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Ask Juan Crespo Your Own Question
Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1523
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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AC clutch won't engate engage, Well I did some T/S and

Customer Question

AC clutch won't engate engage
JA: Has the clutch been slipping? Are you having any trouble shifting?
Customer: Well I did some T/S and worked through the pressure switches and finally got it to engage by providing a manual ground, so I thought it was the ECM. ECM swapped and problem is still there no slipping to date.
JA: What is the model/year of your Chevy?
Customer: 2000 GMC k2500 5.7l
JA: Are you fixing your 2500 yourself? What have you tried so far?
Customer: Check for voltage and relay and everything was good in and through them. then bypassed the pressure switches.
JA: Anything else you want the mechanic to know before I connect you?
Customer: Not really
Submitted: 6 months ago.
Category: Chevy
Expert:  Chad Farhat replied 6 months ago.

Hi, I'm Chad. Welcome to JustAnswer. I'm reviewing your question and I will be posting your reply ASAP.

Expert:  Chad Farhat replied 6 months ago.

are you saying you jumped the pressuer switch by jumping the connector for it and ac clutch came on ?

or the ground was missing ,you added ground it came on? where did you add ground to at what point in the circuit ?

please let me know

Thank You *****
ASE Certified Technician

Customer: replied 6 months ago.
Bypassing switches had no effect, proving a ground for the relay that supplies power to the clutch worked, so I thought it was an exam problem.
Expert:  Chad Farhat replied 6 months ago.

if that is the case , and you already changed ecm

You need to do a continuity test on the ground wire coming from ecm to relay and see where it is broke .

follow the color wire all the way back to ecm.if the giving ground is making it run at relay ,then from relay back to ecm must be the issue

do you have auto or manual ac controls ?

Customer: replied 6 months ago.
I'm guessing manual, I have to rotate the dial to the tempurture I want and have a push button on/off button
Expert:  Chad Farhat replied 6 months ago.

ok will it is manual control then

question the blower motor is still working on inside the cabin ?

Customer: replied 6 months ago.
Sorry, yes the blower is working
Expert:  Chad Farhat replied 6 months ago.

if the blower is working and you are missing the ground the ground comes from the pcm based the wiring diagram for your truck

The dark Green & White wire is the wire coming from the pcm to the Relay as you can see in the diagram provided

https://www.dropbox.com/s/lycprb026ugbvvl/00000000000000011111111111111a.gif?dl=0

Please let me know if you need anything else on it, just reply back & we will continue

If you are good , kindly rate me.

A Positive rating is much appreciated

This post will stay open for you even after you rate me, for you to come back and ask any follow up question you may have on this issue

Thanks You
Chad Farhat
ASE Certified Technician

Customer: replied 6 months ago.
Did some testing tonight and the pin out appears to be slightly different then the diagram. From relay (85) my green/white wire goes into the end on white jack pin #2, I have 12VDC to the pin
Customer: replied 6 months ago.
Also have power through low pressure cut out and up to the high pressure cut out switch, which I have previously bypassed
Expert:  Chad Farhat replied 6 months ago.

how about ground

Customer: replied 6 months ago.
It's connecting to pin 1, I think I was on the wrong wire under the fuse block. Looks like I'm getting about 16k ohms removing the relay and testing ground through the ecm, I'm not sure if that's good or bad. Tested with truck running, act button pressed in and out
Customer: replied 6 months ago.
Do you know if the ground provided by the ECM is reliant on something else providing voltage into it. I'm wondering if the AC button on the dash which appears to be functioning (light is coming on, and I'm getting voltage on the low pressure cut out) could play a part in gating a transistor that provides ground or something. I'm going to head to bed, I'll pick this back up tomorrow evening. I'm thinking about jumping the compressor relay 87 -->30 for a 4 hour trip I need to make this weekend, I'm sure you have to tell me I'm overriding my safety switches and that could cause more problems, but other then that is there a potential problem with the compressor staying engaged for 4 hours?
Expert:  Chad Farhat replied 6 months ago.

yes , it needs to cycle ,if it stays on 4 hours the lines will freeze and no more cooling.

what you can do, id drive ,run it and every 10 minutes cut it off 2 minutes .

the compressor will lock up on you if it stayed on 4 hours, so you do the cycling yourself by switching it on & off

The ECM does provide ground to the relay .

Expert:  Chad Farhat replied 6 months ago.

Please dont forget to Rate me before you exit since that is how we get compensated

Thank You

Chad

Customer: replied 6 months ago.
What are your thoughts about the high resistance from the ecm
Expert:  Chad Farhat replied 6 months ago.

you mean you test it out ecm pins and you had high resistance on some pins or all , is this what you mean.?

Customer: replied 6 months ago.
I'm getting about 16k ohms removing the relay and testing ground through the ecm, positive lead of mutimeter had no continuity, switching leads and using negative resulted in 16k ohms. The diagram shows a ground with an open, so I assume there are other circuits that provide the gating voltage to close the ground path, but I can't see what they are in the diagram you sent.
Expert:  Chad Farhat replied 6 months ago.

what is the reading on PIN 85 on the compressor relay ,switch on ?

do you have a ground on that pin ,it should be the one coming from PCM, dont worry about what color it is , are we getting a ground on that pin to complete the circuit and activate that relay ?

