Lift Pump power 94 3500HD ?

[Yukon6.2]

Hi All
I am bringing a 94 3500HD back to life...
Have zero history on the truck,it was unwanted and i had space in my yard.Had a rebuilt engine installed and a decent dump box so i saw value.
Got it running by supplying the lift pump with it's own power source.Starts and runs good while the battery is hooked to the lift pump.
In checking the wiring to the lift pump,i got power at the pump but no ground.
It has the old relay on the firewall for the lift pump which i tried a couple others.I tried to find the ground in the relay wires that went to the pump with no luck.
Then it was the end of the day.
Does anyone know where the ground for the pump comes from? my guess is the OPS fits in somewhere.
I did not check for ground as it was running.
Thanks Thomas

[Yukon6.2]

Hi All
So i did check the leads for the lift pump while it was running yesterday.
No ground while it's running,also no Positive while running.I was able to get positive power yesterday by jumping terminals on the plug for the relay.
Checked the fuel pump fuse on the fire wall no positive power there,or ground either.
Truck runs great while idling,no smoke on start up or while idling.

Then when i went to start it again it would not fire off,checked the shut off solenoid and it has power and is clicking.

It was time to head home so i put my stuff away and called it a day.

[Yukon6.2]

I finished off my winter job on Saturday,no more delivering heating fuel for at least the summer.
Today i get to try and make the 3500HD run again.
I will plug it in to warm the engine again and see if it will start.
If it still doesn't i will try a different FMD.
One step forward and two back.

[Yukon6.2]

Plugged in the truck and put the charger on the batteries
Hooked both batteries in tandem with my towtruck and two sets of booster cables and the 3500 started again with the lift pump on it's own power source.
While it was running i disconnected the power from the lift pump and there was no notable change in the engine while it was idling.It also revved with zero issues with the lift pump disconnected.
But it did not want to start again after sitting for about an hour.
I think it's the crap batteries i have in the 3500.And if they haven't been charged i believe they just drain the power down below the minimum required for the computer.

[a5150nut]

The 6.5 takes two strong batteries. My 94 left me stranded once with what felt and sounded like good batteries. Wouldn't even jump start. Two new batteries and it started right up. IIRC the 6.5 has to reach 200 rpm cranking before thew computer will call for fuel

[DmaxMaverick]

The 6.5 needs about 200 RPM's cranking speed, but I don't think the PCM requires it. If the starting system is so loaded it can't reach ~200 RPM's, by whatever means, it's very likely the 12V system load is dropping the available voltage below what's required to fire the injector driver. I recall a while back, a "newer" driver (Sol-D?) had starting issues if/when the line voltage dropped near 9-10V. I think other drivers are not as sensitive, but probably not far off. Weak batteries, even if able to crank fast with an otherwise healthy starting system, may not be able to maintain battery voltage enough to fire the drivers.

Trying to start without a working lift pump may cause other issues. If the IP is weak, it may not be able to develop pop pressure at lower cranking RPM's. A weak IP and the absense of supply pressure makes for a hard-starter or a non-starter under the best of conditions.

[Yukon6.2]

I have had the lift pump on it's own power source since the first start.The engine sound doesn't change once the lift pump is removed from the power source.
The no start for me was/has happened twice.
Today it's not raining so i will try a couple things to narrow the no start down.I am going to take the almost new set of batteries from my tow truck and install them in the 3500HD and see if that solves the no start.
Then it's just finding out why there is no negative in the lift pump connection.

[Yukon6.2]

Yesterday with good batteries it started OK the glow-plugs didn't cycle normally so i kept cycling the switch.
Idels good with or without the lift pump.
It will start right away once it's up to operating temp,if you shut it off and start it right away.If it sits for 30 miniutes it will not start till it's cooled down.
Tried cooling the PMD with snow which didn't help,it wouldn't start till the engine cooled for a while.Also tried to cool it with propane the next time for a more immediate cooling of the PMD. Nothing.
While it was running i checked the connector for the lift pump and it had no power.
Any ideas on what could be the cause of the no start while warm?
Or the no power to lift pump?
Could they two symptoms be from a common failure?
Thanks

[DmaxMaverick]

They don't sound related. You'll need a diagram to trace back the LP ground. If it simply grounds to chassis, that's simple enough to remedy locally (just ground the harness wherever it's convenient or practical). If it's switched, you'll need to know where. The 94 LP relies on the OPS for Batt+ while running (it should be relayed or failure becomes more frequent).

