Cosmo Kramer

My HVAC died a couple of days ago and I had some time to work on it today. Symptoms are no blend control, no blower fan and AC button doesn't light up when depressed. Fuse 17 is good and I tested the 53 relay for the fresh air blower, it is good. Also swapped relay from the horn to blower and still nothing. I jumpered 30-87 on the fresh air blower and the fan ran full speed no control even with the key off. I tried pulling the defrost relay but it is really in there and I didn't want to risk breaking it (it is expensive).

Any ideas?

S4ordie

Same issue on 90 S4 (53 Relay is in position XII and Fuse in slot 20. Both are good as tested above).

Same question: Any ideas?

GregBBRD

Check for power at the fuse and work backwards/forwards until you find power. Make sure that you have everything turned on, when checking for power. A bad contact can show 12 volts until "load" is applied and "tricks" lots of people into thinking everything is fine.

Alan

Ted - the rear defroster is expensive (I-II), the DEF relay needs only a std Bosch ('53) SPST/SPDT relay. You may have a fuse tester in there now - you don't need it - its basically cosmetic and a replacement in relay function for/by any '53 relay.

Are you sure you are dealing with the right relays. The fan should run full speed if you link 30/87 on the DEF relay - make sure you are reading the labelling correctly - look for the arrows pointing to the slots. hese two are side by side.

DEF = VIII 4th in from left on top row
Blower = X 5th in from left on top row

Could be the connection to the HVAC head unit has worked loose...

Alan

Cosmo Kramer

Yep, linked 30-87 on relay X and blower ran full speed. VIII is a longer relay with no tester, different part number according to 928 SP, but I believe you if you say a 53 will work. Having a real bitch of a time getting it out. Maybe it is stuck?

Edit: Went and double checked and I was jumping the defrost relay VIII, need to get my eyes checked! . Jumped relay X and blower fan and A/C button worked properly. Looks like the those two relays work in unison. Going to get a pair of new 53 relays and call it a day.

S4ordie

Hi Greg, When you say make sure everything is turned on does this also mean have the engine running as well or just the switches for the fan and AC turned on. I replaced the 53 relay in position XII and the fuse in slot 20 as directed. No change.

finally!

check power at the freeze switch, if power there then could likely be the HVAC needs new relay

S4ordie

In my case fans and AC worked fine. Car sat for a week and next time I took it out for a drive and neither the fans or the AC button worked.

Alan

Dan - could be your ignition switch - do your fog lights work?

Alan

Alan

Maybe not - swap the relays and see if the blower works from the switch.

Jumping the 30/87 simulates the relay being on. Relays not working can be due to a contact issue or a coil control issue (e.g. head unit issue). You don't really know unless you have a known functioning relay to try...

Back to the big/tall relay - if this was a mistake due to the labeling - OK

But the DEF relay should be either a '53 or a fuse tester (fuse tester is 928.615.203.00) this is a short fat SPST relay with fuse contacts and an LED on top.

Alan