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It never turns on in my truck during cranking not until I added the relay mod.

The 96+ that turns on during cranking to prime.
That is odd. On my 1994, I can see fuel pressure build on my gauge during cranking with the key in START. The gauge goes to 0 until oil pressure builds after starting the engine.

I also have an "idiot" light wired to show the LP is getting power. The light comes on during cranking, goes out after the key goes to ON after engine starting until oil pressure builds, then the light comes back on.

IIRC, 1996+ factory LP wiring powers the LP during the glow plug cycle, too.
 

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Someone mistaking crank for run :confuzeld


'92-'94 Lift Pump Circuit
The electrical circuit for the lift pump involves several main components (refer to Figure 8-25):
• The oil pressure switch/sending unit (threaded into a lubrication system passage at the rear of the cylinder case)
• A relay (mounted near the junction block on the vehicle cowl)
• An in-line fuse (also mounted near the junction block)
• The lift pump itself (mounted under the vehicle on the inside of the left framerail)
• Terminal G of the Assembly Line Data Link (ALDL) connector (vehicles equipped with the 4L80-E Transmission Control Module [TCM])

When the vehicle driver moves the ignition switch to the CRANK position, the lift pump circuit is completed through the relay contacts (refer to Figure 8-26, view A). During this time, oil pressure is building to the point of closing the contacts of the pressure switch/sending unit. A minimum of 28 kPa (4 psi) is required to close the switch contacts.

When the ignition switch is returned to the RUN position, the oil pressure of the running engine maintains electrical power to the lift pump (refer to Figure 8-26, view B). If engine oil pressure drops below 28 kPa (4 psi), the engine will run poorly or stall when the lift pump circuit opens.

Source- GM 6.5L Turbo Diesel DB2 Student Handbook. Feel free to download your own copy. Most educational and although NOT intended to be a substitute for the proper service manual(s), it contains alot of helpful troubleshooting tips and diagnostic procedures

Largely the same for 1995 models with a couple notable differences.
'95 model LPR and fuses were relocated to the UHFRC, and the DLC's 'Fuel Pump Test' terminal is 'F', not 'G'.

Also what isnt clear above and worth noting is that the OPS serves 2 very different functions, neither function dependent upon the other.
The oil pressure 'sender' portion can appear just fine while the lift pump 'switch' portion fails.

In '96 began the 'new and improved' OBD2 system. GM added a PRE-PRIME feature to the PCMs start up mode that activated the liftpump while the vehicle driver waited for the WTS lamp to extinguish before actually cranking and starting the vehicle. If after 20 seconds the vehicle wasnt started or failed to start, the lift pump would turn off to protect draining the batts unnecessarily until the ignition was cycled again. But lets not digress further and focus on OPs '93 technical difficulties..
 

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1995 GMC Suburban
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So, the 95 is probably the weakest of all the model year.
Sort of GM's attempt to fix it but never did.

Glad I did the relay mod, though.
I know HC does not like it but it has never give me any trouble starting except when the o-ring was leaking.
 

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The '95 model operation is practically identical. GM shuffled shat 'round to prepare for them OBD2 upgrades. Somewhat flawed but same as it ever was..


As for the OPS mod, I thinking 'bout it...... OK Im done.. :hehe:

Seriously.. If theres electrical gremlins, or the harness is damaged or hacked up by previous owner. Id think much harder about it, most likely.


If the PMD relocate kits temporarily mistreats minor fuel deficiencies.
Then the OPS mod is equivalent to LP viagra..:hehe:

Seriously, dont mind me.. Ya'l do whatever helps ya get your diesel on.. :coolnana::coolnana:
 

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I don't think there is fuel deficiency at least not in my truck.

There was a study here a long time ago on OPS, this was done before I was even a member.
Somebody actually measure the Amp or current needed to run the LP.
Essentially, the OPS cannot pass through the correct amp/current to run the LP RELIABLY.
Over time, the OPS becomes weaker due to overloading.

