When headlights are turned on, PM3 fuel light goes out and/or starts to flicker, and truck bucks. Turn lights off and it smoothes out. It is wired according to instruction, all connections soldered. No codes are displayed. Any ideas?

 

Alternator ??? Are you maintaining 13 Volts or so when your lights are on?? Can you get the TST to repeat this problem with, say the fan blower on high, seat heats on etc etc...

 

Well, I'd say that it might NOT be wired properly. Odds are that you are sharing a ground with something, and the ground isn't very good.

Otherwise, what Zino said-- alternator.

These trucks aren't known to be hard on alternators, though.

jlh

 

It sounds like there is a bad ground somewhere in the system, and not necessarily on the TST box. The ground straps running from the engine block to the frame have been known to corrode on these trucks. Do you have any other strange behaviour on your truck like an inordinate number of blown fuses or anything along those lines? You may want to pull those ground straps off the engine and frame, clean them up, and give that a check. The headlamp switch itself may be on its way out. Do you run any extra driving lights off the main headlight power without using relays?

Rod

 

Rod- no other glitches with the truck. I had been having the bucking probs constantly since I got the box March'ish. You sent it back to TST with the harness and it checked out fine. I replace MAP sensor as suggested by TST, tried grounding to battery, resoldering connections and it did not change anything. Then just recently without doing anything the box started working great! til I turned on the headlights lastnight. I'll give the grounding straps a look.

Grant

 

the ground straight to battery sometimes gives problems,,put your ground to the black w tan tracer wire in the injection pump harness as tst instructions state,ive seen this fix problems with the powermax problems before.......rods suggestions are a good idea also.............bama

 

The ground is back to the proper ground wire. I only tried the battery ground for trouble shooting.

 

You also may want to try a different power source. Turning on the lights might be making your current source "dirty" and is causing the erratic fueling.

John

 

Cleaned up all the grounds I could find. Also found that when I turn on a bunch of electrics it will start to buck like when turning headlights on. Kind of suspect alternator, have it booked into shop next week. Any quick easy test for the alternator? I only have a volt meter.

 

A volt meter should be all you need. Just test to see how much voltage it is putting out at idle.

 

Had charging system checked at the *******. Every thing checked out fine. Any idea what to look at next for the TST. Still bucks when I turn other truck electrical services on.

 

I'd check the voltage myself.

 

Just a thought ... Check your lift pump pressure with a mechanical test gauge. See if the fuel pressure is dropping when you turn on the extra electrical loads. I've seen my pressure drop in the winter when the grid heaters cycle, so maybe you're experiencing something similar. It could be a weak or dying lift pump that can't keep up with the PM3.

 

Good thought. Mine does the same thing (with the fuel pressure that is).

 

FP guage is mech. Press doesn't drop below 10psi. 15 psi idle. Pump was replace a few months ago. I am confident that it is not the LP.