Oil pressure switch life

[Don Whitlock]

What length of service are you getting from the oil pressure switch? The last two I have replace averaged 30,000 miles.

If this is significantly short, any ideas as to the cause?

[tom.mcinerney]

Don, a 'search' will unearth some relevant commentary. Always list model-year-mileage-upgrades-etc, or include the info in your signature.
The pre-96 electronic engines rely on undersized contacts in OPS to drive lift pump directly. '96 and up use OPS contacts only to signal PCM, which energizes lift pump thru relay, so here better MTBF of LP.
Hot engines at idle drop oil press enough to cutout OPS signal, fries points.
There's an article detailing fix somewhere on site or in reprints.
Dirt in fuel line, can degrade LP valves, too.

[ucdavis]

My OEM '95 OPS died somewhere between 20 & 25K. I dissected it & found miniscule burn marks on the contacts. Lift pump hadn't run for a few thousand miles, then the IP died (terminally in love w/the lift pump, I guess). The OEM unit is an old part number, and the replacement has had some re-engineering. Still not a great device for the 94/95 years. I'm going to do the OPS Relay fix when the weather turns a bit friendlier.
p.s. Click "My Profile" and edit your "signature" to permanently input your rig data so you don't have to re-enter.

[HowieE]

I have a copy of the 1996 GM factory manual and it shows the oil pressure switch still carring the full load of the lift pump once the engine reaches the set oil pressure.
Yes there is a relay in the circuit but it is only engaged during the "start".
The oil pressure switch is a safty to kill the engine if oil pressure is lost and thus is the primary control for the lift pupm.

[ucdavis]

Somewhere after '95 (my PCM couldn't care less if the lift pump lives or dies) and B4 '98, the PCM got involved w/the LP. '98 manual describes DTC 0231:
"The status of the lift pump is monitored by the PCM. This signal is also used in order to store a DTC if the fuel pump relay is defective or if the fuel pump voltage is lost while the engine is running...
CONDITIONS FOR SETTING THE DTC-
~The PCM performs this DTC diagnostic continuously.
~The fuel pump is commanded ON.
~The difference between the ignition voltage and fuel pump voltage is greater than 4 volts.
~The condition was met for > 2 seconds."
So the PCM is in the relay link by '98, and monitoring for an LP circuit failure. My guess is that GM figgered some of the early IP failures resulted from fuel (i.e. lubrication) starvation of the IP via dead OPS or LP, and wanted to cut down on service issues, so it put the PCM in charge of LP operation.

[HowieE]

Yes the discription of the P0231 code is correct, but it does not remove the fact that the FULL CURRENT of the lift pump goes through the oil pressure switch while the engine is running. The relay only supplies current to the lift pump during the glow plug cycle and is completly out of the circuit once the glow plug cycle has stopped.
If you want to extrend the life of your oil pressure switch do the oil pressure switch mode.
Cut the white wire coming off the OPS and connect the end coming up from the switch to one side of a relay coil. Connect the other side of the coil to ground. Connect the other side of the cut white wire, the end leading into the engine harness, to the normaly open connect of the relay. Splice the common of the relay into the orange wire going to the OPS.
Now when the OPS sees oil pressure while the engine is running it will close the contacts on the relay and supply current to the lift pump through the relay. The only current that will flow through the OPS is the coil current of the relay, which is ma.
This mode will not effect any of the original function of the PCM. it will still monitor the lift pump during the glow plug cycle and running. You will still see an 0231 code if the voltage to the LP drops as mentioned above. However if you do get an 0231 code in the future I would look at the $3.00 relay first before replacing the OPS.

[HowieE]

There is a mistake in my above post. The wire to cut coming off the oil pressure switch is gray not white.