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Old 01-15-2019, 11:04 PM   #8
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Join Date: Jul 2014
Location: North-central Virginia
Posts: 353
Re: GMT400 Wiring Woes

Originally Posted by Andy4639 View Post
Most late models run a oil pressure switch to the fuel pump so if loss of oil pressure the fuel will shut off before engine damage.
I don't know about the later ones, but the early tbi's(87-ish) used the oil switch a bit differently. If I read the diagrams correctly, the oil switch was simply a second path to the fuel pump.

The fuel pump would be energized through the fuel pump relay, but if that quit, it could also be energized through the oil switch. Thus, a customer might complain that he had to crank his truck for quite a while before it would start. This would indicate the relay had burned out, and simply needed replacing.

Since the oil switch was in parallel with the relay, and not in series, it could open from lack of oil and the engine would continue to run on the relay.

Evidently GM didn't trust their relays...

Edit to add: The "diagram 3" posted above agrees with me on this. Look closely at the oil pressure/fuel pump sensor and the fuel pump relay. You'll notice they are in series.

Also, you likely don't have that hot fuel module mentioned. That was a work-around for heavy duty trucks or big blocks. These were more susceptible to vapor lock due to underhood temperatures, and needed to keep the fuel pump running longer than usual in a hot-start situation to expel the vapor lock.

Last edited by kipps; 01-15-2019 at 11:16 PM.
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