Blast from the past reprint - This article first appeared here in The Diesel Page in mid 1999.

The electric fuel lift pump used in the 1989 and newer GM diesels receives 12 volt electrical power through an oil pressure switch while the engine is running. This function provides a degree of safety in the event of an accident since fuel won't be pumped unless the engine is running.

Some 6.2/6.5 owners have experienced a high rate of either oil pressure switch or lift pump failures. Some of these could be attributed to the design of the associated components and not having a suitable rating for the electrical load. This article will show what you can do to increase the reliability of the lift pump system.

During the course of my testing the lift pump circuit and making the following modifications, I noticed that the existing oil pressure switch would "chatter" while my engine was idling. Obviously, the set-point for my oil pressure switch was not correct for the hot idle engine oil pressure level. I also disassembled a defective oil pressure switch and saw that the switch contacts appeared to be too light for the job of powering the electric lift pump since it draws ~1.3 amps at idle. I believe this constant contact chattering at idle may contribute to oil pressure switch and lift pump failures.

What I've done to improve the situation is to incorporate an add-on relay that is actuated by the oil pressure switch. The relay I used has a coil rated at 100 ma (.100 amps), which means the original oil pressure switch only has to handle a 100 ma load instead of the normal 1.3 amps. This should improve the life of the switch contacts and prevent oil pressure switch failures. The 1.3 amps the fuel lift pumps draws is now being carried by the new heavy-duty relay contacts.

Key points:

1. We only had to cut one wire from the oil pressure switch; the gray wire.
2. We obtained another source of 12V DC to power the lift pump.
3. We used a 3 amp fuse on the hot 12 V side (at idle, lift pump draws ~1.3 amps).
4. The relay we installed had a clear plastic housing, which makes it easier to see if the contacts are latching.
5. The switch side of the gray wire (after cutting) provides power for the 100ma relay coil. The opposite side of the new relay coil is grounded.
6. The pump side of the gray wire was connected to one side of the relay contacts. The other side to the 12 V DC source. I am going to re-wire this step so the hot lead (which is now always hot) will be wired to an ignition circuit. This way, when switching off the ignition switch, power will be removed from the lift pump. I believe this will provide the most safety.

The cost of the materials to incorporate these changes came to about $10.00. The relay is a heavy duty part, which we hung on an "L" bracket from the firewall. It just happened that the relay we used had an anchor screw hole into it's core, so we used this to mount the relay to the base of the "L" bracket. We used Teflon insulated wire and sealed all of the connections with either silicone or hot-glue.

As mentioned earlier, my oil pressure switch chatters at idle. Before this modification, the oil pressure switch contacts were carrying the full 1.3 amps to power the lift pump. With the new HD relay in place, the chattering still occurs, but the 100 ma drawn by the new relay won't stress the original oil pressure switch. (Editor: A 100-200 uf electrolytic capacitor connected across the relay coil terminals may reduce the chatter.)

The approximate time to complete this project was about 1 hour. I am hopeful GM will look at this modification as something that can be used to upgrade existing systems.