Right now I'm only getting 26 or 30 psi fuel pressure, depending on the gauge I use. Supposed to be minimum 39 PSI. Bogging on acceleration, and occasional bucking.

I want to test the voltage to the fuel pump at the harness connector near the tank. See photo. Black wire on passenger side, then purple wire in middle, then gray wire on driver's side. What I plan to do is 'backprobe' the connector, as it is in place, with the engine idling.

I would insert the black (negative) probe from my multimeter (see photo) alongside the black wire (ground) and insert the red probe (positive) alongside the gray wire (power to pump).

I would expect to see at least 12 volts, maybe more since alternator is working. From looking around the net, I see there are some people only getting 8 volts going to the pump, which they think is causing them to have low fuel pressure.

Are the probes on my multimeter too thick?? Will they damage the connector?

I'm afraid I'll fry something electrical. Any chance of that if I just do what I described?

Is there anything wrong with using the black wire slot on the connector as a ground, as opposed to using the frame of the car?

Is there an easier way to test for voltage to the pump?


Thanks

GTA88 350 TPI

 

Anybody? Anyone test the voltage back at the tank? How?

 

You want something called a "spoon"

 

Back probing weather pack style connectors sucks, pull the connector and try it from the front and use something to get in to the connector's contact like a small wire and measure that. Your pump wont start but you can get your reading.

 

That won't work because there is no load. If you want to see voltage drop then pump has to be running.

 

 

Thanks, I like that idea of testing it at the relay. It's right up there where I can see it and touch some probes to it. I'm really paranoid about ruining something electrical though. Not sure what you mean about jumper wire and spade terminals.

How about this: I get out the voltmeter and backprobe the hot wire on the relay, which I think is in position E (the orange wire on the edge of the relay) and touch the negative wire from the meter onto the frame. I would not be opening the relay for this procedure. When should that read 12 volts? Always when key is on? Or just for the first 2 seconds? And then if engine is running, it should be 12 also, of course, right?

Would it be better to open (unplug) the relay for this test?

I'm just hoping, hoping, hoping to find some low voltage, like 8 volts, that is causing my fuel pressure to be low, instead of the pump. Ugh.


GTA88

 

Just an update: I backprobed the orange wire on the edge of the fuel pump relay, and got 12 volts, with key OUT. Kind of surprised me but I guess it makes sense. Next I will probe the yellow and white wire on the other edge of the relay, which according to the diagram, sends voltage to the pump. I hope it gets zero volts with key off, 12 with key on (at least for 2 seconds) and constant 12 with engine running.

 

You are on the right track, that's exactly what you should see or at least in that neighborhood. I was referring to pulling the plug from the relay and making a jumper wire bu you can backprobe the connector there like you did, same thing.

 

I went ahead and did some more testing of the fuel pump relay.

I backprobed the tan and white wire on the end of the relay, which sends voltage to the pump.

Results are:

Key off : zero volts. (as it should be)

Key on: 10 or 12 volts (too fast, couln't be sure) for about one second, then back to zero.

Wait 30 seconds with key off.

Key on: zero volts and no noise from pump(hmmmm) Computer not ready to give me the prime yet.

Start engine : 14 volts. This is the most important finding, I think.

Interestingly, the voltage slowly and smoothly dropped down to zero a couple times, but came right back up to 14 if I moved the negative probe around on the ground screw. I guess the electrons get tired of the probe being kept still. lol.


Good news is I'm happy with how the relay is working.

Bad news is there is only one more test to do, to rule out 'weak fuel pump' as the cause of my low fuel pressure and bucking and bogging problem.