oil pressure warning light to gauge switch/return to normal

[riv1973]

hey all!

so i have an issue., when we bought our 1988 blazer we were told that the engine was replaced a while back. when we test drove it, the oil gauge was pegged at 60 psi which i thought was a bad sensor. i removed the sensor the other day and it appears that the sensor is for a 1991 350 with a warning light. so i figured since the cluster has a gauge they just put the wrong sensor in the back of the motor. so i put the right one for a gauge ( since this is what is in the cluster, and it is a three prong sensor needing a three prong plug) into the motor and notice that the plug coming from the wiring harness is for a warning light also ( the two prong connector vs the three prong gauge plug. ). it seems to come right out of the firewall and plug into the sensor. i could be wrong, but that is what it looks like.

is it possible they replaced the whole engine wiring harness for this 1991 motor and removed the original 1988 factory harness?

i ordered a GM three prong plug but how would i hook it up to the factory gauge? i only see two wires on the current plug in the truck. how can i utilize the factory oil pressure gauge. are there wires on the back of the gauge that i can run right to the new and correct sensor?

thanks and would like any suggestions!

john

 

[BoondocK5]

Watching this, as I'm having a similar issue with my 8.1 swap

 

[76zimmer]

Is it possible they replaced the gauge cluster?

 

[dyeager535]

I dont think '88's had three wire sending units/switches. I could be completely wrong, but I want to say GM started going to those around 1990.

You may be misunderstanding what you are looking at? A two wire plug may be the oil pressure switch for the fuel pump, which means there would be a singular spade-type connector for the older oil pressure sending unit somewhere. Or should be. Tan wire IIRC??

The earlier setup can easily be rewired to the single switch/sending unit setup of course, but you'd need to know for sure what you are looking at.

Check voltage on either terminal of the existing plug with the key off. If there is 12v on one side, you know it's for the oil pressure switch, which is part of the fuel system. Not an idiot light switch. I could be wrong about that, theoretically GM could have set the idiot light switch up the same way, with battery voltage. If you find 12v on one of the two terminals in the connector, check for continuity from the NON-12v wire in that connector to the fuel pump wire that comes off the fuel pump relay. Not the test wire, remove the relay plug and it will be one of those terminals. Can't recall which position, but the diagram printed on it should tell you.