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Hi folks!

My father in laws truck - a 94 3500, has a working lift pump, but it never runs, not even when starting.

We were going to check the oil pressure switch which I've heard of before, but I'm not sure exactly where it is?

Thanks!
 

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It is to the right side behind fuel filter, but can be kind of a pain to get to.

Approx cost..

Switch from a GM dealer is about $33
Oil Pressure Switch socket is around $9 at a local auto parts store. Makes it a lot easier to remove it.Edited by: hoot
 

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Ah, good stuff, thanks!

I am not sure why this would keep it from running while starting though? Shouldn't that be overriden.. usually there's no oil pressure when cranking.
 

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Slow,


I might have this wrong a bit but here goes, near as I can remember description, during start PCM energizes fuel pump relay on OBDI truck but all power is still fed through OPS at any time. OPS contacts are not really strong enough for sustained 5amp ??? draw of pump and go bad even though pressure is indicated through a separate path on dash giving false impression switch is good.


A mod used by many had the OPS contacts energize a relay with only <500ma??? draw and power pump from the relay on dedicated circuit.


OBDII truck PCMs control this during start and thru OPS once running under separate circuit feed controlled by either once GM figured out what was happening
 

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Hello Everyone, Just FYI, I am not trying to impersonate DP the moderator, I am DP from the other page.






Actually the only correction to your descrition is that of the ODB I trucks.





On a '94 or '95 the PCM has no control over the lift pump. Th lift pump os powered by the crank fuse via the lift pump relay. During starting proceedures the lift power never passes through the OPS.


You are very correct in that the contacts of the OPS do seem to fail alot, because of the excess current of the lifter.
 

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The lift pump relay should be in the under hood fuse box. On my truck after she starts the power switches through the oil pressure switch. While cranking the power comes from a 10amp fuse. My ECM does come into play through the ECM B fuse under the hood.
 
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