How To Test The Oil Switch

fearless · May 23, 2007

it has been suggested that my uncles oil switch could be shutting the fuel pump off on his 88 gmc. how can i test this to determine if it is in fact bad?

Matt

RCH · May 24, 2007

I found this at another forum, hope it helps. I used to have the factory service manual, but I gave it to the person who bought my old truck. Oil pressure switches are only about $30 at napa, if it is the original, it wouldn't hurt to try. If you buy a new one they will ask if you have a gage or a light.

"I have a 92 GMC 1/2 ton 2wd w/TBI that stalls when you let off the gas. And then it's hard to start again. I've replaced the fuel filter. Fuel pump is about 40K miles old and seems to be working fine. What's next? Thanks for the advise.

Answer -

You stated: "seems to be working fine".

Question: How did you determine this ?.

What was the LAST repair on the vehicle ?. ANY repair of any nature ?. In most cases problems that arise days, weeks or even months later can usually be traced back to the previous repair.

Try this TEST:

Cycle the ignition key from the OFF position to the ON position 3 times, Do NOT try and start the engine during this test.

On the fourth cycle try and start the vehicle and let me know.

autohelp

Answer -

OK. Locate the fuel pump relay of which is usually mounted in the engine compartment. Look at the electrical harness jsut before the relay and you will see a female connector taped to the harness that is NOT connected t anything. This female connector is the FUEL PUMP TEST LEAD.

Using a standard 12 volt test light, probe this connector and turn ON the ignition switch and observe the test light, It should illuminate for 2 seconds and then go out.

If it does this, We can proceed.

Leaving the test light installed start the engine and observe the test light, It should stay illuminated all the time that the engine is running.

If it DOES, We can proceed.

Take a TEN (10) AMP FUSED jumper wire and attach one end to the battery POSITIVE terminal and the other end to this TEST CONNECTOR, Thus providing battery voltage to the TEST CONNECTOR at ALL TIMES as long as you have your TEN AMP FUSED jumper wire installed. You can hear the fuel pump running constantly.

Leve this TEN AMP FUSED jumper in place and start the vehicle and see if your problem is resolved.

let me know so we can proceed.

autohelp

Answer -

OK, Then locate the fuel pump relay and here is what you should have at these fuel pump wire locations at the relay:

Turn the ignition key to the "ON" position ONLY. DO NOT try and start the engine. OK, With the ignition in the "ON" position via the key:

Locate the GRAY wire at the fuel pump relay and using your 10 AMP FUSED jumper wire, Connect this GRAY wire to battery voltage and you should hear the fuel pump run constantly as long as you have battery power to the GRAY wire.

Now see if the vehicle will start and remain running.

let me know.

autohelp

Answer -

A bit SKEPTICAL is a GOOD thing.

OK, Here is how the fuel pump operates:

1. When you turn the ignition (key) past the ON position to the start position, There is a 2 second fuel pump run as soon as you turn the key to the ON position on it's way to the START position.

This 2 second pump run is provided by the fuel pump relay.

Once the engine starts, The fuel pump relay is no longer needed and battery voltage is supplied to the pump via the vehicles ECM (computer) and on SOME engines, Through the oil pressure switch, Then back to the fuel pump."