2fas2c

Hello everybody,

Today I am calling on all you C4 A/C Gurus out there. I have a customer who has been trying to diagnose a problem with the A/C on his 90 ZR1. I have already rebuilt his climate control head and tested good but he is till having some other issues, I will post the email he sent to me hopefully one of you can help. Here is his email :

Hi Tyler
I managed to display the Diagnostic Trouble Codes on the heater control.There are about 37 different menus to scroll through.Almost all have some kind of a code number.Where do i find information on the code faults?
Below is the a/c issue that I am working through.

Help my a/c on my 1990 ZR1 #1675 is not working. It was working one day than just quite blowing cold air.

I have replaced the a/c relay (under the driver dash, second one in from the outside of the car).Also sent out the controller to have checked and repaired. A/C still not working.

The compressor will engage when I jump the low pressure switch.

The compressor will also engage when I ground the grey wire on the a/c relay plug.
With the grey wire grounded the air will blow cold (44*F).As soon as I disconnect the ground the air temp rises. What should I check next?

Thank you for your help Roy

Thanks for you help guys Roy is not the best on the computer and I want to see the guy enjoy his ZR1 this summer lets help this guy get back on the road! TIA

FASTAZU

Check the AC programer on the firewall inside car just above the gas pedal. head unit sends signal to it than to (PCM may be ECM on 90 my book is a 96) and on to relay. Two common issues with the programer are vents will not change or no AC. or could be a pressure switch.

2fas2c

thanks for the reply that is what I had recommended. I did have Roy run the test by holding the up/down buttons and the displayed code was 00 which from my understanding means there is no fault. I will take a look at the A/C Programmer next. I was also thinking, stuck blend door motor? or collapsed vacuum line? unfortunately I am not close enough o him to have a look myself, but I suppose those are the things to check just wanted to be sure we were on the right track.

miker2

if its low on freon ,the compressor won't engage .also ,the low pressure switch that you mentioned may be bad.

Hooked on Vettes

Could be he's not retrieving the code correctly.

When you hold the up and down arrows on the control head for
a couple of seconds it goes into diagnostic mode.

-00 is displayed. The - sign indicates you are looking at a
Parameter. To see what's stored in that location you press
the Auto button.

To reset a code 09 (Low Freon) you disconnect and reconnect the battery cable.

After that when the ignition is turned on, the AC Programmer recalibrates the blend door which takes around 30 seconds.

When he's grounding the Grey wire, he's manually enabling
the AC Clutch relay. The normal signal flow is from the AC Programmer to the ECM and the ECM grounds the Gray wire.

If you have a volt meter you can monitor the AC request signal from the Programmer. When AC is selected the AC Request voltage signal level should change.

The ECM above the battery has 4 plugs going to it. The AC request signal is on the Green ECM connector Pin 2 a Grey/Red wire. You have to pierce the wire with a needle or sharp probe to measure the voltage on it.

dizwiz24

Amen to this...

This is what my guess is.

Very simple.

Do a quick check for leaks (with uv goggles and light)

Do an evac, add dye, oil, freon.

Good luck

FASTAZU

If it low on freon not only should you get an 090 code on the head unit the off button light should be flashing.

SunCr

The path to the Low Pressure Switch is from the Relay so if it runs when you jumper that harness, the connection at the Pressure Switch is fubar or it's the Switch. Switch contacts are closed above 50 psi so take a DVM and verify that the Switch Contacts are a completed circuit. If not, and no low gas code, replace it. If so, the Connector needs to be diddled with or replaced.

desertmike1

He wouldn't be blowing 44° air with an empty/Low system... but he mentioned; "The Compressor will engage When I jumped the low pressure switch" Thats were I would be looking.

Churchkey

Unplug the E connector @ the low pressure switch.

Test the low pressure switch with an ohm meter or a battery operated test light.

No ohms or no light = a faulty switch.