'91 L98 coupe Auto basically stock , well maitained 150 thousand miles.
Basic Complaint : Huge loss of power after 10 -15 minutes of driving. Cars runs hotter than usual and sometimes in stop and go traffic reaches temps approaching boil over .
Dealership diagnosis : secondary fan circuit shows 25 volts ( not possible in a 12 volt system) , fan not running . Recomended replacing ECM.
ECM replaced , still has same issues. Cleaned cooling system in every immaginable way internally and externally .
Still same problem ,Techs at Dealership stumped . Now it becomes a challemge and they stop charging me for diagnosis.
They commit to finding the problem no matter what it takes at thier cost . Cool Huh ?

Turns out to be the pressure switch on the High side of the A/C not turning secondary fan on and not cycling A/C clutch off . Temp sensor in right side head ,did turn fan on at factory setting .But too little too late .
Engine was hot plus trying to turn the A/C compressor with pressures in the A/C system now at 400 PSI .

Replaced switch , cars runs like a scared rabbit now .

I had not seen this particular issue on the tech boards so I thought I'd share .

 

Interesting, though 400 psi should have shutdown the a/c. Similar has been suspected around here, but not with any frequency. If you want to see more of it, go to the board at www.ackits.com where the pros deal with it from time to time - though usually the question is "why is my a/c blowing hot air at idle? - opps it just overheated too!".

ECM monitors the pressure sensor for a couple of things, one of them being fan operation and it should be turning on both fans whenever the compressor is engaged (and it's above about 70 degrees and the speed signal is below 40mph). Otherwise, fan signal is from the Engine Coolant Temp Sensor which is located below the throttle body on the intake manifold for the '91. A separate switch, on the Evaporator outlet, cycles the compressor on and off based on the pressure at the Evaporator or low side of the system. That keeps it from making ice cubes. Your faulty pressure sensor is also used to turn it off once it gets to 400 psi, that keeps it from blowing a hole in the hood. Glad it's fixed!

 

So who pays for all the stuff they did, that it didn't need, like the ECM? They are still blowing smoke up your skirt, because the temperature sensor in the right head is the sender for your IP temp gauge. Both fans are controlled by the ECM based on the temperature information sent by the ECT sensor in the front of the intake manifold. It IS possible that the A/C switch was defective and it didn't signal the ECM that the A/C was on, delaying the fan activation. And that excessive A/C pressure was sucking all the power from the engine, in an attempt to turn the compressor. What did this $50.00 pressure switch cost you?

RACE ON!!!

 

I did all the other stuff myself. The temp gauge in the car was out by 20 degrees so the sensor was changed and the temp gauge is now out by 13 degrees ....oh well.
I paid $57.00 Canadian Dollars for the pressure switch.
I am happy that the dealer did behave pretty well . I have given them plenty of business over the years , so they helped out a little.
I have had my bad experiences with dealers , this just happened to pretty good for a change.

 

ECM monitors the pressure sensor for a couple of things, one of them being fan operation and it should be turning on both fans whenever the compressor is engaged (and it's above about 70 degrees and the speed signal is below 40mph).


My FSM says fans should come on at speeds under 55 MPH with A/C on and pressure switch should activate secondary cooling fan at 240 psi A/C system pressure.

Does this differ for earlier years ?

 

Yes - up until '90, a fan switch was mounted on the high side that opens at 220 psi signaling the ECM to ground the main fan relay - the optional aux fan isn't used for a/c. It recloses at 170 to 190 psi and that signals the ECM to turn off the fan. A switch mounted next to it opens at 400 psi and that cuts power to the compressor. '90 and up use a thermistor (the pressure sensor) in place of these switches and the voltage returned (it rises with pressure) is proportionate to the actual high side pressure. That gives the ECM more info which it uses to turn on the fans and to monitor system operation and to keep it safe if things get out of control.

R12 - the charge until '94, works best when the high side averages 200 psi, so if your FSM shows the specs you've outlined, that sounds about right if it's an R12 charge. The specs for R134 are usually lower - generally about 150 psi or 1.5 volts from the sensor. On the other hand, that 55mph # seems a bit high - hard to believe that even a bottom sucker like a Vette can't get enough air to the Condensor once it hits 40 mph (I'm pretty sure my '89 pulls the plug at 40 mph - though it's awfully hard to tell what it's doing at that speed).

The key is keeping the high side under control so that it can pour copious amounts of cool air out of the vents. You can troubleshoot the pressure sensor with a scanner and most will display the voltage return and the pressure. The switches used on the earlier Years can only be wrung out with a manifold gage set.