my ac compressor will only shut off if i unplug the compressor from the wiring harness. the compressor has been changed and uses only the hot wire that is supposed to lose power when the ac is turned off. the problem is not only is the hot wire hot anytime ignition is on...the ground wire in the plug is also hot even though idt is not being used... any help will be greatly appreciated

 

If Electronic Air, ground is provided by the Blower Module and that could be defective. However, if you've disconnected that side of the
circuit, then the Coil must be shorted internally to ground; ie, it needs a new one. You can buy individual parts through NAPA or replace the clutch assembly.

 

for a little verification the ground wire from the clutch goes directly to chasis ground now... originally the (+) and (-) wires both came thru the wiring harness now only the (+) wire from the wiring harness is being used to engage and disingage clutch but the(+) wire has 12 volts to ground anytime the ignition is on and the(-) wire in the molded plug also has 12 volts to ground anytime the ignition is on... resulting in compressor running all the time

 

Manual air circuit is wired that way, but it all runs through the coil meaning the coil is shorted to the compressor (and since the compressor is grounded (attached) to the engine, that completes the circuit and the clutch engages. You can verify with your meter, engine off. Also verify that the diode is good or in place (inside the connector - on top there's a small cover that you remove to access). If it's not good - or missing, it can short out the coil; damage other parts.

 

..i think i am a thread hijacker here!...in an 86 with electronic, i replaced the fan relay near the fan shroud in my brothers vette up north a few weeks ago....the fans apparently were for some unknown reason, not working one day and my brother heard a loud "POP" and that was all she wrote...

....when i shorted the the low pressure switch and turned the selector to max air, i assumed that the fans (2) would start regardless of absence refrigerant, they did not and while doing this i had my hand on the relay to"feel" it "click" and did not...no fuses blown or similar...

...so, w/out the manual here now or at the time i was there, where should i tell him to look and where is the power source from the relay...the simplest thing as to applying power directly to the fans was not done by me while there and is unknown if down by my brother since.....thanks!

 

Only one fan is used for a/c (through '89) and if it's functioning correctly (proper charge, clean system) it should never get close to the threshold for the aux fan (controlled by a switch in one of the heads). For this Year, the ECM gets it's signal from a switch screwed into the high side. It (the ECM) sends it 12 volts and it's normally closed so the ECM is seeing 0 volts. When the high side rises to somewhere around 220 psi, the switch opens and when the ECM senses the voltage, it grounds the Main Fan Relay. It keeps it grounded until the pressure falls to about 190 psi. At that point, the switch closes and the fan should turn off. Jumpering the Low Pressure Switch does nothing if there's not enough gas in it (or if it's too cold to make the threshold pressure - usually something below 50 degrees ambient). Simple test is to disconnect the harness at the switch which simulates the open and the fan should turn on. Rule out the ECM by grounding the Diagnostic Link with the ignition on and that should also turn on the Main Fan. Otherwise, hook up some gages so that can see what it's churning - if it's making the pressure and relay and ECM are good, replace the Switch (Dealer, Autozone, NAPA, etc).

 

THANKS FOR THE REPLY!!...

Only one fan is used for a/c (through '89) and if it's functioning correctly (proper charge, clean system) it should never get close to the threshold for the aux fan (controlled by a switch in one of the heads).
***there is an aux fan on the outside of the condenser as well as the engine cooling fan on the "inside" of the radiator (not actually the inside but ya know what i mean) that i thought that when the ac came on BOTH fans start up, am i mistaken?...on my 88 i have the same set-up...

For this Year, the ECM gets it's signal from a switch screwed into the high side. It (the ECM) sends it 12 volts and it's normally closed so the ECM is seeing 0 volts. When the high side rises to somewhere around 220 psi, the switch opens and when the ECM senses the voltage, it grounds the Main Fan Relay. It keeps it grounded until the pressure falls to about 190 psi. At that point, the switch closes and the fan should turn off.

