Voltage drop at fuel pump relay

[clinter36]

Hello all,

 

The vehicle is a 1989 R3500 (bucket truck) with the 5.7 TBI engine.  Although its technically a square body, I am posting this in 88-99 trucks because its probably more relevant to the tbi.

 

I have had issues for years starting the truck and it would take a shot of ether, to get it running. but then it would run and idle perfectly.  I had always known it was something with the fuel pump(s) but didn't feel like digging into it as I rarely use the truck.  So I finally decided to tackle this.  

 

After reading about many issues with fuel pressure plus the fact that I have never cleaned or checked the fuel tanks in the 10 years I have owned this, I decided to pull the right tank and replace the fuel pump (I have only used the left fuel tank for fear that whatever was in the right tank was so old).  The tank was relatively clean, the hoses in decent shape and now it has a new pump, new filter and 10+ gal of fresh fuel.  But that has not fixed the starting problem.

 

So I moved on to the fuel pump relay and ECM temp sensor, and I also replaced the idle air control - for no great reason.  Still no start.

 

So here is the strange part...  if I pull the plug off the fuel pump relay and put a jumper directly from the battery to the tan/white wire.  I can hear the fuel pump run and the engine will start and run without issue.  When I check voltage at the orange wire on the fuel pump relay plug (its us supposed to be fused directly from the ECM B fuse terminal) I get 2 volts.  If I unplug the ECM under the dash, and check voltage on the orange wire, I get 12 volts.  SO... something else in that orange wire circuit is robbing voltage when the ecm is plugged in.  The only thing else I see in the wire diagram is the oil pressure switch - but I am really only looking at a partial diagram.  I didn't think about it until this morning at the office, but if I were home,  instead of unplugging the ecm and checking voltage I would unplug the oil pressure switch and check voltage. 

 

So I am thinking its either a faulty ecm or a faulty oil pressure switch.   I did not suspect the oil pressure switch initially because the engine would run after a shot of ether.  Another clue I should mention is that when I checked the ECM b fuse it was blown, but I did mess around a bit with the relay plug so is its possible I did something.  The oil pressure switch is already on order.

 

Any brilliant minds out there have any suggestions ? 

 

Thanks

clint

[Richard Stratford]

Amazingly, I seem to have an identical problem.  1991 V3500.

 

Any chance you can share what you found?

 

Thanks,

 

Richard

[Brian Lindsey]

fyi , its almost never the ecm , what he is describing sounds to me like a short in the wiring , or maybe a bad ground, check your powers and grounds and i think you will find your problem