928 obsession

After reading extensively on replacing the AC clutch relay in the HVAC unit I decided to buy the materials necessary to fix my AC. Once inside I realized the '82 HVAC unit is different than the newer units described in the repair. Instead of 6 connections to the relay, the '82 only has four. The relay also has a different part number. I also pulled an extra HVAC unit from an '85 and opened it to compare.
'82 unit


'85 unit



So this leads me to my question has anyone repaired an early HVAC unit using the Radio Shack relay? If so what solder joints correspond to the relay points? Any help would be great!

SQLGuy

First question I would have would be: Are you sure your relay is bad?

If you are, then can you post a marked up picture of the bottom side of your board with numbers on the pins that connect to the relay? From there it will be much easier to say, for example, that "1 and 2 connect to the coil, with 1 being ground."

Radio Shack's catalog number 275-248 would be a suitable part, electrically. Whether it will fit in the case, I can't tell from their site.

dr bob

Christian--

Way back when, I needed a bigger relay to run my AC clutch. The 'fix' was finding and installing a bigger relay. I looked at/researched the pins on the existing relay, and wired my external relay to the existing connections using the corresponding pins. You can do it too! Find the two pins that connect to the coil on the original relay, and connect them to the coil on the new relay. If your old relay installed uses only one set of Normally Open contacts to run the clutch coil, by all means wire a pair of Normally Open contacts on the new relay to the existing connection points on the controller board where those contacts from the old relay connect now. There's nothing magical or marvelous about this 'fix', just duplicating the original relay functionality with a similar unit in a bigger case.

WallyP

dr bob is an electrical expert, and he has trouble relating to us mere mortals...

It is rare indeed when I can add to or even (gasp!) correct anything that he says!

So, this is a picky point, but what dr bob means when he says a "bigger relay" is a relay with a higher current-carrying capacity, not a bigger case...

dr bob

Wally is correct of course. But in my '89, the original relay is a tiny DIP flat-pack device. There's just not enough room inside for large enough contacts and adequate room to get them open far enough to avoid arc damage when they open.

In fairness, I was hunting for a brute-force, never ever need to look at this again solution when I did mine. The ratings of the relay in my '89 weren't anywhere close to what the clutch coil requires. So in fact I needed a bigger case with bigger contacts rated for both the current and for DC inductive-load service, yet small enough to fit in the console too. I happened to be headed to 928 International that day, so on a whim I stopped at one of the electronics surplus junk dealers nearby and found a nice industrial-rated relay that would fit. It's just small enough to fit in the console, stuck to the outside of the control head case with double-faced tape.

Meanwhile, others found the Radio Shack relays that potentially fit inside the case. This is easier/cleaner for most, and the contact capacity seems sufficient for the clutch coil duty.


The relay in the OP's pictures is way bigger than what was in my later car. Wonder what they were thinking when they decided that the tiny later one would make the grade?

It also turns out that different years have different relay contacts arrangemnets on the circuit board, most probably changed as certain relays became unavailable.

StratfordShark

I got this relay from a supplier in the UK. It's a direct replacement physically for the original yellow a/c relay on my 91 S4, and despite compact size is rated for 5 amps which is comfortably over the clutch draw.

dr bob

The 5A @ 250V --AC-- contact rating is deceptive. DC circuits with inductive load components mean there is a significant risk of arcing across those contacts as they open. With AC, the voltage drops to zero 60 times a second, so that contact arcing is not anywhere near the problem that it is on inductive DC loads. Not saying it won't work, because it's certainly better than the 2 amp AC rating on the relay that was in my car originally. The industrial relay I found at the surplus place has contacts rated to 10 amps@250V DC. I probably should have searched out a nice 200 volt-rated diode to install across the contacts for reverse-current arc protection, but so far (10+ years now) it's been great in SoCal AC-alot duty.

StratfordShark

That's enlightening Dr Bob - I didn't think of AC v DC rating.

Will see how this relay goes. GTS-owning friend has used same relay for a few years without issues, but if it fails I'll think about beefier outboard solution.

928 obsession

Thanks for the suggestions everyone. I had orginally purchased RS 275-218 but sice I only need four pins I'm returning it and purchasing RS 275-248. I'll probably pull the HVAC unit again in the next day or two and replace the old relay.
Also how common is it that this relay burns out? Both of our 82's and our 85 have a blown relay, so I assume it's fairly common due to the insufficiently rated relay .