This is the original Gruner 202 relay in the control module. As you can see, the relay coil is located between pin 5 and 8. Quite uncommon. The relay is a form 2 relay, DPDT style. The compressor clutch takes 2,5 A so.
This picture (Dwayne's) shows the overview of the relay in the pcb. (actually he made a mistake, the pin numbers on the relay were reversed, I corrected this)
He also states that pin 2 and 3 are not used. That is actually not true. In the rest-state pin 2 and 3 make contact to their pivot contact... which is #6 and #7. But since the pcb itself links pin 2-6 and 3-2 are connected. In rest state the relay connects 7-3 and 6-2 and since 3 and 2 are connected always, 6 and 7 are connected in rest. The picture below clearly shows the pcb tracks. Not connecting pin 3 will result in failing to have #6 and #7 connected in rest-state. That seems not a problem but I'd rather do it as designed as I'm pretty sure they did this for a reason :)
Bottom line: take a proper (5A?) physically small but capable relay, fit it in if possible and use wires to connect the signals to the pcb. It is unlikely that you will find a relay that is a drop-in.
Many thanks to all for guiding me on fixing my AC. I replaced the little blue relay (LBR) on the AC control board with a relay from Radio Shack. All is cold now. I'll be watching for leaks from the compressor.
Here's some notes from my experience replacing the relay that isn't covered or is different in the info from Theo's website. Hopefully, this will help save someone some time. This is for an '87 928.
The RS part number is 275-0218. It is made made by Tyco with P/N PCLH-202D1S. Cost was about $9.19 with tax.
I found that the schematic printed on the LBR did not match up with reality. After making the translations from one relay to another, the fix did not work. The relay did pull in when the AC button was pushed, but nothing else. After checking voltages at the new relay's prongs there was at least 3 wires in the wrong place. I needed to refer to the wire diagrams to determine the proper hookups. I'll try to describe the board layout and the connections to the new relay:
With the controller board turned over and looking at the back you'll see the 8 LBR holes, 2 rows of 4. They will be numbered as such:
1 2 3 4
5 6 7 8
The connections I made to the pin numbers on the new relay:
1 4 (switched on)
4 5 (12v)
5 7 (coil)
6 6 (12v)
7 3 (switched on)
8 8 (coil)
I hope this helps.
'87 928S4 with working AC fixed for $9
HVAC control unit modified with an upgrade relay since the original is too flimsy to operate the AC clutch properly.
Power flow of the Relay:
So I've been tracing the factory schematics to try and determine the correct
orientation of the Radio Shack 276-0218 relay when replacing the odd pinout of
the '83 and '84 model year Gruner 970 relay. The Gruner 970 does not have a
schematic on the case like most relays. Furthermore, the pattern of the relay
itself is different than the parallel 8 pin configuration of the 87+ relay. It's
more of an offset 6 pin relay with 2 separate pins that are indeed the coil.
What I have found is that although the boards are completely different in design, the pins on the connector are identical in where they go on the control board. So I created a drawing of the original mod provided by Dr. Bob that includes the connector pin numbers. This 1st drawing shows the 8 pin original location with the Radio Shack relay on the right. I've included photos borrowed from Dwayne's HVAC relay repair thread, others that show the Gruner 970 relay for reference, and closeup photos of my 84 HVAC board. I've also attached the schematics from the 87 and 84 model year AC head.
I then created another drawing that traces the relay pin holes that correspond with the connector pin in the opposite direction to determine the correct location for the new relay.
I'm not completely sure this is correct. I've poured over drawings and schematics from the different model years and I'm pretty sure it's right, but I'd really like to get a confirmation from any or all of our resident electrical geniuses before I start soldering.
I've got a new diode to replace the burnt one on the board and I'm itching to get going. Hopefully this process won't give you as much of a headache as it's given me...
If this all works out, I'll post the results to Dwaynes HVAC thread to help future victims. I sure hope this is correct... I'm ready for cold air again!
This shows a close up of the GTS style PCB and you see that the middle pins are merged as one, which makes selecting a replacement relay easier
This shous the earlier pcb with the middle contacts separated and functionally different. IŽd say select a relay that is identical.