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Can you give very specific part numbers and sources for the DIODES? And extremely specific instructions on how to put the wiring together?

It's all Greek to me and many others. Though I know it's really not that hard.

You don't need the 2 relay stuff just need to use the original relay. Just use 2 diodes and will be just fine. glow plug wire to one diode and cut the relay feeder in my 94 it's purple. hook it to the other diode. The 2 feeds go in on dual side and the other to the relay. it works just fine.













 
Will having the LP run during glow time help with starting or is this to mask fuel system problems? Reason I ask is because my 94 starts after 2 seconds of cranking like it should and the fuel system is up to snuff. Will I benefit from this mod? Opinions experience?
 
IMHO, the pre-prime feature is a bandaid for weak lift pumps (w/lazy check valves) and/or mask airleaks that often occur when the vehicle is parked for extended periods, overnight for example.

'Air-ports' are driven by atmospheric pressure, which is constantly assaulting the fuel supply system, relentlessly searching out any system breaches regardless if the system is in use or not..

Lots of physics but in a nutshell.. When not in use, the fuel supply system should remain sealed in order to maintain a slight positive pressure. However with time and use, the liftpumps one way check valves become worn and lazy and may not remain sealed as it should do when new..
As such, the liftpump will allow the fuel to drain back to the fuel tank but only if atmosphere (14psi) breach's the inoperative fuel system and displaces (replaces) the fuel thats otherwise would remain 'suspended' above the fuel tanks lowest fuel level.

Atmosphere is always trying to equalize the fuel's level to the lowest point.
When the fuel system is operating at minimal fuel pressures, the 'suction' line is subjected to an extremely low pressure zone created by the liftpump pressurizing the fuel supply system. Atmosphere, always trying to equalize the negitive pressure in the suction line pushes the fuel from the tank towards the liftpump. So in a sense, atmosphere pressurizes our fuel tanks during normal operations..
IF theres any weaknesses in the suction line, atmosphere will expose them creating an air leak.. Sometimes air leaks are only exposed during max fuel flow, when fuel pressure is at its lowest, many capable of opening and closing like a 'zipper'... Suction line is the worst for hiding 'zippers'..

IF theres any fuel restrictions (ex- clogged fuel strainer or FFM element, or kinked or crushed fuel line) causing fuel pressure to drop below 3psi at any time, especially during high fuel flow demands, it exposes the entire system to atmospheric assaults as well as entrained air too...

The DS4 IP is a serious binge drinker. Most often it sips the fuel but its capable of demanding instantaneous big gulps of fuel when engine suddenly loads up while your already cruising 75mph for ex.. Its during these MAX FUEL FLOW demands with minimal fuel pressure that atmospheric assaults are greatest..
 
I'd rather mask an air leak than be stuck on the road because I found out about it or developed the leak 300 miles from home.

I would not bother doing the lift pump relay upgrade without the prime feature
 
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I do have the ops relay for the LP done if thats what youre talking about. I wont waste time with this then now it makes more sense to me. Right now my fuel holds and truck pops off well even after sitting for a week or longer. Id rather leave it instead of possibly masking a future problem. Thanks guys
 
'Rather mask an air leak' is exactly what the PMD manufacturers want us to do. Worst possible thing you can do is mask an airleak, it only fuels future PMD tales err sales.

Even the most observant operators may not be aware theres an air leak or other fuel supply system deficiencies occurring. Fact is, often the operator remains completely oblivious, CLUELESS to the problem(s) as the show goes on without any obvious glitch(s).. This unknown undiagnosed condition could go on for days, weeks, months even before any physical signs or driveability symptoms surface and by then the damage to the PMD is already done, most likely.

Frankly, you want to know sooner rather than later if theres any lingering fuel supply deficiencies to avoid damaging the FS's poppet valve seats or its driver, the PMD....




Say, wheres your PMD. Best Buy two.. <sarcasm
 
Or it could even mask a tiny air leak that could be problematic for years. Sometimes it can mask a problem that has been ongoing and then last for years.
 
Pre-prime feature exist in 96 and later model.

All we do is to have that with 96 and before model.
So why is it masking for before 96 and not masking after 96?

Also, please don't tie it into the GP circuit.
In the summer, GP only glow for less than 2 sec and that is not enough to prime the LP. It just enough to energize but not pump.
It may be fine in the winter but even then the max is only around 6-8 sec unless the ECM has been reflashed. Just a thought.
 
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IIRC, the pre-prime feature was added to 6.5TD OBD2 models, starting in '96 or '97, for sure by '98.

Under perfect conditions, the pre-prime feature isnt necessary as I find my recently restored fuel system still pressurized after sitting unused for days.

But as the liftpump loses its ability to maintain a positive seal with normal use, it becomes important for the fuel supply system to remain positively sealed to prevent fuel draining back to the tank..

GMCorp anticipated undiagnosed air leaks and other fuel deficiencies were a serious threat so they added the pre-prime feature to the PCMs Startup Mode programming..

'START UP MODE' was given a whole sentence to describe it, so no wonder its largely misunderstood..