The relay mod is just directing the current using the relay as supposed to using the OPS. Sort of relieving the load of OPS.

I am sure you probably knows that also.
Whatever you want to call it but if it helps, it helps and there is no sense bashing it.

To make things worst, the aftermarket OPS does not even have half the capability of the ACDelco OPS to pass the current to the LP.
 

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Cant recall ever bashing the OPS mod but I do advocate performing all the proper diagnostics first to ensure your not putting new part(s) right back in harms way.

Want to add the mod, fine but make sure your not applying a bandaid to a growing problem..

Theres several youtube videos detailing exactly how to check for fuel pump voltage drop and amp draw.. Not hard really but does take some time so it too is routinely skipped, assumed good..

Restoring lift pump power via Relay Mod may quickly solve the repetitive OPS failure caused by excessive LP voltage, specifically AMPS.. Remember that cause Ill be returning back to that..

Lift pump mod does nothing to address a restricted fuel supply (clogged fuel strainer for ex) line, or patch an undiagnosed breach in the 'suction' line (heavy corrosion, fuel sender connection o-rings damaged for ex) that can cause a new liftpump to work that much harder and increasingly hotter. As the liftpump starves and/or struggles to keep up with DS4 binge drinking fuel demands or cant ever slow down (relax a bit) due to suction line breach, it quickly heats up and draws ever more AMPS that eventually exceed the OPS intended max rating of 5 amps and the OPS lift pump switch quickly fails..

But why would they do that, the liftpump relay circuit is already on a 20 amp fuse thats hot all times so what gives? Simple really..
The power source for the liftpump also powers the PCMs memory. Blowing that fuse wipes out all the PCMs learned memory.

GM intentionally designed the OPS to fail when lift pump amperage draw exceeded 5 amps, kind of like a non serviceable circuit breaker.
OPS failure was intended to be a sign, big red flag to the diesel tech indicating trouble may be brewing in the tank, not regarded as an annoying weak link its often mistaken for and always misrepresented as...

A HEALTHY OEM liftpump (ACDelco EP158 for ex) thats not in any distress should not exceed 3 amp draw so GM nearly double that spec allowing for some distress thats expected to occur with age and normal use..

Popular aftermarket liftpumps not applicable. The norm for our solenoid type lift pump is 1- 1.5 amps per 10psi read our system when all is healthy normally draws between 2- 2.5 amps.

FWIW My new lift pump operates right at 2 amps but I am continuing to monitor the s'ituation, more so as my pre-lift pump vacuum indicator starts registering as then Id expect the amp to increase.



Been busy testing and conducting on-going field experiments myself.
Stay tuned for the report.
 

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Discussion Starter #27
After a few days of driving around since the repair I can tell its quite a bit more responsive (throttle wise) and seems a bit peppier around town especially 1/3 throttle and less. Im having more fun driving this truck than my 11.
 

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I have what sounds like a similar issue as discussed in this thread, so perhaps it's best to post here ...


I have a '94 Georgie Boy RV that has a '93 or '94 6.5 turbo with mechanical fuel injection. From reading here, the LP should be on when the engine is running, but it does not come on. I can run it from an external 12V source and it runs nicely and produces a steady stream of fuel (I disconnected the fuel line to check). I measured the voltage at the fuel pump plug when the pump was disconnected with the engine running. I got 14V. But when I plug the pump back in and measure, from exposed terminals, it is only 8.8V. From a bit of reading I just did, does this suggest a bad OPS that can no longer handle the load of the LP?


Interestingly, I noticed a distinct change in engine note even at idle when I jump the LP to the external 12V source while the engine is running.


As part of the troubleshooting for this, it sounds like I should be checking the LP relay, but I am not sure if my RV fuse panel is the same as a regular GM one (my chassis is a Spartan). If I should be checking the relay, perhaps a pic of my fuse panel would tell if mine is like a GM one?