The no-start after semi-cool isn't uncommon. Some remedies include manual GP cycling (add a momentary switch and relay), or hacking the PCM temp sensor (resistor to lower the perceived temp). Next time it doesn't start after a partial cool-down, disconnect the temp sensor before key-on (PCM will see -40). That should force a maximum GP cycle. If it glows and starts, there's your problem. Also, verify all the GP's are actually glowing when power is applied. If a few are MIA, it can cause starting issues in the gray areas.

Starting and idling w/o a working LP isn't unusual, if the fuel system and IP is otherwise healthy. It will fall flat under load, though.

[Yukon6.2]

Yesterday
Started the truck not plugged in with good batteries everything worked good,temps were above freezing when i started it.Got to -6 C overnight.
Idled it for 1/2 hour or so,disconnected the battery from the lift pump and took it for a short drive.Basically a wide open throttle romp to see what happened.
Good power for a 6.5 in a HD truck no excessive smoke,no hesitation.
Parked it,shut it off and it fires right up.Left it for an hour,no start.Two hours no start.
I did try disconnecting the temp sensor on the thermostat crossover,no start.I did the continuity check on 6 plugs,but didn't remove them for the ultimate check.
There is no function on the relay for the fuel pump when the key is on,and no power at the fuse for the fuel pump.Both the fuse and relay are on the firewall.I did check to see if power will get to the lift pump by jumping the relay plug and the wire is good to the pump from the relay.
No SES lights at all.

[a5150nut]

There is no function on the relay for the fuel pump when the key is on,and no power at the fuse for the fuel pump.Both the fuse and relay are on the firewall.I did check to see if power will get to the lift pump by jumping the relay plug and the wire is good to the pump from the relay.
No SES lights at all.

IIRC relay powers lift pump for 10 seconds then switches to Oil pressure to keep it activated. So not to pump fuel when upside down, strapped in the seat and can't get away.

[Yukon6.2]

Yesterday kept me away from the truck
The day before...
Set up a oil pressure sensor with it's own pressure source to see if it would have any difference.
I found that triggering the purple wire on the solenoid would get power to the lift pump, same as keeping pressure on the OPS.With the both OPS unplugged from the system the gauges do weird things while running.The temp gauge will follow engine RPM's.
Still no ground at the lift pump.
Still refuses to start after sitting for about 15 minutes at operating temp.Unplugging the temp sensor changes nothing.
Going to have to find a wiring diagram somewhere.
And i think i need an extension harness and remote mount for the PMD and another PMD to rule out the one on the pump being the culprit on warm starts.
I will rob the one off my burb and see if that cures anything.
I feel the injection pump is OK as it will start and drive without a lift pump, and there is never ant indication when the lift pump is powered down.

[DmaxMaverick]

Back to basics. This is a chassis ground failure. It sounds like you have the systems making their own grounds, through other components. It doesn't take long before really bad things happen. Make sure both batteries are grounding to chassis, and every other ground point (engine, frame, body, etc.).

[Yukon6.2]

Good point...

[JohnC]

Rule #17: when two or more seemingly unrelated systems start interacting, check your grounds.

[Yukon6.2]

The normal grounds are no an issue anymore.
Cleaned them all up and added a couple.
There were 3 wires to the right rear manifold,added one to the right battery
Added one to the left which went to the frame
Nothing changed...
Should have remembered to attack them as soon as i found no ground at the lift pump.
When the truck is running there is nothing at the lift pump,no ground or positive.
I can trigger the fuel pump relay with power to the purple wire and get power to the lift pump.
Still will not start when it warm soaks after 15 minutes
Starts cold with or without the lift pump.
Maybe today i will have time to rob the extension harness and cooler from my Burb to rule out the PMD as the hot soak issue.

[Yukon6.2]

Hi All The saga continues
Yesterday i robbed the spare PMD on a cooler and extension from the Burb
The pmd is defiantly not the component that doesn't work after a heat soak.
Once at operating temp with both PMD's it will start if you shut it off and re start right away.Leave it sit for 15 minutes and nothing with either PMD.Unplugging the temp sensor makes no difference.
Wait 2 hours and 15 minutes and it fires right up with either PMD

What else could heat soak and not allow the truck to start?

Still no power or ground to the lift pump.When i use the second oil pressure sensor hooked to regulated air 20psi with oil in the fittings and OPS the lift pump will get power.Plus if i trigger the purple wire on the fuel pump relay it supplies power to the lift pump.This sensor max's out the oil pressure gauge even at low regulated pressures.
I am sure i could do some creative wiring to power the lift pump, but would rather fix whats stock.
Thanks for all the tips and directions to check so far.
Fingers crossed i find an issue to correct it.