During startup mode, ie the ignition is first turned to the ON position (engine off), the PCM runs through a preprogrammed routine that includes taking note of ECT sensor and battery voltage to pre-determine the glowplugs pre and post glow times..
During startup mode, PCM initiates the lift pump prime feature via the liftpump relay that typically stays ON for 20 seconds. If the engine isnt started by then, the PCM turns off the liftpump to avoid draining the batteries.. Its during this pre-prime period that the PCM notes liftpump voltage or amperage draw. Once the engine starts, the OPS powers the liftpump while the PCM simply monitors voltage via the relay...

Like OBD1 model, the OBD2 model is flawed too.. During startup mode, the PCM notes LP voltage. IF the Liftpump is already struggling or in distress or already failing when the startup mode is iniatated, the PCM notes the skewed reading so it often wont set any DTCs or trigger SES lamp....


People suggest the OPS is redundant power source on OBD2 models, personally I am not convienced but I have no way of checking it out for myself...

But were digressing, this thread is about '94 models... :confuzeld
 
Pre-prime feature exist in 96 and later model.

All we do is to have that with 96 and before model.
So why is it masking for before 96 and not masking after 96?

Also, please don't tie it into the GP circuit.
In the summer, GP only glow for less than 2 sec and that is not enough to prime the LP. It just enough to energize but not pump.
It may be fine in the winter but even then the max is only around 6-8 sec unless the ECM has been reflashed. Just a thought.
Tying into the glow plug circuit has seemed to work fine on my vehicles.

I've never had an issue with the prime feature activated by the glow plugs.

As far as I know the lift pump still comes on when the Key is in the crank position.

What exactly is your fear of using the glow plug controller to trigger the relay for the prime feature?

My glow plugs run a lot longer than 10 seconds.

There might be a huge difference in glow plug time between Texas and Illinois
 
Under perfect conditions, the pre-prime feature isnt necessary as I find my recently restored fuel system still pressurized after sitting unused for days.
Got to agree with you on this one :thumb:

Put a new IP return regulator on for a few bucks and make sure your lift pump is up to snuff and you don't need pre-prime BS. It'll hold 3+ psi for DAYS, WEEKS, forever.

I ignored all the posts below about diodes and other stuff. 94 is stupid easy to do the relay mod on (added it to my signature so I never have to type it again). That's all you need because the more random wiring you add, the more problems you'll have later.
 
I never did any diodes either. I just used 2 relays.

That's good to not need the prime feature. But if you are on the road and suddenly have a leak issue it can save you some grief.

Do whatever. however you wish. I do the prime feature when I work on a 1995 or older.

I use only an AC Delco lift pump for a 1993 if using an oem style lift pump.

I'm definitely not against using an upgraded lift pump
 
I never did any diodes either. I just used 2 relays.

That's good to not need the prime feature. But if you are on the road and suddenly have a leak issue it can save you some grief.

Do whatever. however you wish. I do the prime feature when I work on a 1995 or older.

I use only an AC Delco lift pump for a 1993
All you need to do is open up the cover on the firewall and tap the fuse holder to one of the power lugs to run the lift pump. It's a 94 specific thing and still works that way after you do the relay mod. I suggest this way because it's only one splice and real easy to implement.
 
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i am against splicing. I do source my 12 volts from one of the studs on a 1994. I do install an inline fuse.

Run down to the lift pump plug to trigger the relay and back from the relay to the pump to run it.

It seems my 1989 was similar to the 94 for wiring the lift pump
 
'Rather mask an air leak' is exactly what the PMD manufacturers want us to do. Worst possible thing you can do is mask an airleak, it only fuels future PMD tales err sales.

Even the most observant operators may not be aware theres an air leak or other fuel supply system deficiencies occurring. Fact is, often the operator remains completely oblivious, CLUELESS to the problem(s) as the show goes on without any obvious glitch(s).. This unknown undiagnosed condition could go on for days, weeks, months even before any physical signs or driveability symptoms surface and by then the damage to the PMD is already done, most likely.

Frankly, you want to know sooner rather than later if theres any lingering fuel supply deficiencies to avoid damaging the FS's poppet valve seats or its driver, the PMD....




Say, wheres your PMD. Best Buy two.. <sarcasm
That's where the fuel pressure gauge comes into play
 
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All you need to do is open up the cover on the firewall and tap the fuse holder to one of the power lugs to run the lift pump. It's a 94 specific thing and still works that way after you do the relay mod. I suggest this way because it's only one splice and real easy to implement.
I don't splice. Make it plug and play.

The numbers for the connectors are here on the forums
 
Just curious what was the sudden motivation to reply to 8 ish year old thread? That seems to be happening a lot recently. Heavy Chev hasn't been online since around the time of this thread and Quadstar hasn't been online since 22 so its not like they had recent activity that prompted anything. I ain't saying its good or bad just curious.
 
It popped up on.my notifications for some reason and I never looked at the date.

I'm very under the weather right now.
 
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Very strange. Anyone familiar with the software have any theories?
 
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