Lift Pump Operation

[afgunn]

I have read everything I could find on the operation of the lift pump that would apply to the 93 listed below on this forum and another one. I fully understand how the lift pump works for a 98 with OBD-II and the oil pressure switch. But how does it work with a 93 with OBD-I? Is there an oil pressure switch? I need to know because as best as I can tell mine is not working.

From what I have read on the 6.2 and OBD-I the lift pump does NOT cycle on when the ignition switch is first switched to the ON (run) position but DOES when in the START position. And, I suppose, does operate when the engine is running and the ignition switch is in the ON position. Thanks for any help.

[Robyn]

92 through 95 used a similar system.
The lift pump is powered on as the key is cycled to the start position.
As soon as the engine builds oil pressure the oil pressure switch sends power to the lift pump to keep it running.

On the 92 the fuse for the lift pump is in the junction box on the RH upper side of the firewall by the AC (if equipped)
Remove the black plastic cover and the little black unit on the far left (looking from the front) is the fuse holder. Release the little clip and pull the wire harness and the fuse holder from the little retainer box.

The fuse will be right in the end of the wiring harness device you unplugged.

You can test your lift pump easily by touching one of the open slots in the fuse to the large terminal (Bat +) in the junction box (This is the stud closest to the fuse)
The lift pump should start to buzz as soon as the connection is made.

(This is also very useful in primeing the system after the fuel filter is changed)

Once the engine is running the lift pump should run continuously. If the pump is not running then the oil pressure switch is a definate suspect.

If the pump does not run any time then the pump may be bad.

Let us know

Robyn

[afgunn]

Thanks for your help Robyn. It is very difficult to troubleshoot a system when you do not know how it is supposed to work! My brother currently has all my DP books and repair manuals and it will be the end of the month before I can get them back.

My 93 does not have the junction box that you are referring to or else I do not know what I am looking for. I looked on both fenders in the engine compartment and did not see any junction box. I am not sure where the fuse for the lift pump is. IIRC it is not in the fuse block inside to the left and below the steering column.

I was not sure that the 93s had an OPS. But I believe I saw it today at the rear of the manifold on the drivers side. It will be a few days before I can check the system out since it was raining today and the truck will be in for alignment the next 2 days. I will post with what I find out and what the conclusion is. If I am not able to find the lift pump fuse I will jumper the OPS, if necessary, and see if I have voltage at the lift pump connector. Of course, I will check for voltage at the connector in the START position and while the engine is running first.

Thanks again! Very helpful!

[Robyn]

The junction block is on the firewall right above the heater/ac box.

[afgunn]

Yep, the jucnction box was exactly where you said it would be! I was looking for a more traditional square or rectangular box. It is very well disguised! I thought it was a cover for electrical, heater or AC lines. And the tip on how to check the lift pump was great as well. And what an interesting fuse holder!!!!??

The lift pump was bad! Replacing the lift pump has made the engine run/accelerate very smooth and predictable and the auto tranny likes that very much as well. Before it used to surge and stall, which the auto tranny did not like.

I plan to install an auxillary fuel filter before the lift pump as soon as I get a chance. And then I will change the main fuel filter. The aux filter will help save the lift pump and help me when I start running B20 or Schur Fuel.

Thanks much for all your help Robyn!!!!