Fuel Pump Relay Hot

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Sep 14, 2011 | 11:20 AM

Andy1

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From: SoCal

Fuel Pump Relay Hot

I just realized that when I wired my fuel pump relay, the switched +12V (not the signal, but the hot to the relay coil [86]) is wired to key on, power off durring cranking. It has always started just fine, but I'm wondering if the stock Fbody circuit runs the fuel pump relay +12V when key on, power on durring cranking?

Andy1

Sep 14, 2011 | 07:30 PM

S10xGN

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From: Port Neches, TX

Quote:

Originally Posted by Andy1

Without knowing anything about your engine or PCM or what body it's in or what wiring you have... Most PCM's send a 2 second pulse to the FP relay when the key is turned to "on". It then shuts off the pulse and awaits a start condition before re-energizing it.

Sep 15, 2011 | 11:17 AM

Andy1

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From: SoCal

The reason for my question is that in the current scheme, there is no power to the fuel pump durring cranking; power is only in run. Yes, I'm aware of the 2 second pulse (though I always heard it was 3 seconds). I just was wondering if the Fbody has power durring cranking. This is a 914/LS1 swap; motor is '01 LS1. The change to make it so that there's power durring cranking is pretty simple.

Andy1

Sep 15, 2011 | 02:10 PM

bczee

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From: Concord, CA

I would wire the relay hot term (#86) to a fused +12v battery source and not switch Ign. That well leave switched power from the IGN to control the relay. That way as the pcm sends a 2 second activation, the relay will have +12v to prime the fuel system.

BC

Sep 15, 2011 | 05:13 PM

chrisfrost

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From: phx the cactus patch

No , none of the GM's have power during cranking unless You turn the key on and as quick as possible turn it to start . Turn You're key to on don't start and listen to the fuel pump run and turn off . It won't turn back on until the engine is started .

Sep 15, 2011 | 11:22 PM

busta9876

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From: West Plains, Missouri

As mentioned above, feed the load side (switching circuit) of your fuel pump relay with 12v+ directly from HOT AT ALL TIMES (fuse protected) This takes the current draw of the fuel pump away from your ignition switch. You should do the same with any FAN relays as well.

The PCM supplies 12v+ to trip the relay coil for the fuel pump. Make sure other side of relay coil is GROUNDED. Keep in mind ANY other devices the PCM controls, like fan relays, the PCM supplies GROUND for relay control.

The fuel pump is energized for 2 seconds or so on initial KEY ON, then is commanded OFF until there are ignition reference pulses received from the crankshaft position sensor. (You can just bump the starter and fuel pump will activate again for 2 seconds and stop) This means as soon as you start cranking again, the fuel pump will activate. (this also serves as a diagnostic aid for a no start condition. If fuel pump activates when cranking, you know the PCM is reading the crank sensor)

Good Luck

-Brendan.