1988 OBS LS swap wiring

[Armstrong4826]

Hey guys, I have an LS swap project going and I would like to use the original gauges. However, the previous owner had cut the original harness and I am not sure what wires are going where.

What I have so far:
1-single green wire=coolant temp
2-oil pressure=orange and red wires
3-alternator=single brown wire and B+
4-speedometer=twisted pair green and purple

Unknown:
1=single grey wire that goes to cluster pin adjacent to coolant temp pin(see images)
2=tachometer…diagrams show a white wire but I can’t find the coil wires at all.

Thanks

 

[mrrsm]

You've probably already --Scoured YT Spanking Clean-- of these Videos...but...jik...THESE Videos are Kinda-Kool as a lead in to another *Spot-On* Resources and will give your Readers-Followers a better idea of what you are dealing with:

 

[TJBaker57]

This diagram is really hard to follow.... but I think it contains useful information.

I 'think' I see the connector you have pictured first, a 34 cavity connector in these cut up images.

Indicator Lamps — 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L.

charm.li

 

[TJBaker57]

If I read the diagram I just posted a link to correctly the two connector cavities you show your red probe in are circuits 31 and 35 (35 has a typo in the drawing calling it 36 at in point).

#31 is a tan wire, oil pressure, in cavity A13.

#35 is a dark green wire, engine temp, in cavity A14.

 

[Armstrong4826]

If I read the diagram I just posted a link to correctly the two connector cavities you show your red probe in are circuits 31 and 35 (35 has a typo in the drawing calling it 36 at in point).

#31 is a tan wire, oil pressure, in cavity A13.

#35 is a dark green wire, engine temp, in cavity A14.

Ok, so the 2-wire “oil pressure switch” is not reporting a value to the cluster? The harness in the truck only had 2 wires going to the oil pressure switch, a red that goes to the fuel pump relay and an orange that goes into the cab. Where does the tan wire play into the oil pressure sensing?

Are there really 2 oil pressure devices on the 5.7?

 

[TJBaker57]

Ok, so the 2-wire “oil pressure switch” is not reporting a value to the cluster? The harness in the truck only had 2 wires going to the oil pressure switch, a red that goes to the fuel pump relay and an orange that goes into the cab. Where does the tan wire play into the oil pressure sensing?

Are there really 2 oil pressure devices on the 5.7?

I cannot say that I know this!

All I have to go on is the diagram I linked to.

Does the IPC connnector seem to match what you see in the truck?

 

[TJBaker57]

Ok, so the 2-wire “oil pressure switch” is not reporting a value to the cluster? The harness in the truck only had 2 wires going to the oil pressure switch, a red that goes to the fuel pump relay and an orange that goes into the cab. Where does the tan wire play into the oil pressure sensing?

Are there really 2 oil pressure devices on the 5.7?

I will see what I can find in the diagrams.

I remember my 86 Trans Am with the 305 had a fuel pump oil pressure switch.. There was a fuel pump relay connected to the starter that got the pump running during cranking but then dropped out and a oil pressure switch then ran the fuel pump.

EDIT: Research seems to indicate the relay did NOT drop out and the fuel pump oil pressure switch was a backup system in case of relay failure.

 

[TJBaker57]

It "looks" like you may have two oil pressure switches/sending units.

A fuel pump oil pressure switch plus a oil pressure sensing unit that reads a pressure.

Fuel Pump Control — 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L.

charm.li

Electrical Diagrams — 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L.

charm.li

 

[TJBaker57]

More research leads to a correction. It seems the fuel pump relay is the primary means of fuel pump operation and this oil pressure switch is a backup system.

Anyway, have a look at this image...

Oil Pressure Sensor: Locations — 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L.

charm.li

 

[Armstrong4826]

2 oil pressure devices haha, I guess that’s where we were in 1988. Not a bad design but it seems strange today. Well I guess I will be bypassing the fuel pump oil pressure switch since I won’t be using the PCM at all. Thanks for the help, I was wrong in my original thinking.

Also, per the diagram does that mean 39 or 250 is for the tach? Both are labeled ignition

 

[TJBaker57]

, per the diagram does that mean 39 or 250 is for the tach? Both are labeled ignition

Does your cluster have a tach?

I can't find any reference to a tachometer in the literature.

Speedometer Head: Diagrams — 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L.

charm.li

 

[Armstrong4826]

Does your cluster have a tach?

I can't find any reference to a tachometer in the literature.

Speedometer Head: Diagrams — 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1988 Chevy Truck C 1500 Truck 2WD V8-350 5.7L.

charm.li

Well I thought I had a tach in the cluster this whole time and that’s why I was looking for a tack wire, I must have confused the newer OBS cluster with the one out of this truck. I guess that’s not needed after all on the 88. Oopsie, sorry about that

 

[TJBaker57]

guess that’s not needed after all on the 88. Oopsie, sorry about that

No worries!! The exercise allowed me to learn how to follow these older wiring diagrams a little

 

[Armstrong4826]

No worries!! The exercise allowed me to learn how to follow these older wiring diagrams a little

Well I appreciate the help, I was on another forum and got no help. I was ridiculed for LS swapping an OBS lol.

Anyway I would like to get opinions on the alternator wire. The TBI engine used a smaller maybe 12ga wire from the alt to the bulkhead distribution block, however the LS has a 1 or 2 ga wire going from the alt to the battery.

Should I mirror the LS system since I will need to use an LS alternator, or would the TBI wire be enough to power the limited electrical load on these trucks?

 

[Armstrong4826]

Well I finally got the gauge sensors installed, I had to do some research on what senders this truck is supposed to have since everything was missing. I went with the SMP-TS76 for coolant temperature and the SMP-PS154 for oil pressure.

Also the required adapters to install these two…

Let’s hope they are correct haha, if not no big deal really. Next up I need to get a starter and battery to see what kind of compression this 5.3 makes….fingers crossed