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Intermittent no start, 8 volts on ground wires,


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Truck decided not to restart once Saturday, started Sunday and died once, one restart since then.  No rhyme or reason.

but. . .

I have 8 volts on the grounds at the Aux. battery for the PCM and ECM.

 

Disconnect them with the key on and everything comes on like normal but no start. (no voltage on VP pin 7)

Reconnect and it will start.  (voltage on VP pin 7)

 

Will restart everytime after you disconnect and reconnect grounds at Aux. battery.

 

Any thoughts?

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  • Administrator

Back check the passenger side battery voltage. Could be the red bridge lead across the radiator I've seen this before just weak connections.

 

Second I highly suggest the W-T ground wire mod and my protection fuse mod.

 

 

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Update:

alternator and starter are disconnected 

batteries are separated

2 plugs at driver side are disconnected 

12 volts only going to PDC

 

voltage still present unless PDC fuse 11 is pulled

or plug closest to engine on PCM is unplugged

 

I cannot fine PDC fuse 11 on the diagrams.

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 fuse 11 in the PDC is for the anti-lock brake controller.

 

Here is pin 7 at VP.   It is the communication from the pump relay.  Your electrical problem is not allowing this relay to reset properly.  (first picture)

 

Second picture is how the relay is triggered.   This is from a 2001 manual.  your mileage may vary.

image.png.3af5bcbe1fab2824176dc415526fc612.png

image.png.61d078703b69d273a1a2028bbd6fc0d1.png

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Posted (edited)

Fuse 11 in the PDC is labeled "50 AMP IGNITION RUN/ACC" under the cover.  Can someone assist with some diagrams on this circuit?

 

 

IMG_20200518_180640045[1].jpg

Edited by puppy
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Posted (edited)

Not sure what year yours is.

From my 2001 manual (my manual says 40 amp): It is the power for the ABS controller.

image.png.8ea055f0b96b08fea1a97896bad3b17b.png

 

My fuse 10 is 50 amp and goes to the ignition switch.

image.png.25eeb14b68415365875406f1ebd8f23c.png

HTH

 

Hag

Edited by Haggar
added the 50 amp circuit that may help. Year of truck makes a difference....
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Thought you were speaking of a 98. Just noticed you have 2 trucks in your sig. My 02 is the same as yours, fuse 11 ignition, run, acc. 50amp

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  • 2 weeks later...

Update 

 

The truck can be made to run every time by unplugging the ground plug at the auxiliary battery several times and getting the fuel pump relay in the PDC to close, which turns on the lift pump and gives the IP power.

 

Is it possible that this exercise is resetting the ECM somehow and getting it to operate properly?

 

I suspect an ECM problem.

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The relay in the PDC does not give the lift pump power, only the VP. Stock the power for the lift pump comes from the ECM. Most after market pumps that come with a wiring harness have a relay in them that the oe lift harness is used for a trigger and power is derived from the battery or another 12v source. 

After having my batteries disconnect for work I am doing, if I connect the passanger battery first it will power on the lift pump. Never have figured that out. Mine is an aftermarket with the relay added.

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So the relay in the PDC labeled fuel system relay (#51 in the picture above) is the power supply to the VP only?

 

The lift pump is in the tank with a separate relay, battery power from PDC main stud, triggered from original source. 

 

The lift pump comes on when the ground is unplugged, cycle the ground plug a few time and the #51 relay clicks, the truck starts.

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That is correct on the relay. The oe pump power came thru the ECM. The whole purpose of the relay harness is to get the load off of the ECM to run the lift pump. 

If relay #51 is not energizing the truck will definitely not run. The truck can run without power to the lift pump. 

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