Is it ok to do this?

[flipster125]

First info on my truck...

1989 4.3 TBI 4x4

Ok, a few days ago the fuel pump decided to just stop working... the truck would still run fine, but would take 10-15 seconds to start... Also kept getting a code 54 (low fuel pump voltage) Tested the fuel pump by suplying it directly with voltage (hooked a small 12 v battery up to the wires right before they go into the tank), and it worked fine... So I thought it must be the relay... Replace it and same thing... so I tested the wires going into the relay... now bear with me as I don't know all "terms" involved with relays (though I *DO* know how they work), but I'll do my best to describe it...

One wire has constant 12V
One wire goes to the pump... when the relay is activated it bridges these two wires (giving power to the pump)

(when I remove the relay and bridge these two wires the pump ran fine)

The two other wires are what activates the relay... One is ground, the other is the "trigger".

Ground tested fine, however, no matter what I did (turning key off/on, cranking, running) I couldn't seem to get any voltage on the "trigger" wire, which comes from the ecm... tested the wire to see if it was broken or sorted, but it's fine... So that left (as far as I know) the ecm... though I happen to have a spare ecm and it does the same thing. Well with no other leads here's how I fixed it... (This is what I want to know if it's ok to do this)

I took the trigger wire and cut it at the relay, and then I took a length of wire and attached the trigger wire to the pink ignition wire at the coil (which only have 12V when the key is in the "on" or "crank" positions)... With it hooked like this, the fuel pump turns on (and stays on) whenever the key is on or cranking... Now the truck starts up like it did before, and seems to be running fine... I just want to make sure I'm not going to fry something etc... I'm leaving for a 1000 mile trip on Friday, and would rather not be stranded

So, is it ok to use the ignition wire as the trigger to the fuel pump relay? Or does anyone know the reason the ecm wasn't triggering the relay itself?



Thanks

[BlazerLT]

test your battery voltage.

[flipster125]

Originally Posted by BlazerLT

test your battery voltage.

It's good... Not sure how that would affect it though

[BlazerLT]

because you fuel pump is powered by the battery.

If the battery is low or in poor shape, the fuel pump cannot supply the fuel pressure needed.

What do you mean "it is good"?

Have you even tested it?

[Rmbodie]

I just converted my 89 from TBI to a carb and I installed a pressure regulator on the fuel line . I noticed that when I cut the key on to purge the air out before connecting everything , the pump only runs for a second . But when I cranked the engine it runs continuously . Yor battery has to be good if you can turn it over for 10 seconds . I think your relay is sticking , worn internally . I had a GM car that would do that and I swapped matching relays around and it fixed it but the wipers adopted the bad habit until I bought a new one .HAHA. Rob

[BlazerLT]

Originally Posted by Rmbodie

I just converted my 89 from TBI to a carb and I installed a pressure regulator on the fuel line . I noticed that when I cut the key on to purge the air out before connecting everything , the pump only runs for a second . But when I cranked the engine it runs continuously . Yor battery has to be good if you can turn it over for 10 seconds . I think your relay is sticking , worn internally . I had a GM car that would do that and I swapped matching relays around and it fixed it but the wipers adopted the bad habit until I bought a new one .HAHA. Rob

Completely wrong, if the load from the starter is taking all the remaining juice from the worn out battery, there will be little left for the fuel pump.

Hell, he even got a low fuel pump voltage code.

[flipster125]

Originally Posted by BlazerLT

Completely wrong, if the load from the starter is taking all the remaining juice from the worn out battery, there will be little left for the fuel pump.

Hell, he even got a low fuel pump voltage code.

the Battery is fine... plenty of juice.. I can leave the headlights on while cranking and they don't even dim... I also point out that since I did my "fix", the fuel pump runs fine... so I can rule out a dead battery... The problem was/is that the ecm isn't switching on the relay... my fix was to use the pink ignition wire to trigger the relay... Let me illustrate...

This is a crappy wireing diagram I just made... forgive my crudeness


I want to know if it is safe to leave it like this... The truck runs fine like this... I just want to make sure I wont fry anything (such as the coil etc) by tapping into the ignition wire...

[BlazerLT]

test the battery voltage, you don't judge it by the headlights.

Take a voltmeter out and test the battery voltage directly with the engine off and the engine running.

[Allbert]

I don't know exactly why your ECM (or a spare) would not have wanted to turn on your fuel pump while cranking, but it sounds like the sort of thing it would do if it were getting bad information from somewhere, like if the fuel pressure sensor were telling it that fuel pressure was too high when it was actually not. I'm really not even sure an '89 has a fuel pressure sensor, but I'm guessing it does. Anyway, the most likely downside I can think of to hooking your relay straight to ignition is that your pump may run continuously during extended key on engine off periods, but I would hope the fuel pressure regulator just bleeds extra pressure back to the tank while this goes on.

[rksnc]

Well to give more info, the battery wire is the same wire to ecm and also to oil senging unit. If you would look at wiring diagram for fuel system you could see this. The ecm sends power to relay to close switch. I would check oil pressure switch and make sure that is not the fault and the next thing I have seen is the ecm to cause same thing. The ecm in 89 are junk and fail all the time. I have replace more ecm for fuel pump problems than anything else other than bad fuel pumps.