Question about 1999 GMC Yukon

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Where do I measure the voltage to the fuel pump? I parked the car, and the next morning it would not start, until I added gas to the throttle body -

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I measure it right at the connector at the fuel pump at the tank. Problem is this is pretty tough to get to in most cases so you have to be pretty flexible to reach up and get the connector disconnected from the pump module plus you'll only have power to the connector for about 2 seconds each time the key is cycled "on" so you'll need help when testing it. I would have the actual fuel pressure tested before heading to the tank as its actually quite rare to be low on voltage back there [I've only found about 3 with bad wiring in several years] I see a lot of pumps get weak or fail on these at about the 120K mark so if you are close to that mileage this could be your problem. If the fuel pressure is good however you probably have another issue that's starving it for fuel.

Posted on Jul 14, 2010

Testimonial: "Thanx for the info - I was thinking of measuring the voltage post relay (Fuse Relay Center), as the relay "clicks", but I was wondering if the contacts were "dirty" (high impedance) - Any thoughts?"

  • mclaen Jul 14, 2010

    I've yet to have to replace a relay in one of these but when in doubt I usually just swap the fuel pump relay with another identical one in the fuse/relay junction box. The test I use to see if I'm getting full 12 volt power with enough amps behind it to run the pump is to connect a 12v sealed beam headlamp between the pump power feed wire [usually gray] and the ground wire at the pump connector at the tank. If the light comes on nice and bright for those 2 seconds when the key is turned on then you know you have sufficient power and good enough circuits to run the pump. If the relay will click but you don't have any fuel coming out of the fuel line at the fuel rail then odds are your pump has quit. It's my personal preference but I only use OEM pumps in everything I fix as I've had the best luck with them. Good luck!

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Try this tips:
1. Check the battery voltage at the battery with the digital volt/ohm meter. Set the meter to "Volts D/C" and touch the black meter lead to the negative battery post and the red meter lead to the positive post. A fully charged battery will measure 12.7 volts or more. A measurement of 12.5 volts is considered discharged, and the battery should be charged before any further testing. Any measurement below 11 volts indicates a severely discharged battery--the battery is most likely damaged and should be replaced. If the battery measures above 12.7 volts, proceed to the next step.

2. Check the starter for battery voltage by touching the red (positive) meter lead to the battery cable terminal on the back of the starter and the black (negative) lead to the chassis. The voltage reading should be approximately the same as the measurement at the battery. If the reading is lower than battery voltage, check for bad connections at the cables on the battery and the starter or ground cable.

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