[DmaxMaverick]

Back to the grounds. Batteries to frame and engine are important, but the entire chassis needs to be interconnected. It should have a large, bare bonding strap from a head or intake manifold to the firewall. This is very important to provide a ground to the cab and interior, including instrumentation, HVAC, ignition, and lighting. It should also have a bonding strap between the cab and/or frame and the bed (OEM or otherwise).

Have you confirmed operation of the OPS?

When you have a warm/hot soak no-start condition, have you tried to manually cycle the glow plugs? Many of the 6.5's won't start well without GP's when not at full operating temp. Is the WTS/GP lamp coming on during this time? If so, how long?

A tired IP is the next suspect, despite the cold water test results.

[Yukon6.2]

I added grounds and cleaned all points in the engine bay.Left battery ground large ground going to the engine then i added a large ground from battery to frame Right battery large ground to battery to engine, small ground to fender i added another small ground to the back stud on the manifold which had 3 wires going to it.Cleaned all points and soldered all rings to wires.I have a dead 94 in the back yard i compared the grounds to and i think they are all accounted for.Have robbed the fuel relay from it.
With the original OPS the gauge reads while it's running,but no power to the lift pump.
With the OPS and regulated 20 lbs pressure rig the lift pump gets power but the gauge pegs out to max.
I don't remember reading that there are different OPS,I will try a third one from a 94
In the warm/hot soak mode the glow plug light cycles normally
The IP is ok i believe as it will start and run, and drives wide open throttle with no hesitation.
When you shut it down at operating temp and restart right away the glow plugs cycle and it fires right up with or without the lift pump.
Nothing changes with or without the coolant sensor plugged in, and when changing PMD's
It's like there is a switch off once it's warm soaked.
Have been by myself which limits some tests.Going to try and rule out the fuel shut off with some help.I tried to remove it last week but was unable to that try.It has a recessed allen fitting instead of the nut on top .It was getting power when it doesn't start,maybe it's the culprit
I am assuming the IP was installed new with the new engine it had paint marks on the connections to the PMD and other points. It has a green tag on it which i think i read is the 94 IP.
Yesterday i spent no time on it,had another project.

[DmaxMaverick]

With the original OPS the gauge reads while it's running,but no power to the lift pump.
With the OPS and regulated 20 lbs pressure rig the lift pump gets power but the gauge pegs out to max.
I don't remember reading that there are different OPS,I will try a third one from a 94

The same OPS is used for 1992 - 2002, all models. It is dual function, a sender for the oil pressure gage, and a switch for powering the fuel lift pump. If both functions work, but only one at a time, the OPS is bad. The gage pegging when the lift pump gets power is an indicator. The two functions are independent, and only share the ground. If one function effects the other, the switch is failed. All this assuming the remaining associated electrical is correct. If you're still having power/ground issues, bets are off.

[Yukon6.2]

Yesterday
changed the OPS and now have power from it,but no ground at lift pump still.
I think the actual OPS should have been grounded to get proper gauge operation while it was on my test rig.
Brought up to operating temp.Unplugged the temp sensor while at operating temp and the engine changes RPM and sound,so that appears to be working.
Will restart warm without glow plugs or lift pump within a few minutes of shutdown
Wait 15 minutes and nothing.I can feel and hear the shutoff solenoid when at operating temp when i have the switch cycled or if the switch is on and i unplug it.
Head scratcher...
Is there any other electrical component that could heat soak while the ignition switch is off that would interrupt the IP function?
Thanks Again

[Yukon6.2]

Spent a couple days without being able to try anything on the truck
Yesterday i had it in a no start condition.I cooled the IP with liquid propane and it started up.
Wanted to replicate it when a new problem arose, bad connection on the starter system and it was just clicking.
Fingers crossed that fixing the loose connection may solve some issues.
Once i fix the loose connection i will replicate my test to see if i get the same results.
If i do i am assuming that i will have to change the IP, being a 94 i believe the options to replace the IP is limited to 94 year only.
Thanks all that have given me ideas to try.

[Yukon6.2]

I finished off a project that i had in my shop, and brought the 3500 in and put it on the hoist to clean up a few items.
I was able to trace the ground wire for the lift pump.
For some reason GM grounds the lift pump to somewhere at the back of the frame by the fuel tank.Soldered the broken wire, now the only issue is the no start after a heat soak.

