Oil Pressure Gauge Wire Route

[MTCK]

So when I replaced the motor in my V3500 I replaced the loom on a bunch of the harness. The 93 motor I put in had a plug in the block (I think newer style TBI fuel pressure switch / gauge sender up by the distributor?), but I pulled it and installed a sending unit from Napa.

Now with the cleaned up harness I can't find the oil pressure gauge wire. When started the gauge pegs at 3:00 consistent with an open circuit. Can anybody give me a hint where the wire comes through the firewall so I don't have to pull the dash out. Thanks in advance.

[chengny]

Here is it's position in the firewall cable gland. Not much help - but at least you can start there and trace it till you find the end:

[MTCK]

Perfect thank you. I appreciate the help.

[MTCK]

Looking at the wiring diagram in detail jogged my memory a bit. The truck that my junkyard motor from was a few years older than mine. It had the separate oil pressure sending unit down by the oil filter on the side of the block where my 1991 just had the one up by the distributor.. When I had the two motors next to each other on the stands, I put the plug in my block into the junkyard motors sending unit hole.

The oil pressure fuel relay switches look identical but are not. They even have the same plug end, but the later unit (1991) has a built in gauge sending unit. I confirmed three wires, not two on my truck's harness going to the back of the switch including the tan culprit. So more harness work and that tan wire now goes to the sending unit down by the filter that I had already installed. I had a 50/50 chance and grabbed the 1988 or whatever oil pressure switch. Hope someone else can learn from that adventure.

The part that gave it away for me was the diesel engine codes shown for the stand alone sending unit route.