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Lift Pump Problem

In OBD-IIs, with OPS unplugged lift will continue to run, been there done that just test the theory if it's really needed or not, test condutcted at GMCTD's place both of us learing the ins & outs of the lift powering routine of a OBD-II, unplugged the PCM feed lift still runs, unplug both or the common gnd and lift loses power.

See, here's where my theory kicks in... if the OPS is plugged in, but dead, will power flow through that side? When the contacts fry in aftermarket OPS modules, this seems to keep coming up. Along with blown fuses, on occasion.

Sure would be nice to get a good handle on this - I know they are SUPPOSED to be ECM-controlled, but then, why are they (OPS) still there? Too many anomalies...
 
It is a back up Jim, I never seen the write up on it, but my theory is that GM in OBD-II wanted power all time to both except with key off or no oil pressure, as I think an attempt to make sure IP mounted PMD does not die via overheat from limited flow in the IP with OPS only power source, as we know a IP can pull its own fuel thru a dead lift but that does supply IP with full fuel required for PMD heat rejection.

The parrallel circuits splits electrical load, primary power on is thru PCM/fuel relay, which IIRC is how we killed PCM controlled power in our test, and OPS is independent of the relay, we did not take good notes I'm afraid and CRS kicking in :eek: I guess I'll have to go duplicate the test again.
 
"I guess I'll have to go duplicate the test again."

I'm glad I'm far away from the Doc,, he's looking for Somebodies OBDII to tinker with!! =)
 
Okay that was easy, unplugged OPS lift came on with key on, plugged in OPS, removed fuel relay, key on no lift, cranked & started engine, got oil pressure then lift pressure, removed both and no lift pressure, I don't know how it's wired where the pararallel portion of power goes to series, but so long as you have one or other in OBD-II the lift does get power.
 
That I don't know, guess you'd have to check your setup maybe the dual power came into being at same time theft deterrent came into play, that was last major revision to power train management software.
 
Okay that was easy, unplugged OPS lift came on with key on, plugged in OPS, removed fuel relay, key on no lift, cranked & started engine, got oil pressure then lift pressure, removed both and no lift pressure, I don't know how it's wired where the pararallel portion of power goes to series, but so long as you have one or other in OBD-II the lift does get power.

Yep... one or the other. But then WHY the OPS? If the oil pressure drops, because of the parallel system, the LP would still get power. But it doesn't... when there is no oil pressure, the OPS should kill the circuit.

What I'm thinking is that with a fried OPS (yours are both good), it mimics the 'no oil pressure' condition.

Or maybe I'm just nuts... like I told AK, this condition keeps appearing when aftermarket OPS switches die. And I still don't have a good reason why, so I'm grasping...
 
Jim you missed my theory, IMO both are there to insure PMD always has power even with a bad OPS, easier to add PCM control than to remove the OPS control, so they built in redundancy, I think GM & Stanadyne knew the marginal design of the PMD, and marginal heat rejection/transfer via IP so wanted to make sure loss of lift flow was not contributing to the problem.

PCM in accident would secure list via loss of engine rpm signal (not sure if it would be loss of CPS or rotation of optic), and OPS would secure power in loss of oil pressure; even OBD-IIs still have running lift after ignition is switched off until oil pressure bleeds off like some OBD-I trucks do.
 
So my ? is on TDs 98 they must have done away with ops as a safety device?

Nope see above post 30, it won't shut down on loss of OPS alone, has to lose both, will stay running with OPS only though.

I think they just added to the basic OBD-I logic if rpm & oil pressure are present, energize the lift pump,

With key on and no ops but reading crank speed or optic I'm still not sure of the logic interplay, PCM keeps lift energized, making sure lift is always getting power so long as the engine is running. At 1st I used to think a control handoff PCM energizes lift at start, then hands over authority to OPS, but if that were the case the OPS unplugged the lift would stop, it doesn't.
 
No, I got the theory, but it only works as an 'ON' thing, not as an 'OFF' thing, or else I just can't figure out how to draw the schematic. If either can shut down fuel flow, then the fried OPS theory works. But not if they are parallel. OTOH, if either one can power the thing, then neither can shut it down, if they are parallel.

I'm just not smart enough for this... arrgghh!
 
AKDD

Kinda sort of, if PCM control/relay power isn't working then OPS would be operating alone, and then be the safety device again, both operate independently, at least from observations when plugged in and not plugged in independently, the GM training manula, nor the factory manual explain the operation of it very well which is what inquiring minds like me & GMCTD to run the experiment with his GM T2 hooked up so we could see if there was anything that would flag via the diagnostics, it didn't so I'm not exaclty sure how it works, but unplug one or other and lift pump stays running, unplug both and lift dies.

With OPS only though the lift does not come on until oil pressure builds to the setpoint level of the OPS
 
Damn. Another perfectly good theory shot down...

Oh well. I still bet that Steve's issue is fried OPS.
 
No, I got the theory, but it only works as an 'ON' thing, not as an 'OFF' thing, or else I just can't figure out how to draw the schematic. If either can shut down fuel flow, then the fried OPS theory works. But not if they are parallel. OTOH, if either one can power the thing, then neither can shut it down, if they are parallel.

I'm just not smart enough for this... arrgghh!

May be parallel all way thru, split out in the PCM to think as a OBD-I while running thru the OPS, and no relay, and 2nd path as OBD-II thru the realy, maybe that is the kicker in OBD-I doesn't lift power go OPS then relay then lift?

And if in OBD-II its parallel running OPS into lift direct, may be why a non OEM OPS causes so much grief even in OBD-IIs, but I think common denominator is ground either system which being dirty would cause excessive draw on a relay or a OPS controlled circuit, or maybe I'm just overthinking it :mad2:
 
Sort of a chicken or egg Jim, when PCM shuts down truck you get no fuel so eventually no oil pressure and no closure of OPS contact keeping lift running I'm not sure which is the primary method of keeping lift energized, since both routines will keep the lift powered without the other being active
 
The power to the LP is purely parallel, the PCM would not know if the OPS is working or not. I have the schematic attached, no need to hypothesize ;).

I would expect the ECM B fuse to make more stuff not work, but that fuse will prevent the LP from running. The PCM would know if there was no power going to the LP, and likely give you a SES light with P0231. So the break would probably be on the ground side or after the PCM feedback tie in.

Like TD said, likely the ground is broken or something like that.
 

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