hacksaw

I would like to get mine back for the $75 core. Or to experiment on.

G8RB8

If both radiator fans run when the A/C button is depressed is that an indication the head unit relay is good?

soontobered84

Roger is still trying to get the head unit from my GTS back after a year and a half.

dr bob

The little relay is a DPDT, or "double-pole, double throw" relay. Essentially it has two sets of independent switch contacts inside, both activated by the same magnetic coil. In my car, one of those sets of contacts drives the compressor clutch, and the other tells the fans-and-flaps controller that the AC system is running. Bottom line: Having the fans and flaps operating is not a guarantee that the other set of contacts is still working correctly.

Consider that the original relay, at least the one installed in my car, was under-rated for the compressor clutch load. So it's merely a matter of "when" the original will fail, not "if". If you find that the compressor doesn't run when it should, having the fans run removes everything upstream of the control head from your diagnostic protocol. Meanwhile, you know you will need to do a relay replacement at some point regardless. I suggest that you do it now and remove that from the diagnostic protocol too.

My original effort and documentation of the modification of my own car is the basis for Dwayne's excellent procedure. The recommended test procedure is passed through to his, in an easier-to-follow format. Some meter-sleuthing per the instruction will help you quickly isolate the problem. Hopefully it's just the relay.

G8RB8

I was going to just do the replacement / upgrade, but the relay specified is nla as far as I've been able to find.

dr bob

The relay requirements are pretty simple. You'll need to have a double-pole double-throw relay, with a 12V coil and with contact ratings better than a minimum of 6A@12VDC. Finding 10 amp or greater contacts is not at all difficult, and higher ratings are easy too. The contacts MUST be rated for DC current. The next requirement is that the relay you buy needs to fit in your target space. If you follow my original guidance with the relay attached to the outside of the control head, this requirement can be critical.

As for the wiring... The original relay has a contact diagram printed on the top. The new relay you buy will have a contact diagram printed on it. As you put jumper wires in to connect the new relay to the board, just connect the relay terminals to the matching connector on the board where the original relay was connected. There are eight pins on the new relay, of which you'll need 6. 2 for the coil, and two pairs for the relay switch contacts.

G8RB8

The little arm holding the contact on mine was burned completely in two and rattling around inside the relay case. All sorted now with

Jeffbeingjeff

Christopher Zach

I cheated and used a 30a SPST relay up by the freeze switch. No soldering, just crimping and building an adapter to take +12 from the plenum temp switch. Works well since the relay in the HVAC head may not be able to handle 3amps, but it can probably handle 150ma to trip a relay.

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

John Speake

I found a small 5A relay to go inside the head (pin wiring is different to original) - it's available in the USA I think, it's
OMRON G6B 2214 P 12v

FredR

John,

Be careful with those electrical thingamybobs- you how much they confuse you!

John Speake

I'll get the hang of them one day :-)

dr bob

Hi Jeff and welcome to the group.

Your relay needs are the same as the S4 discussion. No worries.

My post 171 above has a couple sentences that may help you. You can use almost any DPDT relay with coil and contacts rated for the duty. I wrote the original diagnostic and documentation on which Dwayne based his (much better), and that included some guidance on relay choice. I found the one I used at an electronics surplus store conveniently close to 928 International. It's a piece that was specific to something and has a non-standard pin arrangement. But unless you find an exact plug-in replacement to match the original, you'll be wiring a relay to the board with pigtails anyway. So getting the correct connections is relatively easy. just by looking at the little contact arrangement diagram on each relay. That's how the whole things started, when I had to document the non-standard repair done to my otherwise-standard car..

Jeffbeingjeff

John Speake

The presence of volts is good, but the other requirement is sufficient current drive capacity of the relay and wiring. The drop out you saw might have been to the relay contacts getting hot and failing after supplying the high clutch current for a few minutes.