Scott
 

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Ya who knows how they did the lift pump. I doubt it was the same as GM did but they could have taken the bad design cue from GM and run it directly through the oil pressure switch. Who knows if there is a relay for starting or not. Do you know what kind of transmission this has? Any signs that there is any sort of computer? Like a check engine light? Just thinking if its a 4L80e like the trucks had maybe its got mostly a truck harness and maybe the fuses?

A bad ground could cause the voltage drop you are reading. Maybe try finding another ground at least for testing?
 

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Low voltage at lift pump.

I have what sounds like a similar issue as discussed in this thread, so perhaps it's best to post here ...


I have a '94 Georgie Boy RV that has a '93 or '94 6.5 turbo with mechanical fuel injection. From reading here, the LP should be on when the engine is running, but it does not come on. I can run it from an external 12V source and it runs nicely and produces a steady stream of fuel (I disconnected the fuel line to check). I measured the voltage at the fuel pump plug when the pump was disconnected with the engine running. I got 14V. But when I plug the pump back in and measure, from exposed terminals, it is only 8.8V. From a bit of reading I just did, does this suggest a bad OPS that can no longer handle the load of the LP?


Interestingly, I noticed a distinct change in engine note even at idle when I jump the LP to the external 12V source while the engine is running.


As part of the troubleshooting for this, it sounds like I should be checking the LP relay, but I am not sure if my RV fuse panel is the same as a regular GM one (my chassis is a Spartan). If I should be checking the relay, perhaps a pic of my fuse panel would tell if mine is like a GM one?


Scott

Scott, I'll try to copy/paste the GM service bulletin that I believe addresses your problem. You've got exactly what I had on my '93 C2500, but I was down to 7 volts at the lift pump with it connected. I put in the later OPS part no. about 10 yrs. ago and have had no problems since. I'm not saying the relay mod. that the guys here like to do is a bad idea, but the heavier duty contacts in the GM modified OPS have worked for me. Hope this helps.


Number: 93-286-6A
Section: 6A
Date: AUGUST 1993
Corporate Bulletin No.: 276101R
ASE No.: A1, A8
Subject:
STALLING/HESITATION/LOW POWER/WHITE SMOKE (CHECK LIFT PUMP/CONDITION: NEW OIL PRESSURE SENDER/SWITCH AVAILABLE)
Model and Year:
1992-93 C/K TRUCKS WITH 6.5L TURBO DIESEL
(RPO L65) OR 6.2L DIESEL (RPOS LH6 AND LL4)
THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 92-309-6A, DATED OCTOBER 1992. THE 1993 MODEL YEAR HAS BEEN ADDED AS WELL AS A NEW OIL PRESSURE SWITCH PART NUMBER AND FURTHER INFORMATION TO THE CORRECTION SECTION. ALL COPIES OF 93-286-6A SHOULD BE DISCARDED.
CONDITION:
Any one or combination of the following:-Low power (continuous or intermittent)-Occasional stalling or hesitation on cold start-White smoke at high engine speed
Above conditions may occur on some 6.5L turbocharged (VIN F) and 6.2L naturally aspirated (VINs C and J) diesel-powered vehicles.
CAUSE:
Improper electric lift pump operation, due to an inoperative lift pump electrical circuit OR a faulty pump. THE MOST PROBABLE CAUSE IS A FAULT IN THE OIL PRESSURE SENDER/SWITCH CIRCUIT which operates the lift pump during engine operation. The oil pressure sender/switch is located on the left bank, rear of the intake manifold. A new oil pressure sender/switch, P/N 10243574, will be available from GMSPO August 30, 1993. This new part provides increased switch contact durability, and was introduced into vehicle production late in the 1993 model year.
CORRECTION:
Check the operation of the lift pump first. It is located under the vehicle on the driver's side frame rail. Lift pump operation can be checked by feeling its pumping action while the engine is idling. You should feel the rapid end-to-end oscillation of the hollow plunger within the pump. Care should be taken not to confuse this oscillating with normal vehicle vibrations. If the pump is not operating, be sure to check the pump's electrical supply circuits along with the actual condition of the pump.




