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Jumper wire works great, several NOS choke heat relays don't. Is it possible that those several relays are bad? There are no new manufacture relays that I have been able to find which fit.
I assume they're 12v relays, have you tried testing them? Just takes a battery and some alligator clipped wires. What about the rest of the circuit? You have power coming up to go back out but what about the power and ground to activate the relay? Are you getting those? Where do they come from?
Most specifically, ARE THESE REPLACEMENT RELAYS EVEN OPERATING? That is, do you hear them click when they're powered up & down?
Meanwhile, a relay is a relay is a relay is a relay, at a certain level. It's nothing more, or less, than an electrically operated switch. Just like in your house you can replace a regular light switch with a Decora style, a Hubbell with a Leviton, or a dumb one with a voice-activated one, you can replace the one in your car with any other one you can find, as long as its ratings are adequate, it has the contacts needed, the coil voltage is the same, etc., by also replacing the connector. If it comes to that, it's just not that hard.
But I suspect your car isn't telling the relay to operate when it should, or is telling it to operate when it shouldn't. If "several" relays don't work, then I'd start looking beyond the relay itself. Relays don't go bad all that often to begin with; so if more than one is not working in your car, the problem is more likely to be the car, than that you have a pile of defective relays.
Last edited by sofakingdom; 07-11-2023 at 10:22 AM.
you can also plug in the gm part number over at rock auto and 4 different options appear. the plugs on the relay look really dirty, if so, that won't help any. Or maybe its just the photo.......
Charlie, not the sending unit but the "switch". It's just an off/on pressure switch.
However, it could also be the ground for the relay, the FP/Inject 2 10 Amp fuse being blown or missing, or any one of those wires broken.
I would check for 12 volts on the Tan/White wire (Tan is the primary color, white is the stripe). If the oil pressure switch is working while the engine is running, there will be 12 volts passed to that wire at the relay harness. If not, the oil pressure switch is not closing, the FP/Inject 2 10 Amp fuse is missing or blown, the relay ground is bad, or there's another wire somewhere in there that's broken.
12V at the OPSU and FP relay; green wire (this means the FP/INJ fuse is good... pretty sure the FP wouldn't work if it wasn't)
12V coming out of each/either of those 2 things (they're in parallel; either/both should provide 12V at their "output"); tan/white wire
12V reaching terminal A of the CH relay (tan/white)
If the fuel pump is working, all this stuff is prob OK
0V (gnd) at the black wire for the CH relay coil (see JT's comment about "relay ground bad"... not likely, but certainly not impossible either)
The relay should click when the fuel pump is commanded and/or the OP reaches some threshold value
If all that is OK, and the relay clicks, then the coil ckt is good. Time to move to the contact portion of the circuit around the relay.
12V at the brown/white wire at the CH relay
12V at the CH wire (light blue)
If all this is OK, the CH relay and all its wiring is working. Now look at the wiring having to do with the light.
12V at the pink/black wire to the instr cluster... if the gauges work, then this is OK, and of course the gauges fuse is OK (Do the gauges work?)
Check for each of those conditions and report back.
It's really not that hard. Battery & ground. As we used to say in the telephone switchgear business, everything in life is just battery and ground.
12V at the OPSU and FP relay; green wire (this means the FP/INJ fuse is good... pretty sure the FP wouldn't work if it wasn't)
This is one of the questions. I have less experience with GM's carbs and this setup, but people claim the mechanical pump can draw fuel through a bad in-tank fuel pump. I don't think it's been determined if the in-tank fuel pump is working in Charlie's car, besides the mechanical one.
Looking at this design, the oil pressure switch is there to cut the power to the electrical in-tank fuel pump so that the fuel pump isn't spraying fuel while the engine is off in case of an accident or maybe to prevent the pump from straining against a no flow engine off. I don't think this is used to stall the engine in case of oil pressure loss. GM didn't do that on the MFI applications. Besides, the amount of fuel in the fuel bowl would keep the engine running for a while after the electric in-tank fuel pump stopped anyways, so not like it would completely save an engine from oil pressure loss. On a MFI the ECM cuts the fuel pump if it doesn't see an ignition pulse.
My suggestion was to check for 12 volt at the Tan/White wire at the relay harness because the oil pressure switch is hard to get access to. Then trace back.