Jumpering the Low Pressure Switch does nothing if there's not enough gas in it (or if it's too cold to make the threshold pressure - usually something below 50 degrees ambient).
***i realize that but, in the absence or refrigerant or in applying refrigerant to a newly rebuilt or serviced system, one usually jumps the lps out to engage the compressor which is what i did and in my thinking at that time since i was unfamiliar with his year, was to eliminate anything obvious (although probably wrong) that would prohibit the fans from coming on...(outside ambient was 82 degrees)..

Simple test is to disconnect the harness at the switch which simulates the open and the fan should turn on.
***the high side switch?...

Rule out the ECM by grounding the Diagnostic Link with the ignition on and that should also turn on the Main Fan.
***i didn't even think of that however, when the ac is on don't BOTH fans come on?...the main fan will run fine when "up to temp per t-stat but, the one in front does not...which pin to ground on aldl?...

Otherwise, hook up some gages so that can see what it's churning -
***you forgot, the system is void of all refrigerant at this time and we were trying to assess as to why the fan(s) were NOT coming on with the ac on max air...

if it's making the pressure and relay and ECM are good, replace the Switch (Dealer, Autozone, NAPA, etc).
**see above

.....Thanks Again...

 

Aux Fan was initially installed as a Dealer Option (your '88 was from the Factory) following complaints from autocrossers and high altitude users. It's a pretty meaningless option that should rarely if ever come on (or if it is, that's good indication that something is wrong with the cooling system).

A/c doesn't rely on Coolant Temperature - it would blow up if it did. Rather, it maintains the temperature of the gas condensing into a liquid in the condensor at about 100 to 140 degrees (with subcooling it's actually a tad less, but that's another discussion). The fan is going to come on sooner and run longer to keep it there. As a side benefit, coolant temps will be/should be 10 to 12 degrees lower (idle and low speeds) with the a/c cranking assuming it's the way it left the Factory.

High side line from the Condensor to the Evaporator should have 2 switches next to each other. One is high limit which opens above 400 psi - the other is for the fan. Whatever turns on the fan when disconnected is the Fan Switch. Whatever turns off the compressor when disconnected is the High Limit.

 

Aux Fan was initially installed as a Dealer Option (your '88 was from the Factory) following complaints from autocrossers and high altitude users. It's a pretty meaningless option that should rarely if ever come on (or if it is, that's good indication that something is wrong with the cooling system).
***humnnn, i switch my ac on and all fans come on regardless of temp if i recall correctly..will check later when it stops raining..

A/c doesn't rely on Coolant Temperature
**yes, i know and agree.
- it would blow up if it did. Rather, it maintains the temperature of the gas condensing into a liquid in the condensor at about 100 to 140 degrees (with subcooling it's actually a tad less, but that's another discussion). The fan is going to come on sooner and run longer to keep it there. As a side benefit, coolant temps will be/should be 10 to 12 degrees lower (idle and low speeds) with the a/c cranking assuming it's the way it left the Factory.
**i see...

High side line from the Condensor to the Evaporator should have 2 switches next to each other. One is high limit which opens above 400 psi - the other is for the fan.
**only recall 1 but, after a few weeks i do not recall..

Whatever turns on the fan when disconnected is the Fan Switch.
**isn't the main engine fan switch located in the block between 1+3 cylinders and shorting to ground will activate it?..somewhere i think i recall something (a signal?) has to be present from the ecm for the fan to work(?).

Whatever turns off the compressor when disconnected is the High Limi

 

Aux Fan is controlled by a temp switch in the head. Main is driven by the ECM from the Coolant Temp Sensor (non a/c) or a/c switch. If both of yours are coming on at the same time, someone spliced the ground side of the Relays together.

 

....nuts!, i just remembered in my car i put in a kit for that, geez....however, dealing with my brother's car, no fan comes on except the engine radiator cooling fan that comes on "normally".....other fan doesn't so, have to "trip" the other fan to come on manually for test purposes after i tell my brother to manually apply voltage to fans to see if they work to begin with...ok, i'm done.....thanks for the reply

 

Assuming the system is clean, he will need to block air flow or run it up Pikes Peak in first gear.