[DmaxMaverick]

I finished off a project that i had in my shop, and brought the 3500 in and put it on the hoist to clean up a few items.
I was able to trace the ground wire for the lift pump.
For some reason GM grounds the lift pump to somewhere at the back of the frame by the fuel tank.Soldered the broken wire, now the only issue is the no start after a heat soak.

The ground location is universal with the gasser counterparts, with the fuel pump in the tanks. We like the Diesels, but the majority of models were not Diesel. They probably saved a million or three avoiding the engineering and tooling accommodations.

[Yukon6.2]

It ends at a bolt through the left rear frame rail.
My plan was the same but 6" from the pump
They could have saved 10 ft of wire with my change order that didn't go through.

[Robyn]

Reading over this issue......
Once the engine gets up to temperature EVERYTHING expands a little.....Including the Plunger bores in the Injection pump.

How old (Miles on the IP) ?????
When was the fuel filter last changed ????

A hard / no start on a warm engine can be related to NOT ENOUGH PRESSURE BEING MADE in the IP to pop the injectors

GET A GLOW PLUG ON THE DRIVERS SIDE OUT and spin the engine....see if fuel mist blows out......

If fuel blows out it should run....especially when warm.

NO fuel at glow plug hole..... WHY ?????
IP WORN OUT...
FUEL SHUTOFF SOLENOID FAILURE....

The fact that the engine will start from cold and run OK indicates that the systems SEEM to be functioning.....

An electronic components failure SHOULD show issues while the engine is running...

A short shut down should NOT cause an issue

My suspicion is the IP is just not making POP pressure....

The reference on the SOL D unit was correct.....The unit would not lite the fire when it was cold and line voltage dropped off....

Jump the rig with some big batteries and BOOM lite right off.

SOL D never addressed the issue and faded into the mists of time and 6.5 history.....


I have started a few 6.5 with low crank speed.....Computer does not care so much about speed.

Cranking speed has more to do with getting compression in the cylinders up .....And if the IP is worn ....a low speed will simply not get the pressure in the plungers up to pop level....


When it is warm and will not start....Yank a plug...roll it over and see wasssssup

[Robyn]

Another thing

These rigs have two points that the positive battery feed connects..

Early GMT400 trucks fed the RH battery to the starter and to the junction box on the firewall
The LH battery was connected to the alternator and to the cross connect cable between the batteries....

Later Rigs connected the RH battery to the starter ...The LH battery to the relay box on the driver inner fender....The alternator connects to the LH battery
Cross connect is similar.

THE GM BATTERY CABLES are crap.....Crimped connections on the side terminals.....IT IS VERY COMMON to see the cables corroded down in the strands of wire..

Same with the ground cables....JUST BECAUSE THE CABLE ENDS ARE CLEAN....Does NOT mean the circuit is good.

I have seen grounds on these that looked perfect....but when the cable was dissected were just nasty.

The cross connect cable is also suspect too.....You can have connection all the way.....but once the load is put to the system the voltage and amperage goes in the toilet

Check with the engine running and see what the voltage is at each battery.

14.5 volts is good.....It is common to see the LH battery (Alternator connected) at 14.5 and the RH battery at 12.5 or so.....

Used cables are suspect......They can't be examined without cutting the insulation jacket......I have seen the ground on one battery absolutely dead ZERO ZIP NAPA and the other one good....

CHECK THE VOLTAGE....The system voltage at the relay box lug should be 14.5 running....

There should be very similar voltages at each battery....Alternator lug and the main power lug in the relay box...

Any odd ducks are a sign that cables are to blame.....

The only cables that are close to satisfactory are ones that have the copper eye soldered onto the cables and all the strands in the solder.....
All crimped battery cables will corrode over time....and you can't see it...

[More Power]

When one of these engines won't start after sitting for 30+ minutes, it's usually due to one of a few items (not necessarily in any order):
1- Poor fuel atomization.
2- Slow cranking speed.
3- Low cylinder compression.
4- Injection timing.
5- Cam timing.
and maybe others... always begin your troubleshooting by eliminating the cheapest/easiest possibility first.

Generally, these engine will start normally without glow heat down to less than about 80 degrees F. As an aside... Sarah's 1994 6.5TD Blazer is running an aftermarket PCM chip. This chip glows the glow plugs hot or cold. It's either a Heath or Kennedy chip.

[Yukon6.2]

I am pretty convinced it must be the injection pump.
When it doesn't start i can cool the head of the pump with liquid propane for 20 seconds and it fires right up.
The rest of the health of the pump masks the heat soak.
It starts and runs without the lift-pump,drives good no hesitation on WOT, and no codes