docmirror

You can send me your HVAC head unit and I will fit a bigger relay to it and send it back. It's most likely failed, and if not will fail in the near future. My car did the exact same thing. I charged it with freon, it worked for 2.3 miles down the road, then the relay fried.

PM me for shipping details if you want it refurbished.

jthwan22

You can check the HVAC head unit relay quickly without getting your hand dirty.
1. Turn switch to on but don't start the car.
2. Slide the lever away from off.
3. Push AC button in and out
4. listen for AC clutch click with each AC button push.

I just did the head unit relay job using Dwayne's write up.

steve_p

thats interesting that a recharge caused your relay(s) to fail, I guess there's more load on the compressor with extra gas/liquid in there, so it draws a bit more current than before that causes the relay contacts to finally give up the ghost.

Thanks for the rebuild opportunity, but apart from the fact that I'm in the UK, I build electronic control gear in my job, so I'll be fine replacing relays (once I find the HVAC head unit..!)

Very nice of you to offer, though.

dprantl

Good. So now you know your refrigerant system is good and you just need to chase down the electrical problem. The little relay in the A/C unit, the freeze switch, the low pressure switch, any of those or a combination could be causing your issue. It's easy to find out which ones by tracing 12V as you already have. Since you don't have 12V on either side of the freeze switch with the A/C on, the head unit relay is the likely culprit.

Dan
'91 928GT S/C 475hp/460lb.ft

JHowell37

I would check the low-pressure clutch switch and make sure the wires are still connected and free of damage.

steve_p

I took out the head unit today and opened it up (very quick job)

Apart from the relay replacement job, I could see that the curved PCB tracks that the wipers run along were covered in grime, the little plastic linkage that connects the lower slider **** to the rotating section wasn't even connected, and the little numbered cylinder that shows the blower setting was broken off.

The slider ***** were dropping off too, but I already knew this. So I fixed all these little faults (I ended up glueing the ***** back onto the sliders, and used a tiny set screw to act as a shaft for the numbered cylinder to rotate around.)

As to the relay, I had the blue Gruner 202 version, so I rummaged in a box of bits and found I already had a DPDT 10A PCB mount relay, so I used this, on flying leads as others have done, but I just got it to fit inside the case. A couple of the PCB tracks had slightly lifted off the board near the relay pads, hence the wires running to other, more secure, pads on the board.

Put it back and.... coldness!

The lower slider actually does something now, I'd better read the owners manual to see what it's meant to do. The demister now blows when the main blower **** is off, which obviously it didn't do before, considering the linkage was off.

Anyway, thanks again to all who posted help.

steve_p

By the way, for people doing the 'flying lead' relay replacement, you only need 6 wires- the NC (normally closed) contacts are unused in the circuit, so you actually only need a double pole relay (or two single pole). It doesn't necessarily need to be double throw, or changeover.

I was mildly surprised to find that the head unit is effectively an entirely electrical device- I was expecting the sliders to have some kind of mechanical connection to flaps etc. It made me think it wouldn't be too hard to make a complete replacement drop-in unit using the same connectors, with perhaps push-button digital controls, and maybe a digital temperature readout.

Perhaps I should start a new thread to gauge interest in this idea, since I wouldn't bother designing a PCB for just one for myself, but if a whole lot of people were interested it might be worthwhile. It seems to me that the HVAC head unit is the only thing that dates the interior of the 928, so if there was a modern digital version it would really freshen up the interior.... just a thought...

dprantl

https://rennlist.com/forums/928-foru...h-windows.html

Dan
'91 928GT S/C 475hp/460lb.ft

docmirror

The HVAC system on these cars is about the worse design imaginable. Glad you have the knack to do the job, looks fine. BTW, I only use 6 wires, but I do use a DPDT relay with both NO and NC connections simply because they are easy to source.

dr bob

It's possible, at least on the later cars, to replace the relay with a SPST relay for the clutch, and use a diode for the second contact to the setting motor. I won't be redoing the instructions or supplying more documentation, but those with creative analytical electrical minds will quickly see what's involved. That's you, Doc. Perhaps a commonly-available SPST relay can be found that will fit comfortably inside the case.

steve_p

Thanks for the link Dan, I had seen that thread, but from the first few pages it looked to be an integrated carputer type thing, which doesn't really interest me, although now I can see they're also talking about selling an individual HVAC control as a standalone unit. I think this will be of more interest to more owners, since it would be a half hour job to swap out.