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Old 08-05-2014, 08:33 PM   #1
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Default 1999 s10 ac problem

I have a 1999 chevy s10 4.3L with ac problems. To date New compressor, accumulator, high pressure switch, high/low pressure lines, orifice tube, evaporator. Cannot get ac to come on. Jumped compressor to charge system. Currently 45-50: it'* in the upper 90s here. Have current at the clutch switch, high pressure switch. No current to low pressure switch. All fuses and relays good. Even changed out the Underhood fuse block thinking maybe I had a short.
When I had the compressor running had cold air coming though. Also, checked current from control switch in the cab to the ac clutch...works fine. I was reading your forum regarding jumping the low pressure switch but I do not have current there the jump. Also diode is New. Any thoughts?
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Old 08-05-2014, 09:55 PM   #2
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Did you put a good vacuum on the system first, then add cans of R134? It does take several cans to bump the pressure up before the PCM commands the compressor to turn on.
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Old 08-05-2014, 10:17 PM   #3
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Yes. I pulled a vaccum fir 45 mins. It held at -22the for ten mins. I put at least 2a cans of 134you in.
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Old 08-05-2014, 10:19 PM   #4
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I did not have a voltmeter only a test lamp. I bought a voltmeter and i will record the current tomorrow.
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Old 08-06-2014, 05:46 AM   #5
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Per Alldata for my 2002 S10 4.3

Quote:
The following conditions must be met in order for the PCM to turn ON the compressor clutch:

Engine coolant temperature (ECT) is less than 121C (250F)
Engine RPM is more than 550 RPM
A/C low pressure switch signal circuit is grounded
A/C request signal circuit to the PCM has 8.5 volts or more
Once engaged, the compressor clutch will be disengaged for the following conditions:

Throttle position is 100 percent
A/C request signal circuit has 0 volts
A/C low pressure switch signal circuit has voltage
Engine coolant temperature (ECT) is more than 121C (250F)
Engine speed is more than 5,500 RPM
Transmission shift
PCM detects excessive torque load
PCM detects insufficient idle quality
PCM detects a hard launch condition
When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike
Quote:
A/C Pressure Switches

Two pressure sensors protect the A/C system. If the A/C low pressure falls below 145-172 kPa (21-25 psi), then the A/C low pressure switch will open. This will disable A/C compressor operation. This switch will then close at 262-290 kPa (38-42 psi) to allow A/C operation. If the A/C high pressure exceeds 2826 kPa (410 psi), then the A/C high pressure switch will open and not allow for the PCM interpret an A/C request. This switch will close at 2413 kPa (350 psi
).
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Old 08-06-2014, 09:08 AM   #6
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First, you are looking for voltage, not current...just a matter of semantics....

Is the 45-50 psi, static or running pressure on the low side, when you jumpered the compressor? If static(compressor not running), then you still don't have enough refrigerant in the system....

Disconnect the harness to the ac clutch....attach your test light to the ac clutch harness....start vehicle...request ac....does the test light come on when you request ac?

If no, disconnect your test light lead from the black wire and touch that end to a good ground....Does the test light light now?

If yes, you have a bad ground....I think that low pressure switch shares the same ground.....

That low pressure switch should have 12 volts on it with ac request...disconnect harness from low pressure switch and attach one lead of test light to the green wire, and the other end to a good ground....does it light? If it does, now take the lead that went to ground and attach it to the black wire in the harness...if it doesn't light, the ground is bad....like I said, I think the clutch and low pressure switch share the same ground......

If the test light did not come on when you grounded the test light to the green wire in the low pressure harness, you either have an open wire, or a bad VCM/PCM......
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Old 08-06-2014, 07:13 PM   #7
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The pressure of 45-50 (on the low side)was when I was jumping the compressor. truck was running, compressor running, clutch engaged. I did hook the test tight to the clutch wiring harness and turned on the ac control from inside the cab...light came on.
I have a lot to test (from your previous thread) I will do it Friday and get back to you.
one question until then: if I have to replace the PCM I plan to get it from the junk yard. Does that PCM need to be programmed at the dealer using my VIN?
for now thany you so much for your help!
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Old 09-06-2014, 05:41 PM   #8
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Ok, After carefully following all your directions I check the rely switch using the votmeter...no voltage at all. So putting the ground on the battery terminal then the pos on the switch I got almost 12 V when the ac switch (inside the cab) was turned on. I have no ground.
One question: instead of tracing the bad ground wire down can't I simply run a new ground wire. You can't over ground something, right?
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Old 09-06-2014, 07:05 PM   #9
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No, PCM controls ground to relays. This means the PCM is NOT commanding the relay on.
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Old 09-07-2014, 02:54 PM   #10
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So this means I need a new PCM? I was doing some reading online and where I buy one (juck yard, online, etc) I have to take it back to the dealer for it to be programmed??
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