New A/C compressor, but engine temp....

[P944GUY]

Just got a new (fac. rebuilt) Denso compressor put in and it works great..after 3 years without a/c it was worth the expense. However now, all my gauges are screwy. Especially the temp gauge, it goes way up to third mark, and even a little above, it always stayed right in the middle. When I have the interior fan on high I can see the needle jump up, so I know there is some sort of something going on--I just cant figure what. Are there any grounds behind the instrument cluster or the a/c control unit (which by the way is new). Could the alternator be going out, figuring there is now a heavy load on it with everything running? Diodes? and causing some sort of electrical resistance--old wiring?? Could the wires that are connected to the tempgauge sensor on the engine be faulty, and causing this? My mechanic suggested removing and cleaning the positive lead to the battery, it also has two side terminals with bunch of wires connected to them. And finally, maybe the negative battery cable needs replacing?

I hate electrical problems, as you can see there are so many things to check...anyone have the miracle answer????????????

[Makenzie71]

A full day with no A/C = constand 195~205* temps

A full day with the A/C off/on or just on = between 200~235*

Where exactly is "overheat" with this engine? I hate it's cooling system. One of the frist big things I do will be to replace the radiator with a 3" aluminum one.

[P944GUY]

by the way it's an 87 944,

[P944GUY]

the engine is not "overheating" it's the same temp. as it always was..new radiator,pump,hoses

[Makenzie71]

How do you know? Do you have an auxiliary guage to double check? The stock guage is pretty accurate.

[Porsche944]

I wouldn't go so far as to say the stock temp gage is "accurate", but I will say that the stock gage indication is repeatable.

With that said, when the you see significant changes in what the gage indicates for a given driving condition, you need to determine what may be causing the change.

Since "all the gages are screwy," I would suggest checking all for the ground and positive battery connections to be sure they are clean and tight.

Start with the battery terminals, then do the grounds on each frame rail behind and below the head lights. Next are the grounds on the bell housing near the heater control valve, and then the ground in the cargo area, driver side, between the driver side tail light and the license plate well. Finish off with the starter/solenoid connections, and the connections to the alternator.

If that does not "solve" the problem, ask your mechanic/A.C. technician for a detailed, step-by-step recount of what was actually done. HINT: A first rate tech will ALWAYS disconnect the battery for safety reasons, and a first rate tech will ALWAYS clean the terminals when he/she reconnects the battery. This may be what is causing your gages to be "screwy."

The reason I say this is because most of the gages (with the exception of the volt meter) work by "measuring" voltage to ground. As an example, the engine temp sensor is an NTC, or Negative Temperature Coefficient device, which is a fancy way of saying the device's resistance goes down as temperature goes up.

The gage is connected in series with the sensor, so as the resistance of the sensor goes down, more voltage is "dropped" across the meter movent, and the gage indicates higher. You can prove this by disconnecting the sensor and grounding the signal lead while the ignition is on (engine NOT running). The gage should peg high. With the signal lead disconnected and not grounded, the gage will peg low, just like when the ignition is off.

To find the temp sensor, look under the intake manifold near #2 intake runner. Early cars ('83 to '85.0) will have a single wire sensor. Later cars will have a two wire sensor. Both sensors use spade style terminal lugs, and the late sensors have one small terminal and one large terminal.

Once you have completed this cleaning/testing, verify that the cooling fans are running properly. For early cars, the fans should run on high speed when the A/C is running. For later cars the fans run at slow speed, and will kick to high speed whe the compressor discharge pressure gets too high.

When you are satisfied that the fans are running properly, verify that the cooling systmem is properly filled and bled. A quick test for this is to run the engine and turn the heater on. If the air temperature goes up when you increase engine RPM above idle, chances are there is air trapped in the system.

Once you have that checked, take the car for a drive and note where the gage reads under different driving conditions, like sitting at a stop light, cruising at highway speeds, and just "puttering" around town. Also note when/if the fans run.