Alan

DPDT - Double Pole, Double Throw (= 2 separate 2 way (NO+NC) switches)
DPST - Double Pole, Single Throw (= 2 separate 1 way (NO) switches)

NO = Normally Open (inactive coil), NC = Normally Closed (inactive coil)

The stock relay is a DPDT but the second (NC) throws aren't actually used so you could use a DPST instead (though these are comparatively rare).

Do test the intermediates as Dr Bob says - be sure what's busted.

Alan

The Forgotten On

Keep in mind this was on a 81 with a simpler relay. I do not know much about the S4+ wiring system. Just giving ideas.

Randy V

Thanks to DR, I have a replacement relay to install on the HVAC controller board.

Reference this thread:

https://rennlist.com/forums/928-foru...l#post11373347

The new relay from Dave has 8 pins, whereas the original had only 6.

The first picture indicates the location of the original relay's six pins in RED.

I've indicated the location of the new relay's pins in BLACK. Most pins share the original location. i relocated that patch wire to an unused hole.

Following pictures show the location of the original relay, followed by the underside pinouts of the old and new relays.

Do I use all 8 of the pins on the new relay or do i need to remove two?

Alan

OK so the old relay was a DPST and the new is a DPDT (as discussed before). You need to ensure that only the NO contact set is used (the ones that aren't connected until the relay coil is activated). The other 2 NC pins can be ignored, folded over above the board or chopped off if needed.

Is there a pinout diagram for the new relay?

If there are no markings:

Make sure the coil (it will have resistance across it ~100-150 ohms) will be located in the same place as on the old relay.

Activate the coil with a 9v battery. Test which pins are now NOT connected to any other pins - these are the two to eliminate.

Alan

Randy V

I've asked DR to jump in with any info he can share.

I guess I just need to know which pins to elimiminate on the new relay.

DR

Randy,

Here is the Schematic for the relay.

Randy V

Thanks, Dave!

Alan - with that info can you guide me as to which pins to delete on the new relay?

Alan

Randy - Delete 11 & 6 (the NC contacts).

Alan

Randy V

Done! Time to solder.

Thanks again, Alan!

MFranke

I just implemented Dwayne's solution with the Radio Shack relay and got my AC up and running again on my 88 S4. I took the blue cover off of the old original relay to inspect it. One of the leafs was burnt out.

I like the external relay solution because (1) don't have to worry about whether the pins fit the board or do any significant mod to the board, and (2) don't need to pry open those little plastic tabs on case again if I need to change it out the relay.

gsmarrara

Guys, considering the relay is working properly supplying 12V to the components but unable to supply current enough to activate the compressor clutch, WHY can't we just install a simple 12V 5 A relay (connected to 12V and ground) in the wire that connects to the compressor and them to the compressor itself? This would boost the current without any change in the HVAC control and minimaly intrusive. Would that work, if not, why? Thank you and best, GSM

John Speake

Several people have done just as you describe, plus a suitable protection fuse.

gsmarrara

Thank you again for the answer John! Will try this in my car, while I import a suitable relay just like the one recommended in the tutorial (amazing by the way...)

MainePorsche

Even though I have a HVAC controller unit from Greg Brown with his upgraded internal relay, I still have this supplying only current to switch a heavy duty weatherproof relay under the hood. My compressor clutch sees voltage/amperage right off the hot post (fused of course).

pjmcgnns

Since it appears the Radio Shack relay is no longer available I found one at allied electronics PN 70178809. Appears to be identical in characteristics to the Radio Shack unit.