JG928S4

a /c anti freeze switch

Any pictures of this so I can confirm I am testing the right thing? Got full A/C pressure, but not working. Have light on A/C button.

88 928 S4 w/AT

Mechanic (not my regular Porsche Guy) confirmed when clutch jumped it work, but he says I need a new $1000 brain.

IcemanG17

https://rennlist.com/forums/showthre...ht=recirc+flap

There is a pic towards the middle of the 1st post.....that shows it really well......

You can bypass the freeze switch by jumpering it...... Do the same with the low pressure sender under the drier.....if the compressor still doesn't kick on then you have a bad relay inside the control brain...so yes a new $1K brain will fix it...but so will a $10 outboard relay from radio shack and some soldering on your part....there are several write ups about it & I've helped out with 3-4.....not hard really...do a search for A/C relay or A/C outboard relay....you can take a wire straight from the hot jump start post to jump the compressor.....then when it runs you can check your pumping pressures & confirm that the relay is indeed dead.....

heres one
https://rennlist.com/forums/showthre...ht=a%2Fc+relay

JG928S4

I have researched and read the write ups, saw a few w/pictures, the picture you just referred me too was the one I was missing. Thanks, I will go try to bypass the freeze switch and see what happens...

JG928S4

That switch tested OK, power/12V on both terminals. Now to the refrigerant pressure switch, can anyone lead me to the picture of that?

dr bob

JG--

Make a jumper with a 10A fuse in it, and use it to pass +12V from the jump-start terminal to the two connections on the freeze switch. Leave the two wires there connected. If you hear the AC clutch click when you apply the 12 to each of the two terminals, it means that your freeze switch is working, your pressure safety switch is closed (you have adequate freon), your compressor is plugged in, and the 14-pin connector under the jump-start terminal is plugged together correctly. Then if your compressor doesn't work when you push the AC switch, the problem almost undoubtedly the little clutch relay in the control head.

JG928S4

I had my daughter push the a/c switch w/the ignition power on and I hear what I believwe is a clutch click and both the radiator fans go on, is this the same test, or do I need to do as you describe?

The A/C guy did a pressure test on the A/C and it is good, but could a faulty pressure switch still stop it from working? and if not then I know it is the relay? How do I by-pass the relay to test for the relay, or knowing the mechanic jumped the compressor and got it to work confirms it is the relay?

If that is the case, I have the link to replacing the relay w/the radio shack one, but do not know how to pull the board.

VehiGAZ

Yes, a faulty pressure switch or anti-freeze switch could keep the compressor from engaging. No picture of the low-pressure switch handy, but I just went through this myself, so here's a fair description:

Pop the hood and find the dryer canister in front of the radiator on the left (as you face the engine from the front of the car). It's black and has a sight-glass in the top of it. On the right side of the cannister is the pressure switch. There are two separate wires running to the top of it, and a modular wiring harness plug into the bottom of it. I unplugged the harness from the bottom and tested there. The wires on the top only gave me 6V or so, so I knew that wasn't the right wires.

As for the compressor, don't go by a click sound, because the fans can engage with a click. Confirm visually: as you stand over the dryer & radiator, look at the belt-driven component on the lower left side of the engine. That's the A/C compressor. In the middle of the pulley is an inner ring that should be static with the engine running and the A/C off. If that inner part starts spinning when you turn on the A/C, then that means the A/C clutch has engaged and the compressor is working. If it is not turning, your compressor is not engaging. If you get good voltage at the pressure switch but the compressor is not kicking in even while jumping the pressure switch wires, then your problem is between there and the compressor clutch.

Good luck!

dr bob

The top device is the pressure transducer that is used to operate the fans and the grill flaps when the AC is running. The lower device, the one with the modular plug, is the pressure switch.

Testing for good voltage at the switch must be done with the connector still attached to the switch. The load of the clutch coil must be there to get a valid reading. There's a very common failure of the clutch relay in the control head, where the contacts in the relay get burned. Testing with the switch unplugged will show the correct 12V, since there's no load on the circuit. With the clutch coil drawing current while you test, the high resistance in the relay contacts beciomes evident when you see a low or no voltage in the circuit. It's significantly easier to test at the freeze switch, one step upstream in the circuit, since those terminal connections are discrete.

JG928S4

I popped the bottom plug off, hooked one side back on, put my volt meter + in the other part of the plug and jumped the - of the meter to the other side of the switch and recorded 11.5 volts w/key on and A/C switch on, car not running. This tell me the switch is operating correctly? If so what is the next step?

Thanks,

JG

hacker-pschorr

I overheated my system (fan failure on my part) that blew a seal on the dryer canaster (early cars do not have a high pressure switch to protect the system). New canaster, refilled but now my A/C will not drop below 66 degrees in the vents. Before this failure it would drop to low 30's. So something is keeping they system from going full cool eventhough everything appears to be working.

JG928S4

How do I get to the relay? I have no problem soldering etc. Just don't want to tear apart the dash if there is a simple way, I read where many of you have done it quite quickly.

Thanks,

JG

Rob Edwards

Here's a thread on getting the HVAC head unit out- pretty simple!

HVAC head unit removal - S4

JG928S4

Thanks, I will give it a go tomorrow in the 108 degree garage.

dr bob

Your description is not clear to me.

Pull the connector completely off the switch. Using the OHMS scale to measure resistance, put the probes on the switch terminals. Zero of very low ohms measn the switch is OK and there/s freon in the system High to infinite ohms means the switch is open, indicating there's isn't enough freon in the system. If the switch is closed, goto next step.

Plug the connetcor back onto the pressure switch. Move the meter up towards the freeze switch. WITHOUT REMOVING THE CONNECTIONS ON THE FREEZE SWITCH, use you meter to measure DC volts to ground from each freeze switch terminal, with the key on and the AC button pushed. What voltage do you see on each terminal?

JG928S4

0 ohms on pressure switch, 11 - 11.3 volts on each of the Freeze switch terminals.