frustrated

89 928 S4
My compressor has stopped engaging. Have followed Dr Bobs guidelines for testing everything apart from relay. Have 12Volts at fuses and 12.3 at the Antifreeze switch (both terminals) But only 5V on one of the pressure switch wires (when disconnected) and 0V on other. And only about 0.3V on both when they are connected.
All the articles I have read seem to address either 12Volts or nothing. Is this voltage drop likely to be the relay even though I am getting 12volts plus at the antifreeze switch terminals?
The flaps and fans only work (with A/C button on) if I bridge the pressure switch or disconnect it entirely, but I assume that has nothing to do with it?

SQLGuy

That's probably the relay. IIRC the relay feeds power through the pressure switch to the clutch.

Here's the thread from when I dealt with mine a few weeks ago (with a couple of pitcures): https://rennlist.com/forums/928-foru...c-leaks-3.html

frustrated

I figured that if the feed goes through the antifreeze switch first and Im getting 12V there then the relay would be ok, or am I missing something?
Thanks for the pics. doesnt look too difficult apart from the fact you cant buy the relay here, the whole control unit is £700 plus tax and we don't have radio shack like the States so have no idea what to replace it with!

SQLGuy

You have other sources over there. Dalbani.uk for one. It shouldn't be too hard to find a relay that will fit and can be connected by jumper wires, like I did mine. A drop-in relay would be tougher.

Worst case, digikey ships overseas. I ordered some parts from them when I lived in Belgium. (Edit) Also, I seem to remember some chain of electronics shops in London, but I don't remember the name. They were rather like Radio Shack, but bigger and a bit more into components (these days Radio Shack mostly sells cell phone and digital cameras).

I think the evaporator temp sensor cuts power to the relay coil itself, while the pressure switch is in series with the clutch.

SQLGuy

Did a bit of a search... I think it was Maplin that I was thinking of. It's been a few years since I was there, though, so I'm not positive. Regardless, they should have something suitable if you do end up needing a relay.

Something like this may be suitable (can't tell for sure wheter it's small enough, but it looks to be): http://www.maplin.co.uk/Module.aspx?ModuleNo=2525

frustrated

Ah! yes that might explain it. And we have a maplins somewhere in the city so will go look. thanks!

dr bob

Dear Frustrated,

The 'obvious' AC pressure switch is not really the switch. It's the pressure transducer for the AC system. The hint is that the flaps and fans come one when you mess with it. The 'real' switch is at the bottom of that little tubing manifold next to the drier, directly under the pressure transducer but upside-down.

The AC clutch wiring passes through the freeze switch, then the pressure switch, then through the 14-pin connector by the jumpstart terminal, then to a single spade connector on a pigtail out of the front-of-engine harness right by the dipstick tube, where the pigtail from the compressor clutch coil connects.

Before you get real deep into the electrical part, beg/borrow/buy a set of AC gauges and see what the charge level is in the system. Many people chase electrical problems because they are so much fun, when the most common cause of 928 AC 'failure' seems to be freon leaking out. Then the next biggest problem might be people who just decide to 'top up' or look for a freon substitute to just 'top up' the system. The leak should be found and fixed, the oil replenished in the system as part of the reseal process, then the system needs to be properly evacuated and recharged with whatever you like (so long as it's R-12 or R-134a).

So my nickel to your beer says you are low on freon, and the pressure switch is doing its job protecting the compressor. It's my story yer onner and I'm stickin' to it!

Dictys

There is a direct replacement relay with a slightly higher amp rating that is available from the UK supplier RS Components. The stock number is 1782214, and the relay is marked DSP2A-DC12V. One of the UK members posted about this a week or so ago.

frustrated

Dear Bob. Thank you for putting me straight. (a little knowledge is obviously dangerous and very time consuming)! Yes have dug out my old dryer and bits and see which one you mean. Thank god I didnt hook up a 12V supply and wack it on the transducer to er test the "switch" I didnt consider the gas as I only had it topped up 2 weeks ago after 12 months but I suppose the extra pressure may have been enough to increase an otherwise slow leak.
And thanks Dictys, the naval yard where I work uses RS so that should be easy to obtain if it comes to that!

StratfordShark

Yes the low pressure switch is next to receiver-drier, and you have to pull the connector downwards from it to remove and check voltage at that point. With connector off you can check for continuity across the low pressure switch - it's a dead short if pressure OK.

Where was the "pressure switch" you checked?

I thought fans came on from separate signal when A/C switch engaged, which does not require relay to be working? I'll take have another look at the wiring diagram.

Check your compressor with engine running, and briefly bridge with wire from the (disconnected) compressor side of connector which you've removed from LP switch, and 12V from the jump post. It should engage and rotate.

I posted about the direct replacement relay from RS but it replaces only the yellow relay, not blue. Not sure when they changed to yellow.

https://rennlist.com/forums/928-foru...tch-relay.html

frustrated

Hi. yes I was testing the transducer with the two brown wires on top. Duh!
But Bob was right, The gas has gone. The condenser has burst at the passenger side mounting point. According to the "Ritech Systems" guy its because the mounting is lower than on later cars and the bolt chaffs at the underside of the condenser until they leak. Very common apparently. Needs a washer fitting on the bolt which effectively shortens it and solves the problem. Too late for me but they do a OEM condenser for £270 inc. so not too horrendous.
I wonder whats left to go wrong on this car. Think Ive had everything fixed bar the gearbox & Diff. I'd better start saving Now!

frustrated

Not sure how the transducer works but something there obviously blocks the signal to the fan and flap circuit when there is no pressure. they only worked when I bridged the transducer or totally disconnected the 2 wires from it.

Rob Edwards

Just wait 'till you (or someone else) try to R&R the condensor. Here are some hints for getting at the lower mounting bolts:

https://rennlist.com/forums/928-foru...-marathon.html

frustrated

Good grief! Well obviously there are some differences between european cars and yours. I didnt need to remove anything major to fit it. Loosened the Rad top mounts and pushed it back and lifted the gearbox fluid cooler out of the way. The condenser dropped in beautifully. And it is only held by the 2 brackets as shown in pic 4 and the two bolts underneath. They are fiddly to reach but there was no ducting in the way and as the cooler coil on the floor pan lies horizontally under the rad they are reachable albeit with a few scuffed knuckles. Hopefully it will come out the same way!

Rob Edwards

Y'know, come to think of it, you're right- I didn't need to do all the disassembly I show in that thread. No wonder there were never any responses, people were being polite....