robot808

My battle for heat continues in one Chicago's worst winters on record. I am still trying to get my blower to work. I have tried replacing fuses, relays and the head unit on my 1982 stick shift.
The blower works with a jumper on relay XXI or XXII. The blower works only on full speed when jumped. I understand why with the defroster relay jumper, but why with the HVAC relay jumper? Shouldn't jumping the HVAC relay send power to the fan control switch on the head unit?
Also, what actually controls the relay switch? Is the fan control switch enough to trigger it, or does the thermostat have to be involved?
I have been trying to understand the wiring diagram, but my little brain only gets me so far.
Thanks as always for your help.

davek9

Hey Bob there are some real good write up's on this and a step by step troubleshoting.. did you search?

davek9

Here is just one of many links, this ons is Wally P's of 928 Specialist

https://www.928gt.com/t-wallyhvac.aspx

the flyin' scotsman

have a look at Johns write up; it explains much also:

http://members.rennlist.com/pirtle/svc.html

robot808

Thanks guys, I had used Wally's write up. It's great but only got me so far with the blower. This write up from John looks like it might be the next step.
Even, if I can get the fan speeds working with a new fan resistor, I don't think that would actually trip the relay, would it?
Thanks again,

Alan

The DEF relay is controlled only by the DEF position on the slider in DEF mode that relay turns on and drives the fan on full.

The Blower relay controls power to the whole head unit - including the fan switch - it should not cause the blower to go on full - unless there is a secondary problem (with the resistor pack).

When the resistor pack thermal protection kicks in it shorts the resistor pack so the fan goes to full speed all the time...

The fact that the blower relay is not coming on anyway suggests a connection problem to the head unit or that you have a problem wity the X-Bus - try swapping that relay too.

Bob you are going to have to start trying some of these things... you almost certainly need a new resistor pack - get that. Test the X-Bus (do your fog lights work?). Are you sure the connector to the HVAC head unit is not damaged? has anyone done anything to hack this up already?

The way the blower relay is controlled is via a simple power loop that runs through the HVAC head unit and back to the relay

Alan

robot808

Alan,
I believe that there is a connection being made between the relay and the head unit. I show power from the red/bk wire and red/wht wires all the way to the head unit when jumped. My fog lights do work, however I show them as relay XII with the horn (also works, weakly) on X.
I will get a new resistor pack.
Also, I think you have the sig line in the game.
Thanks as always.

robot808

Anyone have a resistor pack for an 82 they wanna sell?

bd0nalds0n

928 international does

the flyin' scotsman

Bob.......the original lasted 26 years...........'nough said?

robot808

OK, so I installed the new resistor pack today. It has restored the fan speeds when I have the relay jumped. Thanks Alan. Now the other half of the problem. The power to trip the relay.
Power for the relay seems to go from Fuse #9 to a blk/red wire that goes to the control head unit. Now I keep blowing that fuse. So there is probably a short somewhere, right?
Would that short have to be in a particular place to blow the fuse? That is the same fuse that the reverse lights and power mirrors are on. Does that mean the short could be in any of those systems?
Any guesses?
Thanks again.

Alan

Fuse #9 also feeds the Backup Lights, Rear Window Wiper Relay and Mirror Switches. I presume none of these work either. Did any of them fail before the AC stopped working?

Disable them all and see if the issues goes away:

Back-up lights - remove the connector for the transmission reverse switch (behind the cover above the battery box) Black/ Blue wire.

Rear Window Relay - unplug the relay - behind the tool panel - near rear wiper motor

Mirror Switches - pop out the mirror switch panel (face plate) - and unplug the mirror directional switch.

Do all these and with the AC set to the Off position replace fuse #9

1) Turn the key switch to ignition position and then back to off. test the fuse - did it blow?

If not:

2) Turn the key switch to ignition position and select an operating mode - is the fan functional? - select DEF mode - does the blower default to high speed?

3) Replace the items one by one until the fuse blows... debug whatever the cause is. Most likely is probably the mirrors due to cable flex in the door hinge area - but it could be pinched wiring on any of these...

Alan

robot808

I will run through these all asap. This make so much sense. A million thanks for te advice.

anonymousagain

Alan - going through a similar deduction for no Blower.

You wrote For me, no, my fog lights don't work...<perhaps a silly question> what is the X-Bus? and how do you test it?

As always, your electrical guru-ness is greatly appreciated!!

Alan

The X-bus is the accessory supply from the ignition switch - on later models it feeds the HVAC as well as the foglights. Culprit is usually the ignition switch. You can test it on your car by seeing if the connection at the top of fuses 1-5 is active during the Accessory & Ignition positions. It is not likely a culprit in the blower on your car due to a different configuration (either one).

Alan