebody else does.
Same deal I highlighted Power in Red and Ground in Green
Thank You so much. I will be trying this out. I've been doing different test and it just has me stumped. I will let you know as soon as I do this and let you know. Thanks Again!!!
Ok pin 30 gets power when key turned on, nothing on 87. I jumped them no clutch engagement. I then took volt meter while jumping and checked voltage at connector for clutch. Got a reading of 0.03.
I wasn't able to do a resistance test yet. I need longer wire to reach inside glove box to clutch connector.
I also notice that the relay doesn't click with the new or old or others I switched in place. The one that clicked is the one that turns on blowers etc.
It's time to test the AC clutch. Refer to the wiring diagram. 1 wire to the clutch is Power and the other wire is Ground
.
Grab a couple of wires (12 gauge will work), disconnect the wiring connector from the compressor
.
Put 12 volts from the battery positive post to 1 terminal on the compressor and Ground from the battery negative post to the other terminal
.
The AC clutch will engage if it isn't defective
.
Already did that. It engages fine. Did that early on.
Then that narrows it down. Just check that you have a good Ground on the Ground wire on the compressor clutch wiring connector
.
The easiest way to do that is with the relay still removed and the compressor clutch wiring connector plugged in, backpin the Power wire coming from relay socket 87 at the compressor clutch connector and use a jumper wire to apply 12 volts to the pin
.
If the clutch engages that tells you that you have a good ground on the wiring connector and the issue is in the wiring between relay socket 87 and the compressor clutch
Ok, great! I didn't even think of trying that. I had to get back to work, but that will be my next step. I really appreciate the help and time you're taking with me on this gremlin.
I'm assuming the ac is not turned on while do this and no power on correct?
Yes. You'll even have the compressor clutch relay removed. You want to isolate this to make sure you have a good ground and then move onto the wiring between relay socket 87 and the compressor connector
Will do. Thanks!
My apologies about the delayed response. My sister passed away on 06/23 and I've been dealing with that and still dealing with my brother who passed away on 04/20.
I just did the pin test as described and got the clutch to engage. So I have a good ground. It engaged strong too.
Ok. I found a broken wire going to the cycling switch. Repaired that. When I do the paper clip on 87 & 30 turn key on clutch engages. I put in the relay turned on truck, no dice. So I removed relay did paperclip again and works fine. . 86 has power when I do that and 85 has nothing at the plug when do paper clip. I'm confused 🤔
Correction. 85 has power and 86 has nothing
So I see that's the blue white wire that goes to the control head for heat a/c. I guess it's possible it blew because of the break in the wire, grounded out or something?
Bump
I read through your recent posts so let's see if I'm up on current events
.
By jumping relay sockets 30 & 87 you're able to get the AC clutch to engage
.
So you know that the Load Side of the circuit is good and IF the Control Side of the relay is energized the AC clutch will engage
.
You're also getting Power to the Control Side of the relay on relay socket 85. We'd expect to see Power for the control side of the relay on relay socket 86 but that doesn't matter. There's no law that says it has to be wired that way. The important thing is that the relay coil is getting power on one side (either socket 85 or 86). So we'll go with 85 is the power and 86 is the PCM provided Ground to energize the relay
.
You've narrowed the problem down to the Control Side of the relay circuit. Specifically the PCM provided Ground that energizes the relay coil (because you have power on socket 85)
.
This is the part of the circuit in which the computer uses the pressure switch voltages to determine if it's going to send the Ground to energize the AC clutch relay
.
You didn't mention if you hooked up a set of gauges and checked the static pressure of the system. If the system popped a leak and the pressure is either too low or completely gone the PCM won't energize (Ground) the relay coil
.
At this point I'd want to see if the problem is that the system lost refrigerant before testing the wiring and pressure sensors
.
((Please accept my deepest condolences regarding the loss of your sister and the recent passing of your brother. I know how much losing family hurts))
Thanks for the response. I will check the freon level in the morning and see if that's the next issue. I guess it's possible it has lost some charge because I haven't been using it. I will keep you posted. Thanks again
Ok. It looks like plenty of freon. I tried to do the paper clip on the cycling switch to get compressor to come on and it didn't work. Back to confusion again 🤔
You need to know those static pressures on the high side and the low side
.
I looked through my bookmarks and found the 1994 C/K Truck Service Manual
.
You can download it here:
.
https://www.mediafire.com/file/7d5v4lasgb495ta/1994_NATP_9431_1994_CK_TRUCK_SERVICE_MANUAL.pdf/file
The AC stuff starts in chapter 1B (page 104)
I took a couple of screenshots which contain useful information
.
First is the High Pressure Cutoff Switch
.
It's normally Closed when the pressure isn't too high
.
You can use your multimeter to check for continuity on the 2 terminals on the switch
.
Next is how the Pressure Cycling Switch works. The important thing is that it Closes (at around 40 psi and it Opens below that). So if you have a sufficient AC refrigerant charge, again using your multimeter you'll see continuity between the 2 terminals on the switch
.
.
Here's a screenshot of Diagnosis Chart B. I highlighted to where you are now
.
.
There's a few other diagnostic charts in the service manual with helpful information so look through those too
Will do thanks!
Ok. After off and on working on the unit, traced it back to the AC Heater Control panel. I have cold ac again😀. I want to express my gratitude for all the help and publications etc. I found issues that needed fixing in the process. I just want everyone involved to know it was not a waste of your time. Been a difficult year for me, so this meant a lot that strangers helped me out. Thank You again!