Customer: replied 6 months ago.
Pin 85 has 12VDC on it with the relay installed. I took this measurement on the actual wire going to the ECM, this wire went to the white connector on PIN #1.
Expert:  Chad Farhat replied 6 months ago.

ok what is on pin #1, the one to ecm ,is there ground on it ,with igntion switch on, a/c on switch also ,?

Expert:  Chad Farhat replied 6 months ago.

does pin# ***** wire, does it pass a continuity test between relay & ecm ?

Customer: replied 6 months ago.
it did.
Expert:  Chad Farhat replied 6 months ago.

if your wire is good , then that leaves ecm is not sending ground to the relay.

what is the mileage on the truck, any after market radio or stereo tied it to same circuits ?

all frame to engine block grounds checked ?

Customer: replied 6 months ago.
if your wire is good , then that leaves ecm is not sending ground to the relay.
-- I put a ground wire on the new ecm directly to ground last night to check that theory out becuase from the ECM frame to chassis ground I was getting 90 ohms. The ground wire didn't have an effect on the AC.what is the mileage on the truck, any after market radio or stereo tied it to same circuits ?
--220k with original parts, but I do believe the dash (all instrument panels) was replaced at one point before I purchased itall frame to engine block grounds checked ?
- No, and I did have the engine replaced last year, The AC worked after the engine was replaced though.
- Is this as simple as perfuming a resistance check from the chassis to the engine?
Expert:  Chad Farhat replied 6 months ago.

I m sorry man ,i m not sure which way to go on this one, So i will open up your question & opt out for another expert to help you on it.

Customer: replied 6 months ago.
Chad, I'm trying to rate you on, but the website won't permit me to make any changes, is there a way for you to open that up?
Expert:  Juan Crespo replied 6 months ago.

Hi there. Different expert here - previous expert opted out.

In addition to an A/C request from the A/C control head, the PCM needs to see "good" signals from throttle position and engine temperature sensors before it will switch the circuit to ground the relay. Is there any indication of a problem with either one of those three signals? Check Engine Light on? Engine misfiring or overheating?

Customer: replied 6 months ago.
No problems with any of those items everything is running strong. I have a code reader I can use to see if there is anything reporting if you think that will help
Expert:  Juan Crespo replied 6 months ago.

Can your reader report on A/C request status or just retrieve codes?

Customer: replied 6 months ago.
I haven't used it alot so I'll have to check it when I get home. I will run in "live mode" and report on a list of operating conditions in real-time. I primarily recall a lot of engine specific readings. I'll plug it up and see what it will report tonight. The reader is the innova 3040C I believe.
Expert:  Juan Crespo replied 6 months ago.

Well, according to the Innova website, the 3040C can read live data; they just don't say which specific parameter data indicators. Let's hope it does read A/C request - the parameter might be displayed as a simple YES or NO. Please let me know what you find out.

Customer: replied 6 months ago.
Ok I'm hooked up. AIT is reading 91deg which is about right
Customer: replied 6 months ago.
I have 3 error codes. EGR, HTR & O2S
Expert:  Juan Crespo replied 6 months ago.

OK. I need those 3 code numbers as they might be what's preventing the PCM from supplying ground to the A/C relay.

Also, you'll need to set the reader to display live sensor data, then look for A/C request; is it ON or OFF?

Customer: replied 6 months ago.
Also, you'll need to set the reader to display live sensor data, then look for A/C request; is it ON or OFF?
- I didn't see anything while in Live Data that looked like it dealt with the AC
Expert:  Juan Crespo replied 6 months ago.

I'm not familiar with your reader. I'm just going by the Innova website claims the 3040C can do.

In any case, can I at least get the code numbers so we can be sure that's the root cause of the issue?

Customer: replied 6 months ago.
Sorry for the delay, I wasn't getting any codes and wanted to make sure I was doing everything correctly. the EGR, HTR & O2S icons are red which indicate that the scanner is unable to read them. There were no codes stored in the ECM however.
Expert:  Juan Crespo replied 6 months ago.

OK, then we're left with testing for power and ground at the PCM as our only recourse - problem with that is that we won't know if the PCM is or not processing the signal.

Would you like me to upload the circuit diagram for you to check those powers and grounds?

Customer: replied 6 months ago.
Please, I can go through and perform the tests and provide you the results in batches if that works. I'm going out of town in the morning and will only have brief moments to work on this over the weekend, so I may have long lags in my responses. If you can give me 5-10 things to check I can provide the feedback and we continue to narrow down things. If that works I'm game. I understand your time is important and don't want you to feel like I'm going to rate you negatively if you would prefer to wait until next week when I can devote more time also.
Customer: replied 6 months ago.
Just want to reiterate that this is a brand new PCM, so I would hope the odds are good that it will perform as designed.
Expert:  Juan Crespo replied 6 months ago.

Let's hope is not a defective one. I'll be off starting tomorrow evening and won't be back for at least two weeks - could be longer.

Use the attached diagram for reference - RED means system voltage, GREEN means ground. Perform all checks with engine running at idle and A/C turned ON, blower on HIGH.

Best Regards.