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fuel pump voltage

tbs1967

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94x 2wd. Engine will not start but cranks. I have 9.6vdc coming out of the fuel pump relay. (I removed the back cover of the fuse box that is located just aft of the battery. I was able to to probe a test lead and read the 9.6v.) I swapped relays and still the same 9.6v. The wire color is dark green/yellow.

I checked the fuel pressure and there was none.
All other voltages are normal input voltages to all relays. The inertia switch is good.

My gut feeling is that 9.6vdc is not enough to start the fuel pump. Normally I can hear the fuel pump running a few seconds before starting the engine. But now there is no noise indicating that the pump is running.

My question is is 9.6vdc enough to start the fuel pump? Any other suggestions?
 



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FR-425

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The voltage drop indicates that the pump is drawing power but not turning.

Pump is all done....:(
 






awood

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I'm afraid FR-425 is right. Here's what I'd do to verify...
In the left frame rail right next to the spare tire, kind-of around behind (left and forward of) the left rear shock, there's a connector you'd have to undo to remove the pump. (The pump has a pigtail of about 15 inches). Anyway, disconnect the connector and read the voltage again at the same dark green/yellow wire - key "on" of course. If your voltage jumps up to your battery voltage, then the pump is at fault, since you just removed it from the circuit.

If you still have the 9.6 volts, then you have different problem. I haven't experienced this, but I've read here on the forum where somebody had a problem at the power distribution box (where the relay is mounted), and there's been some wiring/connection problems underneath the box.

I suppose another problem might be chafed wiring somewhere in the wire run, but that's not very likely.
 






Turdle

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Just throwing this in-

Fuel pump voltage is only present for the "prime" cycle ( 1-3 seconds) when key is switched on. You might need a helper to switch the key on for you while measuring the voltage at the pump
 






tbs1967

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Further troubleshooting. That 9.6 v is there after 2 or 3 seconds. It read 12vdc during those 2-3 seconds but still did not hear the pump.
I also did what AWOOD suggested by measuring voltage by the fuel tank. When key is turned it goes up to 12v then after a second or so it drops to 9.6v.

I did some continuity checks of the darkgreen/yellow wire,:
·With the connector on the computer module (PCM) I have a reading of 4.6k ohms. (Test lead on neg batt terminal and the other lead on wire.)
·With the connector disconnected from the computer (everything else connected) module, I get an open reading. This should be expected.
·At the pcm, I checked continuity from pin 8 (where the darkgreen/yellow wire goes in) to ground. I get a 6.1k ohm reading.

Can the computer module be drawing down the voltage and it being bad?

Also at the inertia switch, I see two wires go in: the darkgreen/yellow wire and the darkpink/black wire. What is the second wire for? Cannot find it in the prints.

I also did a KOEO test and got a fault code 542, Fuel pump circuit open, pcm to motor ground).
 






MrShorty

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One thing to note when checking the voltage on the fuel pump circuit: Often there is a ghost voltage (seems like it is usually more like 6-7 V) that is produced by the PCM fuel pump monitor circuit. I would suggest that anything less than 10 V is probably reflective of this ghost voltage.

The 12 V you see for the few seconds after the key turns on is normal. Does the fuel pump run when you see this 12 V?

It has never been clear to me exactly what part of the fuel pump circuit a KOEO/CM 542 refers to. I would guess that it refers to an open between the PCM and ground through the fuel pump.

My suggestion at this point would be to ground the fuel pump test lead (short end of the self0test connector) to close the relay. Then working from the relay to the fuel pump, test for battery voltage along the fuel pump wire. Then test the ground wire for continuity all the way to the body (on my '92 the fuel pump ground attached to the body behind the radiator overflow/washer fluid container).
 






tbs1967

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I chased down the voltage of the green/yellow wire. They all read 12vdc then after a second or so drops to 9.4 to 9.6vdc of the following points:
At power dist box under under relay;
A connector directly under the box;
At pin 8 at PCM; (metal paper clip came in handy)
At inertia switch; and
Connecter just aft of fuel tank.

Seems to me voltages checks out good up to the last point as mentioned above, but at no time did I hear the fuel pump kick in. :( The pig tail from the pump can be bad but I won't know till I drop the tank.

I took the test connector and shorted out the wire to close the relay. IT SHUT so that checks out normal.

I took a jumper wire from positive terminal to the fuel pump relay socket into where said green/yellow wire is. Pump did not turn on. (I did disconnect the PCM in fear the current may cause damage.)

What I don't like is the continuity check on the green/yellow wire to ground:
With the PCM connected, I got a reading of 4.6k ohms.

With the PCM disconnected, I have an open so that checks out normal.

I checked out continuity at PCM pin 8 (where green/yellow wire goes) and pin 60 (ground). I got a reading of 6.1k ohms.

I don't know if the 4.6k ohm and 6.1k ohm is normal or not. May need some feedback on those readings.

I bought a fuel pump and a filter and my next step is to dro the tank and insect the top. Perhaps the problem lies with dirt, gunk, and/or bad electrical connection. I'll also do a ground wire continuity checks as Shorty suggested.
 






awood

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I just repaired a sunk float on my 94. I thought about dropping the tank, but finally decided to try the "access panel" method found here in the forum. I think it was well worth the effort, and the carpet and rear seat hide the new access panel completely.

There's a great write-up with pictures, but I don't have the link anymore.

If anyone can help you, MrShorty can, but here's another thought that might be considered a Hack...How about disconnecting the PCM and Relay (to protect them), then applying 12 volts directly to the DK Green/Yellow wire? The only place for voltage to go would be through the pump to ground, and with voltage applied, the pump should run. If not, the pump is bad. I suppose you could even apply the voltage directly to the pump pigtail at that rear connector.
 






FR-425

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Resistors in "parallel" add like this---

a/2 + b/2 = c/2

Then:

100 ohm resistor + 100 ohm resistor = 50 ohms.

You are just adding "parallel" resistance by checking across the PCM then adding in another circuit and checking again.

Unless you have the engineering specs for the PCM, resistance checks are telling you nothing.
 






FR-425

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:D awood get's the brownie button though.... :D

Put 12v and ground right to the pigtail...

No runny---no worky

Done!

Except for the run to the parts store for the pump, and that whole installation thingy! ;)
 






FR-425

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Hey....

Can I get a Gold Star if my original 13 word diagnosis turns out to be correct!!!?????
 






FR-425

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Mr. Shorty's truck is haunted it has ghosts!
 






tbs1967

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Replaced fuel pump assembly. Originally I was just gonna replace the pump and strainer. Without much effort I broke the rusted fuel line pipe on top of old fuel pump so I had no choice but to replace the whole thing.

It started ok but roungh. It got progressivly better but then went downhill. At times it would start normally and runs good. Back and forth for a while. Right now it's starting up good and runs good.

I've had wiring issues a few months ago that effected my ignition system. In fact in the past couple years I've had a few wiring issues that were nuisances. So there's a wiring issue history. No doubt the electrical gremlins remain.

I did a KOEO test to retrieve CEL codes. I got ,122, 157, & 159. Just by a little paper troubleshooting, sounds like voltage issues. Will do more tomorrow.
 






FR-425

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Gold Star!

Glad it's go'n again with out to much hair pull'n!

Say if you are run'n the 90 amp alt. with a lot of accessories it may not be put'n out high enough voltage. causing erratic PCM.

Ran in to this after the elec. fan install.

130 amp cured it.
 






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