951 A/C issues
 

Before I go throwing parts at this thing I thought I would put it out here to see if I can narrow it down. The clutch isn't engaging when I turned it on for the first time this season. I put a new relay in it but still nothing. I have power to the relay but nothing after. TIA
Jerry

Bypass the low pressure switch (you may have lost all your refrigerant).

https://www.clarks-garage.com/shop-manual/hvac-02.htm

Most likely the interlock due to low/no refrigerant.

These hoses are 40 years old - I had all of mine redone as the low side soft hose was rock-hard and leaking.

FWIW it's a HUGE pain in the butt to remove these lines and put them back in.

But the reward is worth it if you live in Texas or other state where the heat is sweltering and humid.

As I am going through this process I got the HF gauges and after adding a can of R-12 I have 68 PSI on the low side but only 75 on the high. Does this mean the compressor is not functioning?

Update: I hooked a battery up directly to the compressor and it did what it was designed to. It got cold quickly. So there is circuit issue between the relay and compressor. My question now is? What's the best remedy for this??

some of the refrigerants have a dye and I believe it can be seen leaking under UV. some of it has a seal swell incorporated.
maybe someone else can explain it from a more experienced perspactive.

Check your low pressure switch to see if it's closed or open.

You can bypass it with a jumper wire and see if things magically start working again - or not.

If still no love, time to look at electrical, including your HVAC control head inside the cabin.

If the clutch is drawing too much current, it can fry a trace in the head unit.

https://i.postimg.cc/bNq56NCs/IMG-1723.jpg

I've had trouble with the AC relay being old and crappy. It can probably be cleaned up, reflowed, etc. It's not complicated. You can't swap it out for a typical relay unless you really like replacing fuses.

Another update. I jumped the low pressure switch and no change. I tested power to the compressor and the lead is hot but when I plug it into the compressor it shows no power. When I attach 12v to. The compressor it engages the clutch all while by passing the pressure switch and the same while connected correctly

if you sub in a light bulb in place of your volt meter, would the wires providing power have enough current to light the lightbulb?

im not sure if this is the case but maybe there is a self resetting circuit breaker in the circut to prevent a burned trace such as shown above?

At this point I would like to determine if the issue is in the wire from the relay to the low pressure switch. I would measure the resistance from the relay output to the switch input. If it is more than an ohm, you know the wire is bad. If less, it still might be bad since we do not have a spec on it, and measuring that low of a resistance is iffy with a handheld meter. So next, I would want to measure the voltage drop on the wire with the compressor load. I would access the output voltage of the relay using a thin solid insulated wire wrapped around the output pin of the relay, since I have such wire available. You could also build a set of quick connect jumpers to elevate the relay above its socket. Based on your previous comments, you have already determined the voltage at the compressor end to be near zero.

Since no replies to this thread in over a month, I'll jump in with my unique problem and hope I'm not hijacking the thread.

I am a retired EE who's been wrenching cars for over 50 years and who has restored a number of old Porsches.

The A/C on my mint, like new, all original '87 944 S (about 160k miles but does not use oil) was working fine last summer but quit at end of summer. I ignored it until this spring.

Problem is there is no 12v to the A/C compressor clutch when the ignition switch is in the ON position. If the switch is in the ACC position, and you hit the A/C button on the dash, there is 12v to the clutch and the clutch engages. So all the circuitry in the control circuit is working correctly. With switch in the ACC position (just prior to ON), I can turn the power to the compressor clutch on and off with the A/C button.

Power is only lost when ignition switch is in the ON position.

I am assuming the ignition switch is bad, but want to see if I'm missing something someone here knows about.

TIA,

GN

Maybe the AC relay is acting up. It doesn't make sense to me to have a compressor clutch engaged in Accessory position. It also doesn't seem like there would be enough poles in the ignition key switch to control the AC independent of something else you'd notice.

-Joel.

Thanks Joel.

AC relay is brand new. Not acting up.

Seems normal to me that when ignition goes to ACC that the clutch would be energized if the A/C switch on the dash is in the ON position.

The fact that the 12v to the compressor clutch correctly goes on and off with the A/C switch on the dash proves all other "switches" in the A/C system are working correctly. Including the thermostat as I have it set to lowest temp.

Hoping someone here is intimately familiar with how the ignition switch works......

GN

If I'm not mistaken, the ign. switch is same as VW. Not that expensive. I used to have another Porsche and it had some electrical problems, which led to the ign. switch. I pulled it out and it was, indeed, cracked and notch, where the ign. lock "turns", was worn, too. Replacing with a new one fixed whatever the problem was (i just don't remember).

New switch is only $60 buy URO, but the work to put it in is not something I want to do (have done in other Porsches before) only to find out that the problem is somewhere else.

I'd love some confirmation from someone with experience in the issue to confirm my diagnosis is that the problem is in the switch.

Thanks.

GN

Just ordered new URO switch on Amazon for $59........

If that doesn't fix the problem - I'll be back....and not a happy camper........

GN

Ok, you all were correct. New ignition switch did not fix problem and provides no 12v to compressor clutch in the ACC position - or any other position.

AND I find something is burning up the diodes in my new $78 A/C relay. Contacts and resistor are fine, coil is fine.

https://hosting.photobucket.com/ce00...8a9462c4a.jpeg

Sure hope someone here has a clue as to what is going on......

Wonder if I shouldn't start new thread on finding A/C relay diodes burned up?

GN

Going to try to find a couple diodes and repair new relay myself. That's all that's wrong with it.

But I need to find out why the diodes both got burned up first.....

GN