The two circuits powering the lift pump are shown in Figure 1; both circuits need to be checked. Intermittent supply circuit problems may require monitoring the voltage at the lift pump during vehicle operation. This can be accomplished on vehicles equipped with Data Link Connectors (DLC) by probing for voltage at terminal G while the engine is running (charging system functioning properly and batteries fully charged). A difference of more than 1.5 volts between terminal G and battery voltage indicates a problem in the oil pressure switch circuit.
Once diagnosed, the problem is resolved by either replacing the faulty oil pressure sender/switch, the faulty lift pump, or repairing the wiring.


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Thanks 57diesel and 56pan for sharing your thoughts.
 

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Thank you 57diesel and 56pan for sharing your thoughts.






The trans in my RV is an Allison AT542. I'll check the ground again - that's a good point. I wasn't able to trace when I looked before, but I found a common ground mount in the front of the rig, and I loosened it and jiggled all the terminals attached there to hopefully remove any corrosion that might be there.


I'll post a picture of the fuse panel I found under the dash to see if anyone recognizes it. Is there supposed to be another fuse panel near the engine? I think I've seen some references to that.
 

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Suspect you'll find the LP Relay hanging on a bulkhead in the engine compartment.
If you find a faulty OPS, check LP amperage draw immediately afterwards to ensure your not putting the new OPS in harms way...

Enter LPR Mod salesman here but buyer beware. The OPS was designed to fail when loads exceed 5 amps. Thats nearly double amperage necessary for an unhampered unrestricted healthy OEM lift pump

A clogged fuel strainer, restricted and/or leaking supply (suction) line will cause the LP to struggle and fail provided the OPS doesnt fail first, just as it was designed too, repeatedly if left unchecked assumed good. I learned long ago, DO NOT ASSUME NOTHING with the 6.5TD. Not as bad with DB models but the mechanical IP also assumes the rest of the fuel supply system is all good..

Its more important for DB pumps to maintain minimum fuel pressure to the IP. IIRC, its 3psi same as DS pump. Unlike DS pumps, DB pump mechanical advance mechanism is greatly affected by IP inlet fuel pressure or the lack of as you've noted by the change in engine idle, tone and/or performance when LP is working properly vs not..
 

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It's hard to imagine that engineers intended a sensor to perform the function of a fuse. If the OPS is designed to fail, why did GM update to using a relay in later years? :think:
 

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It's hard to imagine that engineers intended a sensor to perform the function of a fuse. If the OPS is designed to fail, why did GM update to using a relay in later years? :think:
Its hard to imagine why engineers do what they do, but there was some method to the madness..

The OPS is not a sensor, its an Oil Pressure Sender / Switch. Sensors typically provide 'input data' to the vehicles PCM/ECM/TCM. I know its semantics but...

The government mandated OBD2 upgrade in '96 included some changes to the lift pump relay circuitry however the lift pump 'relay' has always been part of the circuit since the 6.5TD debut in 1992.
Note that on '94-'95 OBD1 models, the lift pump relay circuit is powered by a 20amp fuse (B+), shared with PCM 'learned' memory. Blow that fuse, wipes out said memory. Enter OPS switch's 5 amp spec. Simple really, only this safegaurd is often misdiagnosed and misrepresented as a 'FLAW' or 'WEAK LINK'. HARDLY, the flaw in the engineers thinking was expecting mechanics to do there jobs well. The 'weakest link' of the 6.5TD be it DB2, DS4, OBD1 or OBD2 isnt the OPS and certainly not the PMD.... Its the lack of properly trained competent diesel technicians who are qualified to perform more than just change the oil.
 
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