1998 Chevrolet S-10 no power to fuel pump

Tiny
TMHANDCO
  • MEMBER
  • 1998 CHEVROLET S-10
  • 2.1L
  • V6
  • 4WD
  • AUTOMATIC
  • 13,000 MILES
I tested the fuel pump and have no power. Then checked the relay under hood and have no power at pin 85.
fuse is good and has power. Do I remove the fuse/relay assembly to find why no power at pin 85? All other pins show ground and pin 85 shows nothing. No power-no ground.
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Tuesday, March 10th, 2015 AT 2:04 PM

7 Replies

Tiny
CARADIODOC
  • EXPERT
"I tested the fuel pump and have no power"

What kind of test did you do and what were the results? Under what conditions are you expecting to find voltage to the fuel pump motor and relay? There should only be 12 volts to one terminal in the fuel pump relay socket. Do you have that? If not, check fuse "ECM B". It's a 20 amp fuse.
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Tuesday, March 10th, 2015 AT 3:20 PM
Tiny
TMHANDCO
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As I stated I have power at the fuse and fuse is good. Method was to ohm
from ground to pin points. All show ground except pin point 85 which should show 12 volts. Have no voltage and is not a ground, shows nothing.
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Tuesday, March 10th, 2015 AT 5:28 PM
Tiny
CARADIODOC
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Dandy. Unfortunately, the diagram I'm working from doesn't list the normal relay terminals. I have to go by wire colors. If you found 12 volts on one terminal, that's all you should have. Two other wires should have continuity to ground. One will be reading through the fuel pump motor so be sure that is plugged in. The other one is a ground wire for the relay's coil.

The clinker with the fuel pump wire is that circuit also feeds all the injectors. The injectors will be turned off so those will be open circuits, but if you don't read to ground through the pump, suspect a bad ground on the fuel tank. That was real common on S-10 steel tanks when the trucks got rusty. To fix that, drill a hole in the tank's flange outside of the welded seam, then run a self-tapping screw in there. Attach a wire to it, and to the frame to repair the ground.

The fourth wire gets 12 volts from the Engine Computer to turn the relay on. That should occur for just one second after turning on the ignition switch. After that, the voltage should reappear during engine rotation, (cranking or running).
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Tuesday, March 10th, 2015 AT 6:19 PM
Tiny
TMHANDCO
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Sorry but your answers do not help. Don't think you understand there will be no power at the pump if no power at pin 85 and shows it is not grounded or powered. It seems to have lost its contact with the 12 volt buss that feeds power to all the fuses and relays at the fuse/relay box (under the hood.)
I think I may have to remove the fuse/relay box and inspect it to see why no power at pin 85 but power to the fuse. Thanks for you trying to help but I think this problem is not normal and may not have an answer in the manual.
Tom
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Wednesday, March 11th, 2015 AT 7:45 AM
Tiny
CARADIODOC
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Please refresh my memory as I've been gone since last night. Do you have 12 volts on one terminal in the relay socket? My diagram doesn't list terminal numbers so I don't know which function terminal 85 is. I only know just one terminal must have 12 volts all the time. If none have voltage, there is a blown fuse or you're right about the internal connection in the fuse box, but that would not be common. I've run into a lot of problems caused by the weak terminals for relays, so it stands to reason one for a fuse could be stretched too. Wiggling on it a little would identify that.

In your original post you stated, "All other pins show ground and pin 85 shows nothing". Does that mean all three pins had continuity to ground? If so, that is wrong. Only two should. The third terminal gets 12 volts from the Engine Computer to turn the relay on.

To be sure I'm not overlooking something, you DO have your negative meter probe on ground, right? There are some tests that require it to be on the positive post, but not here.
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Wednesday, March 11th, 2015 AT 6:26 PM
Tiny
TMHANDCO
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I corrected the problem. Loose connection inside the fuse/relay box.
Fuel pump still not working so have to get at the fuel pump but need to get a tow to where the box can be lifted to get at the fuel pump and check that relay. Will put 12 volts to the pump to make sure it's not the problem if the pump runs will change the relay. Hopefully its got a good connection.
Will check the plugin terms to be sure.
Thanks for your help.
Tom.
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Thursday, March 12th, 2015 AT 6:31 AM
Tiny
CARADIODOC
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Dandy. You found what is probably the least common cause.

Don't forget that if you have a steel gas tank, it is real common to lose the ground on it for the fuel pump. To identify that, ground a test light to the body or frame, touch the probe right to the tank, then have a helper turn on the ignition switch. If you see the test light turn on for about one second, run a ground wire from the tank's flange outside the seam weld to the frame.
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Friday, March 13th, 2015 AT 3:09 AM

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