91 s10 4.3 no power to fuel pump

pole71 wrote: Since you have power on A6, ,B1, and C16, but get no power on the A1(g/w wire) when the Key is first turned on, or while cranking, I believe this is pointing to a bad ECM.

As a final check, I would verify good grounds on A12 and D1.

Does anyone have a second opinion?

Ok I will check ground. Thanks

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Maybe check voltage on grounds if any while your there

chief eaglebear wrote: Maybe check voltage on grounds if any while your there

How do I do that?

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To check your grounds for voltage, put the Negative lead of your multi-meter on the negative battery post and positive lead on the ground wire. You want to see LESS than 100 Milli-volts while the circuit is loaded (in use).

pole71 wrote: To check your grounds for voltage, put the Negative lead of your multi-meter on the negative battery post and positive lead on the ground wire. You want to see LESS than 100 Milli-volts while the circuit is loaded (in use).

Ok thanks I will check that.

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gregster wrote:

pole71 wrote: To check your grounds for voltage, put the Negative lead of your multi-meter on the negative battery post and positive lead on the ground wire. You want to see LESS than 100 Milli-volts while the circuit is loaded (in use).

Ok thanks I will check that.

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Good morning, I checked grounds they were around 12mv I replaced ecm to no avail. The reading is the same at grn/wht wire 11.7 then 0.07 when relay is plugged in. How can bias voltage trip the relay? I just don't understand that. Thanks

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Dyllan.Love334 wrote:

gregster wrote: Hi with key on the ecm green white wire has 11.70 volts w/o relay. With relay same wire is only reading 0.07 volts. Black and white wire is good ground.fuel pump works when org and grey wire is jumped. Any ideas?

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Have you done an ohm test on the control side and the power side of the relay?

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Hi, I did put power to it with battery from cordless drill. It engaged relay. Then I check ohms it read zero resistance. Thanks

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Tyler wrote: pole71, I'm really not trying to be a jerk here. :blush: But, don't the GM fuel pump relay control circuits work this way by design? There's a bias voltage present on the dark green/white wire, as a circuit check on the control side of the relay.

This video is what I'm referring to:

Or, is this old news? 'Cause I'm sorry if it is! :silly:

Will the dark green/white wire light a test light during cranking? Or, during the key on fuel pump prime.

Hi, how does the relay get trip by bias voltage?it doesn't make sense that it can't light a test light, but could trip relay. Thanks

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gregster wrote: how does the relay get trip by bias voltage?it doesn't make sense that it can't light a test light, but could trip relay. Thanks

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The bias voltage is not direct battery voltage. There is a resistor in the circuit that GREATLY reduces current flow. This voltage is there all the time (when the key is on). This voltage does NOT activate the relay. It is there for diagnostic purposes. When the relay is inserted, it pulls this voltage to ground, just like the test light does. When it is time to activate the relay, A NON-resisted voltage is applied. THIS is the voltage that activates the relay.

I will try to find a video that illustrates this better than I can explain.

This is a free ScannerDanner Premium video that addresses bias voltage for an oxygen sensor. I would recommend taking advantage, if you haven't already, of the 14 day free trial of ScannerDanner Premium. There is a wealth of information waiting for you.

So I replaced ecm and have same issue so how do I get non resistant voltage? Or test That? Could it be in module? Thanks for the help

The ECM supplies the power to activate the relay on the GRN/WHT wire.






Turn the key off. Remove the relay. With a test light connected to battery negative, touch the terminal of the GRN/WHT wire. The light will not be lit. Now, turn the key to the ON/RUN position. The test light should light for 2 seconds.

pole71 wrote: The ECM supplies the power to activate the relay on the GRN/WHT wire.

Turn the key off. Remove the relay. With a test light connected to battery negative, touch the terminal of the GRN/WHT wire. The light will not be lit. Now, turn the key to the ON/RUN position. The test light should light for 2 seconds.

Ok I used test light as explained and no light when key is turned on. Test light works with org wire but nothing on grn/wht wire right when keys is turn on. So I'm not thinking there should be bias voltage from what the info you gave me states. Appreciate the help, any other ideas?

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