I have an 88 with a digital A/C Heater Control panel. The heat is always on, fan can be controlled manually. rear defroster does not work, readout works sometimes. I have the A/C working twice in the last 2 months for a short period of time. I suspect the display module but the cheapest I could find was over 400.00. I am a newbie to this site but this is my 4th Vette and my 2nd C4. Help...pkazsr:o

 

..disconnect battery for a min or so and try again....then place the unit to 60* on the display and post what happens...also, behind the distributor is a vacuum "t" , "splice" or "ck valve" with 3 lines going to it...because of heat and age, the valve can break or lines could fail - especially the one that connects to the passenger side of the plenum (ck that)......

 

http://members.shaw.ca/dankai/AC%20Electronic.pdf

 

Today I did some testing and found out the controls work even if the display doesn't. When a/c activated the compressor came on and the hoses got cold. The heater hoses stayed hot. Is there a valve that shuts of the heater core and where is it??? Helms didn't help doesn't cover electronic temp control. The manual one is in the engine compartment but on mine the hoses are complete from the water pump to the block.

 

See Sections 1C1, 8A 61, 8A 66, 8A 67 and 8A 68 in your Helms Manual. Component Locations are shown on the 1st Page of Every Chapter in 8A and there are drawings in 8A 201.

Remove the Blower Module on the Left Side of the Evaporator Case and look inside to see the Temp Door. Have someone work the Controls between 60 and 90 and see if the Door Moves. If so - the Evaporator is probably plugged up on the other side. It came from the Factory with a foam filter and when the Evaporator leaks, compressor oil destroys the filter and plugs it up. If the Door didn't move, the motor and/or the linkage that connects it to the Door may be broken. Access those parts from the passenger side after you remove the hush panel. Removing the ECM helps too. You'll have to lay on your back to get a good shot at it. Have someone disconnect and reconnect the Battery and then turn the key to Run and put the Controls on Auto. The motor should cycle the door from fully open to fully closed. If you hear or see the motor running, the plastic link that connects it to the linkage is broken. If you don't hear the motor running, remove it and using the schematic in your Manual, apply battery voltage and ground from another source to the appropriate terminals. If the motor runs, the Programmer - a postcard size box to the right and above the accelerator - is probably the culprit.

 

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Did you try to pull any codes?

While pressing the Ext. Temp Button, press the Auto Fan button and then release it. Then release the Ext. Temp. button.

a -00 should appear. - indicating what Mode is displayed.

Mode 1 is Program number
Mode 2 is in vehicle thermistor
Mode 3 Ambient correction
Mode 4 Blower number
Mode 5 Outside thermistor
Mode 6 Feed back pot position
Mode 7 Calculated door position
Mode 8 System faults
Mode 9 Ratiometric feedback position
Mode 10 Full hot position
Mode 11 Full code position
Mode 12 Software version

To change Mode # press the Warm button.
To see the value in that Mode # press the Cool button.

A system fault is indicated by a flashing indicator. If there is a problem with the ambient sensor, in
vehicle sensor, or the feedback potentiometer, the Ext. Temp. indicator will flash for about 60 seconds,
upon ignition ON. The flashing indicator will reoccur upon ignition ON until the system problem has been corrected.

If you disconnect the battery and reconnect it, the system will recalibrate and move the door from one extreme to the other and then position the door and stop. The blower motor also comes on and you can manually turn it off. Takes about 30 seconds.


To enter the diagnostic mode, push and hold the fan up and down arrows at the same time for about 5 seconds.

The display should show "-00".

Push "Auto" and the display will show any fault codes that are stored in the computer.

00-Not used
01-Temp. Door Motor Circuit Fault
02-Temp Door Motor Circuit Fault
03- Ambient Sensor Open
04- Ambient Sensor Short
05- In-Car Temp Sensor Open
06- In-Car Temp Sensor Short
07- Solar Load Sensor Open
08- Solar Load Sensor Short
09-Low Freon
10-UART Failure

It could also be the programmer, located under the dash on the firewall near the brake pedal. Check the vacuum lines to make sure they are properly connected. Many time the resistors inside the unit go bad. They are very easy to replace "if you know how to solder."

Also check near the distributor for a vacuum line with a t connector. There are three lines attached to it. Over a period of time the t breaks or becomes inoperative due to age and heat.

 

None of those Codes are applicable to an '88 - Codes are 01 - Temp Door Return Signal open; 2 Outside Temp Sensor Open; 4 Outside Temp Sensor shorted; 08 Inside Temp Sensor Open; and 16 Inside Temp Sensor Shorted.

A Trouble Code would be indicated by a flashing LED below the Outside Temp Readout and as you can see, these Codes are hard faults primarily dealing with the Inside and Outside Sensors. The monitored parameter for the Temp Door Motor circuit doesn't really capture if the door moves - that's a mechanical connection and the motor is being driven by battery voltage. Some sort of solenoid or switch would have been better, but the Programmer merely sends a 5 volt reference to the motor and a variable voltage is returned to the Dash Controls through a rheostatic pot (for lack of a better description) as the motor runs. Essentially, any voltage on that return signal - assuming there's a command to move the door (settings are overridden at 60 and 90) - you won't see the Code. Ditto for busted linkage - motor hums along - voltage is returned, but the door ain't movin'.