What's this relay?

[hdff]

I went to my local old truck parts place to get some stuf and was going to replace this relay, he had no idea what it was. It is on a 71 ac cab on the back of the dash behind the cig lighter. Don't know if it goes with the ac stuff I'm working on but Just wanted to replace it cause it looks old.

[gable74]

Anti-Dieseling relay maybe?

[VetteVet]

Possibly that or the high speed fan relay. Check the wire colors against these.
Top right.

[hdff]

Originally Posted by VetteVet

Possibly that or the high speed fan relay. Check the wire colors against these.
Top right.

Attachment 1490237

ok the wires are orange , green and red so by that schematic it is the anti dieseling relay. was that something that was exclusive to Ac cab trucks? looking in the classic parts book I don't see one. do they not sell one? would a horn relay work if needed? can someone explain exactly what this does. I know what dieseling is but what does this do to prevent it?

and wow I think that is the 1st electrical schematic that I have understood, all of it made sence to me...... thanks vettevet!!

[hdff]

Originally Posted by VetteVet

Possibly that or the high speed fan relay. Check the wire colors against these.
Top right.

Attachment 1490237

another question vettvet, on your schematic if I understand right the line you have highlighted orange comes from the fuse box to the high speed fan relay, so if I check it at the relay the orange wire should be hot. I have a wire disconnected by the fuse block that was like that when I got the truck( the ac also was not complete or working) that is orange. if it is not then probably the orange cut wire needs to be attached to the fuse block somewhere

[Davidf]

Originally Posted by hdff

ok the wires are orange , green and red so by that schematic it is the anti dieseling relay. was that something that was exclusive to Ac cab trucks? looking in the classic parts book I don't see one. do they not sell one? would a horn relay work if needed? can someone explain exactly what this does. I know what dieseling is but what does this do to prevent it?

and wow I think that is the 1st electrical schematic that I have understood, all of it made sence to me...... thanks vettevet!!

Yes, the anti-dieseling relay is exclusive to the a/c trucks. No, a horn relay wont work, not will a fan high-speed relay work. You can still find the relays on Ebay. I believe there was a Service Bulletin related to the anti-dieseling relays that a diode needs to be installed at the voltage regulator (Field terminal?) so that back feed electrical current does not burn out the anti-dieseling relay. I have all the wiring correct on my truck, with new relay, and can't get it to work...gave up.

[hdff]

Originally Posted by Davidf

Yes, the anti-dieseling relay is exclusive to the a/c trucks. No, a horn relay wont work, not will a fan high-speed relay work. You can still find the relays on Ebay. I believe there was a Service Bulletin related to the anti-dieseling relays that a diode needs to be installed at the voltage regulator (Field terminal?) so that back feed electrical current does not burn out the anti-dieseling relay. I have all the wiring correct on my truck, with new relay, and can't get it to work...gave up.

so it is not needed?

[gable74]

The relay sends 12v to the A/C Compressor for a second or two at engine shut down. It puts a load on the engine not allowing it to "diesel." That's probably a crude explanation but it's how I understand it.

So You probably don't "need" it, but I guess it puts you at risk of dieseling without it.

[Davidf]

No, you don't need it. Gable74 is correct in how it operates. So, since mine is not working, and I have my base timing advanced a bit, my truck will diesel. So lessen the likelihood of dieseling, I turn down the idle speed in the winter when not using a/c. I also tend to turn the engine off while in gear (automatic).

BTW, a/c and blower operate normally if anti-dieseling relay is removed.

[VetteVet]

Originally Posted by hdff

another question vettvet, on your schematic if I understand right the line you have highlighted orange comes from the fuse box to the high speed fan relay, so if I check it at the relay the orange wire should be hot. I have a wire disconnected by the fuse block that was like that when I got the truck( the ac also was not complete or working) that is orange. if it is not then probably the orange cut wire needs to be attached to the fuse block somewhere

Sorry I've been out so I didn't see your other question but the guys have covered it. I think I answered this same question a few days ago but if the orange wires do go to the fan relay then they attach at the fuse box to the CIG
terminal. The diagram I posted doesn't show the connection but here's one that does. Bottom center.

[snipescastle2]

yep, that's an "Anti-Dieseling" relay, it's supposed to keep your engine from "Running on" after you shut it off. (Not that they ever worked as intended) I would remove your fuse block and see what wires were cut in the back of the fuse block, bet you'll find the wire you're looking for....you will have to un do the bolt that connects the engine/front light harness from the fuse block (engine side of the firewall) then remove the two 5/16" head screws holding the fuse block to the firewall..... there are two pieces to the fuse block, one snaps together with the other, you should find the suspect wire that was cut off going to your A/C / Heater controls.
Hope this helps!
Ben

[hdff]

So I can just unplug the relay and forget it?


Also the what I thought were cut orange wire actually turned red and had a female spade connector on it, so when I unhooked the red wire on the CIG on the fuse panel there was a place for it to plug into on the orange wire, and then I plugged in the orange wire to the CIG spade. Now both should have power, we'll see how it works when I complete the AC refurb

[tk1251k]

Just checked my 72 GMC and it has one never noticed it before. Thanks.