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Discussion Starter #1
Hi everyone, I have a question about the 6.5 diesel lift pump circuit. According to what I have read, the fuel pump relay is supposed to supply the voltage to run the lift pump and the OPS is supposed to supply the voltage in case the FP relay stops working. So why not eliminate the OPS feed so it does not overheat or ruin the OPS. A light could be mounted in the dash that indicates when the lift pump is being supplied with voltage, and if the relay went bad the light would go out. Any thoughts? Thanks


Al
 

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The light modification is a good idea. :thumb:

Typical fix for 94 and 95 model years. I had to do it (94)

Search out "94 95 lp ops mod"

It does just as you say minus the indicator light. Utilizes a relay to take the lp amperage off the ops. The ops has a 2.2A max. The lp has a 5A max.
 

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Another good fix to the issue is a manual glow plug override. Tie the lift pump in directly to the gp relay. Key on, lp on. Use a single pole switch to ground the relay. Push a button and your glows come to life. Works excellent. Once the engine has warmed up once and ran for a while, glow plugs aren't required to start engine anymore for remainder of the day.

Saves glow plug lifespan.

:thumb:
 

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Back in Aug. of '93 GM issued a service bulletin: 93-286-6A I can print it out and post it if anybody wants it, but it can be looked up on the internet. It was about the oil pressure switch failure problem. There was a new oil pressure switch designed to fix it with heavier contacts for the LP. Part no. 10243574.
My OPS failed about 10 yrs. ago and I replaced it with the updated part and have had no problems since. The LP mod. mentioned here is a good way to go, I'm just mentioning this as the newer model of the OPS has worked fine for me for those 10 yrs.
 

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If your referring to the truck in your sig. The OPS in post 1996 6.5 engines passes no voltage directly to the LP at any time. Voltage to lift pump is supplied by the lift pump solenoid only. The OPS only controls voltage to the fuel pump solenoid closing the contacts in the solenoid to activate the LP. GM changed to a solenoid controlled LP in OBD2 trucks because of all the problems they were having in older 6.5s (OBD1) with the OPS passing full current to the LP burning out the OPS.
It is very simple to tell if the LP is working in a 0BD2 truck. Just turn the key to the on position without starting. You will hear the lift pump run for a short time then stop. If you don't hear the LP then something is wrong. No light needed.
 

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Thanks Tommygunner. With that change in the LP circuit, the updated switch wouldn't make any difference on a 2000 model.
 

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Discussion Starter #7
LP

Hey Tommygunner, I am new to using threads and I tried to attach a pdf of the lift pump circuit out of a chiltons manual. If it works maybe you could tell me if that is a possible wire schematic for a 2000 GMC truck. Thanks
 

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That is the LP control schematic for any 1996 and up 6.5td in a truck or burb.
The OPS switch was eliminated on the later model 6.5td vans.
The OPS sw. is in parallel with the relay, either one will power the LP, both when working correctly.
The main problem is the wire sizing, IMO it should be at least 14ga. and a separate 14ga. ground
to the engine instead of/in addition to the frame ground for the LP.
The LP can run and not pump fuel, and if dead it still has 12v. going to it
so an indicator light would not be much help.
With OBD2 you will get a P0231 code if there is less than about 8 volts
going to the LP.
 

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Discussion Starter #9
Good call Glagulator on the LP still having power but not pumping, so maybe a good idea would be to have a LP output pressure gauge to know that the LP is working.
 
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