If the relay is your issue, try NAPA part number ECH AR221. It is a direct replacement.
Originally Posted by JT
This is one of the questions. I have less experience with GM's carbs and this setup, but people claim the mechanical pump can draw fuel through a bad in-tank fuel pump. I don't think it's been determined if the in-tank fuel pump is working in Charlie's car, besides the mechanical one.
Looking at this design, the oil pressure switch is there to cut the power to the electrical in-tank fuel pump so that the fuel pump isn't spraying fuel while the engine is off in case of an accident or maybe to prevent the pump from straining against a no flow engine off. I don't think this is used to stall the engine in case of oil pressure loss. GM didn't do that on the MFI applications. Besides, the amount of fuel in the fuel bowl would keep the engine running for a while after the electric in-tank fuel pump stopped anyways, so not like it would completely save an engine from oil pressure loss. On a MFI the ECM cuts the fuel pump if it doesn't see an ignition pulse.
My suggestion was to check for 12 volt at the Tan/White wire at the relay harness because the oil pressure switch is hard to get access to. Then trace back.
Please make sure you're testing the Tan/White wire at the harness. If you're testing the Brown/White at the harness, that will also be hot but it's not the relay power.
To make double sure, use your test meter positive lead on the Tan/White wire and the negative lead on the black wire. Should see battery/charging voltage while the engine is running.
To be clear, the relay harness will have two hots if everything is working correctly. With that said, and the fact the two wire colors are so similar, this is why you need to make sure you've got the right "pair" in the harness or you will get the wrong conclusion.
Please make sure you're testing the Tan/White wire at the harness. If you're testing the Brown/White at the harness, that will also be hot but it's not the relay power.
To make double sure, use your test meter positive lead on the Tan/White wire and the negative lead on the black wire. Should see battery/charging voltage while the engine is running.
To be clear, the relay harness will have two hots if everything is working correctly. With that said, and the fact the two wire colors are so similar, this is why you need to make sure you've got the right "pair" in the harness or you will get the wrong conclusion.
Positive on tan/white, negative on black. I get .04 volts.
Positive on tan/white, negative on black. I get .04 volts.
OK. So does that mean you got 14 volts in the previous test because you used a different ground instead of the black wire in the harness? Or because you tested the Brown/White wire by mistake?
OK. So does that mean you got 14 volts in the previous test because you used a different ground instead of the black wire in the harness? Or because you tested the Brown/White wire by mistake?
Both. I grounded to a bracket and tested the middle wire, which if I remember is the brown white.
Both. I grounded to a bracket and tested the middle wire, which if I remember is the brown white.
Yeah, ignore the Brown/White wire for now. That's not the circuit we're currently concerned about.
You need to test the Tan/White wire and black wire. Should see battery/charging voltage with the engine running. If you see nothing close to battery/charging voltage, keep the positive test lead on the Tan/White wire and put the negative lead on any good ground. You need to do both (one test using the black wire and the other test using a good ground) to help make sure this is not a ground wire issue in the harness. I suspect this is not likely, but it needs to be tested to get the correct diagnosis.
If you don't get battery/charging voltage doing the above, then it's either:
1) Bad oil pressure switch
2) Blown or missing 10 AMP Inject 2 fuse
3) Missing or broken wires anywhere in the circuits
4) You have an oil pressure issue, which is not too likely or you'd have other issues.
Yeah, ignore the Brown/White wire for now. That's not the circuit we're currently concerned about.
You need to test the Tan/White wire and black wire. Should see battery/charging voltage with the engine running. If you see nothing close to battery/charging voltage, keep the positive test lead on the Tan/White wire and put the negative lead on any good ground. You need to do both (one test using the black wire and the other test using a good ground) to help make sure this is not a ground wire issue in the harness. I suspect this is not likely, but it needs to be tested to get the correct diagnosis.
If you don't get battery/charging voltage doing the above, then it's either:
1) Bad oil pressure switch
2) Blown or missing 10 AMP Inject 2 fuse
3) Missing or broken wires anywhere in the circuits
4) You have an oil pressure issue, which is not too likely or you'd have other issues.
I'll check again tomorrow, but I'm leaning towards the oil pressure switch.
07-11-2023, 08:59 AM
As we used to say in the telephone switchgear business, everything in life is just battery and ground. 
