AO

Here's what I see. (Sorry for the eye-chart... )

Bill Ball

OK, I missed the splice over to the freeze switch...the line going down to LH 14 threw me. Wonder what that does?

AO

I'll bet that's to tell the LH to bump the idle a bit... just a guess.

Bill Ball

Thanks, Andrew. Makes sense. So, I'll check for 12V at the freeze leads next and trace that back, as Wally says.

Bill Ball

Have any idea what SUPPRESSOR XXI is? It T-'s into the wire from the freeze switch to the pressure switch. I'm going to guess it's not directly invoved but is just getting a signal like the LH.

Bill Ball

OK, with Andrew's help steering me away from the LH, I see this is pretty simple. The freeze switch gets its input from the infamous headunit relay. There are some diodes and other parts in the headunit circuit that could go wrong, but I need to check the freeze switch power first. If this is a relay failure, despite the newer Radioshack unit, it's not a failure mode I've seen.

ZEUS+

The suppressor is for the stereo.

Bill Ball

Ah!

ZEUS+

As Andrew suggested the wire to LH is most likely an input signal for idle. Have you checked for power leaving the switch ? As Ketchmi suggested maybe the relay went south.

Alan

No this "supressor" diode really has little to do with the radio - at least directly - it protects the AC head unit relay when it switches the highly inductive load of the compressor clutch.

This will serve to avoid some radio noise - but thats a secondary effect - there are other supressors (inductors) that really help with that in the radio supply line.

It also capacitively supresses some switching noise (current spikes) on the interior lights - this is a completely unrelated function - this also helps with radio noise.

Bill yes Andrew steered you correctly - simple circuit - LH input is to control the idle when the clutch loading engages.

Test the relay output at the central electric loop though.. what voltage when supposedly engaged? Only the relay drives this - if its drooping its a contact issue in the relay - or upstream

Alan

ZEUS+

I see what your saying Alan. I looked at diagrams and components more closely and the radio suppressor has a cap.

Bill Ball

OK, we bought another relay. But before we installed it, we decided to give the system another try. Checked for 12V at the freeze switch with the motor running. OK. Hooked everything back up and the compressor ran. Maybe yesterday's over-pressure tripped something that required a lot of time to reset.

We kept good airflow over the radiator and the high side never got over 270. Ran in a full charge. but the vent temp never got below 60 on a 69F day. The low side pressure was 10-20. So, to me this sounds like a mostly blocked expansion valve. The rear air blower does not run, so we cannot test that theory.

Where is a R-134 temp versus pressure chart? I can't find it in my Porsche documents.

Bill Ball

Update:

Found no fuse in rear AC slot. Rear AC works and got below 40F while front stayed at 60+F. So, that is consistent with a bad front expansion valve. On order overnight from 928Intl.

Jim bailey - 928 International

Bill the expansion value is on the truck....for overnight delivery

Ketchmi

I would also verify that the heater control valve is closing completely, it will give the same indications.