Question : What Am I Missing? (Wiring Issues)

[HJR1]

Looks like I get to undo the PO's wiring butchering. I was trying to find out where the wires were that go to my oil pressure sending unit and I found a bunch of cut wires taped up out of the way. I imagine some of these wires go to the sending unit, but which ones? What are all the others for?

It's a '87 S-15 2.8L

I looked at a wrecking yard today, but they only had an '86 and two '88s. The '86 didn't look right, and the wires in the '88 appear to be color-coded different.

Here's what I have:

A brown and light pink on the d/s inner fender near the master cylinder:





And six wires that go into a small harness that goes behind the engine and appears to connect to the large harness running behind the distributor:

They appear to be:
Beige
Dark Beige
Orange
Brown
Beige with white stripe
Black



The truck does run, although not well while cold. (I do have some helpful diagnosis hints from another thread on the cold idle issue that I'll try after I get this wiring straightened out.) So far the only thing that doesn't seem to be working is the oil pressure gauge, but I could be missing something.

TIA!

[robertisaar]

i'm seeing an 87 S15 2.8 4WD using black and brown wires on the oil pressure sensor. black is ground. brown goes to the oil pressure relay, which then continues as a pink wire with black stripe and runs through the C100 connector and ends at the check guages light...

[bob442]

you should go get a michlle on demand for your truck. i dont think any of us are that knownledgble about S10 wiring off heart

you should be able to get a michlle print out at an auto zone, partsource, maybe napa.

[HJR1]

Thanks for the replies guys.

robertisaar, I'll try the brown and black for the oil pressure gauge.

bob442, I do have the Mitchell's for this truck, but it's not very helpful, it shows three oil pressure sending units. Two right by themselves and one buried in the middle. It also lacks the ability to show which wires are bunched into a harness as it's just a wiring schematic.

I'll keep on trying, but I may have to wait until the bone yard gets an '97.

Another couple of questions, oil pressure relay?? What's the purpose of that? Sending units are usually wired to the gauge...

The relay center on the drivers side inner fender, some trucks have three, mine has 2, the center position is open. What are these relays and what is their position on that mount?

Thanks again!

[robertisaar]

this pic should help with the relays...

and i can't find a decent description as for why there is a relay for the oil pressure guage...

[bszopi]

Once the engine is started, the fuel pump relay is by-passed and a switch in the oil pressure sensor provides power to the fuel pump. The fuel pump relay can be bypassed manually, and the pump operated on demand, by connecting a jumper wire between the correct terminals of the oil pressure sensor connector, or by applying battery power to the fuel pump prime connector.

Nevermind, that doesn't have to do with an oil pressure relay...

[HJR1]

Thanks for that pic robertisaar, that may clear up some of my mystery wiring. That pic shows the center relay as being the A/C compressor relay which being an A/C truck mine should have. However the PO had 'upgraded' to a Sanden compressor, so it looks like he took out that relay when he took out the original compressor. So some of those extra wires are probably A/C relay wires. Only the coloring does not match up at all with the Mitchell's coloring of the A/C relay... I'm really going to have to find another '87 to match this all up.
I put in the stock style compressor that came with the new engine so I'll be converting back to stock.

Thanks for the info bszopi, I think I'm understanding it now, they run the fuel pump on one power supply when starting and switch over to another when on 'run' and use the oil pressure as an indication that engine is running? So maybe the oil pressure relay is a safety feature so if you lose all oil pressure it shuts the fuel pump off thereby shutting the engine down? I'm just speculating here, but it's interesting.

[bszopi]

Yes, it is a safety feature to basically remove power to the fuel pump on loss of oil pressure.

[Schmieder]

What I would do is disconnect the pcm harnesses and test each pin.

Get a multimeter and place one lead on the wire in question and use the other lead to tap each pin. Eventually you'll get a hit and looking up that pin will tell you exactly what device it wires to.