Fuel pump priming issue

TylerDurden

Jack's Smirking Revenge
Background, I have been trying to chase this down for over 8 years. It started out as the car died while driving one night. Towed it home and attempted to trouble shoot. (its been so long i dont remember what that entailed). Since then I have replaced the pump, walbro 340, I replaced the previous owners hotwire kit with caspers. I have replaced the in tank wiring and pig tail to the hotwire relay. I have also replaced the fender fuel pump relay.

right now the car will not prime.

But I can use the "fuel pump prime wire" at the alternator and get the car to run, but when i disconnect that wire, the car dies.

the original oil pump px sending harness is not in play anymore.
I tested the ground at the front FP relay harness at 4 ohms direct to the battery, and when you turn the key fwd, i hear 2 clicks at the relay, approx 3 seconds between the ON click, and OFF click. I tested the other two connectors at the relay with the key fwd and got 9.4 volts, these are the wires that come from the fuse box and go to the 12v lead to the rear pump relay (hotwire relay). when i took the hotwire kit out of the equation and connect the 3 prong weather pack from the tank to the orig 3 prong connector i still get nothing. I have not had the opportunity to use another person to turn the key fwd while testing voltage with the orig 3 prong. thats next on my list.

My overall question is why will the car run with the FP prime lead at the alt, but die when disconnected? I thought it would have everything to do with the fuse wire to the relay or the wire from the relay to the pump, but seeing 9.4 volts with key fwd kinda shot that theory down. Granted the voltage is low, and that is probably due to multiple start attempts and door being open causing what was a full batt to weaken. I will charge the batt up and do that test again to verify that voltage. But I have had the batt at full charge earlier this week and still could not get it prime.

Another question is should the front fp relay click when I connect the hot lead to the alt, someone mentioned it should but I see no reason why. I would expect the rear relay to click when I do that, but have not had the 2nd person avail yet to test that theory.

thanks in advanced for any help on this gremlin
 

87geeinn

Well-Known Member
Subscribed because I am/was having a verrrry similar issue over the past couple of days and it seems to have cleared itself up, or be a very intermittent thing.

Now, I could be wrong here, but: According to the diagram here, the way I read this is that if you jumper the cable off of the back of the alternator, you are essentially hotwiring the pump and bypassing the relay wiring and oil pressure (backup) wiring. In this case, that should mean that your issue lies not from the grey 12v lead to the pump, but somewhere in the wiring prior to the connector splice located at S115 (lower left corner of the diagram) where the oil pressure, fuel pump relay, and alternator test relay all tie together.

Again, I could be wrong here, but: If you disconnect the fuel pump relay and turn the key on/ignition off, you should be able to run the pump by jumping the middle of the three wires that are in a row (grey) at the harness with that fourth wire that is by itself (pink w/black stripe).

I would love to hear some input from some of the more electrically inclined gurus on this board as I doubt us two are the only ones with this issue.
 

Chuck Leeper

Toxic old bastard
Staff member
"i hear 2 clicks at the relay, approx 3 seconds between the ON click, and OFF click. I tested the other two connectors at the relay with the key fwd and got 9.4 volts, these are the wires that come from the fuse box and go to the 12v lead to the rear pump relay (hotwire relay). when i took the hotwire kit out of the equation and connect the 3 prong weather pack from the tank to the ori".

Sounds like the ecm is timing the prime cycle OK.
The 9.4 v is at least 2.5v low. [Bench testing of pumps/injs on my injector stand showed that at <10V, the pump wouldn't start.]:eek:
As 87G said, look at the rest of the wiring, to determine where the power gets dropped.
Are you using the common wiring that's used in a hot wire kit/ or?? IE: 10/12 ga from alt, thru a fuse, to the relay at the rear. That relay is triggered with the OEM, solid copper wire.
A complete voltage drop test would be a good idea. At 20+ yrs old, the OEM wiring is getting a bit "long in the tooth"...Volt drop tests should be run on the + and - side.
 

TylerDurden

Jack's Smirking Revenge
I charged the battery today, and did another jumper test at the relay, I got 10.5 volts with the key fwd, but no pump prime. I agree that that wiring prior to the S115 junction is what I should be looking at, also, I was able to ohm out the ground from the relay to the battery at .01 ohms. So I am confident at this point it isnt the ground at the relay thats the problem, nor the ecm wire. When i get a chance I will ohm out the fuse wire to the relay next. After that I will tackle the grey wire from the relay to the pump.

Chuck, long time, I think I met you at Reynolds back in 02 or 03. I am using a 10ga wire off the alt to power the pump, I will double check, but I dont think the copper wire is in play. Another question, where does the grey wire from the FP relay up front travel? All I know is it goes down and back under the AC box....where does it go from there? As in where does it join up with the "hot wire lead/oil px lead"?
 

TurboBuRick

Moderator
Staff member
Make a little jumper wire.

Unplug the fuel pump relay on the passenger fender. Jump B and D on the relay plug. See if the car runs.

RL
 
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