domsh

Hi, I have a '91 s4 LHD auto which I bought earlier this year to enjoy.
In July I was run over and now only have one arm that works.
It means I cant do the little jobs on my car anymore.

So I sent my car to a specialist for an overhaul.
They are excellent.
But since return, the AC switch doesn't work, it remains unlit, theres no increase in the ticker revs, nothing under the bonnet at all.

Im guessing a wire or fuse etc up near the switch.
Could anyone help with the starting & obvious places to look/begin please?
I just don't have that ability at the moment, to plan it through.

Thanks very much, Dom

The Forgotten On

Most likely the relay in the head unit failed. Source a good used or rebuilt one and you should be good to go.

Otherwise it is low on freon and the low pressure switch is preventing it from activating.

SQLGuy

What did they do to the car?

Does the A/C come on if you switch the controller to defog?

The relay is a likely cause, though. There are three or four things that have to work for the A/C to switch on from the button. They include the low charge switch, an overpressure switch, a freeze sensor switch (in the evaporator), and the relay in the controller. Might be worth checking the fuse as well.

IIRC, the A/C will come on in defog mode even if the relay is bad. One of my 928s came to me with the smaller plug at the back of the HVAC controller disconnected. I think the center console was out and the guy working on it had forgotten to plug that back in. That was the cause of my A/C not coming on in that car.

Manuel928S4

Hello,
My 88 S4 had a similar problem and the issue was the plug that connects behind the a/c push button in the center console was unplugged. I connected it back and it worked.

Manuel928S4

Oh, I just read the previous post. I had that same issue and fix

Simon928

+1 for the wire to the switch being unplugged. If the shop was doing anything in and around the console then it's easy for this to happen. BTDT myself a couple of times. If they weren't anywhere around the console though then it could also be anything that SQLGuy said as well.

davek9

..."So I sent my car to a specialist for an overhaul."

What work was preformed, this could point us in what area to look first?
The easiest is to see if you have 12v DC going to the compressor, this can be checked at the 14 pin connector on the right side fender under the Bonnet.

Also the same 12v DC circuit should be coming out of the HVAC Head unit to the freeze switch under the plastic where the windscreen wiper mech is located, the switch goes into the Black HVAC box near the center of the car (its two wires) look for 12v DC on one or both if working.

There is an excellent step by step troubleshooting write up here written By Wally P.

Sorry to hear about your arm.

Randy V

If it is the relay in the HVAC it can be replaced fairly easily. However, the root cause of the relay failing is often an excessive current draw from a failing AC compressor.

Call 928 Specialists for a replacement relay.

I've got a thread from a few years ago here that details the repair.

domsh

Wow that was fast! Allthis expert advice in one day, fantastic!!

Ill start with the plug wire first. Theres been a new head unit added 2 weeks ago, they will have been fiddling around there, adding in and pulling old speaker wires out. Now I know thats a real possibility, Im starting to wonder if I even tried the zircon switch between the new radio being fitted and the most recent garage!

If thats not it, then the head unit relay will be hunted down and replaced.

If Im stuck, theres the more technical replies on her which i'll study hard and follow.

To answer a couple of questions, the 'overhaul' was probably a harsh word. The car has come over from Japan to here in the UK. Its a '91 and has done 88,000 miles. Its pristine, but the history is absent. What the most recent specialist did was a full check, a major service and replacement of all belts etc. It was all to give the car a new breath of air and myself a more relaxed mind!

As for the AC working in defrost mode, no Im 99% sure I tried all combinations and it was dead. I hope that helps with the diagnosis.

So, massive reply done, thank you so very much for your replies, just fantastic.

Dom

fossberry

Another simple thing to check is if the AC light comes on if you push the switch right in and hold it. WHen I did that with mine it worked and when I released to where it would normally sit when "ON" it would stop working. Cleaning the A/C switch contacts fixed this problem in my S4.

domsh

Hi,
Problem solved! I managed to wriggle the radio out (with one hand and a few day long breaks). i found the power wires completely unplugged behind the switch.
The radio must have pulled it out when installed.
Thanks for all of your wonderful replies,
Dom