Hi, my neighbors got a 94 with a 6.5 td in it. The fuel pump relay doesn't click there is no juice going threw the 12v fuel pump fuse. If I run a hot 12v to the fuse then the pump runs and truck runs. I even had the truck running and unplugged the fuel pump and it still runs. It wont start normally til after i jumped it and the pump ran. It still starts and idles but fuel pump isnt running at all. He said it hesitated and stumbled the other day and then at work he said it wouldn't start then after a long time it finally started. I noticed the 5 amp crk fuse was blown. I checked the voltage at the fuse and only 12.6 v reads while its being cranked. I was wondering if the fuse that keeps blowing is related to the fuel pump not running. He said he replaced the fuel module, "looks like battery wire runs to it on the back of the motor close to the fire wall" He said he replaced it a few years ago. Any input to what the prob is?
thanks
keith
The '94 only initially powers the Lift (fuel) Pump during cranking, not with only the key ON. After engine start the 12v to the lift pump comes from a circuit through the Oil Pressure Sensor. If the pump doesn't run after the engine starts it's probably a bad OPS circuit. The OPS will still read oil pressure, just not power the LP. Many chose to put the LP on a separate 12v key ON relay for more reliable operation. ~FH
I was actually planning on wiring the lift pump on a seperate relay, but what would shut it off once the engine starts? I was thinking on using a ignition trigger wire for the relay but what would make it shut down. Also... A guy told me from carquest that the lift pump runs all the time otherwise the injection pump will starve and take it out eventually. Is he wrong then?
thanks
keith
It will put a lot of extra work on the IP to run without the Lp, yes. And it can contribute to early IP failure. On a '94 truck that may not be so far away.
There's a forum Stickie on the LP relay mod somewhere. Do a search. Essentially, what you want is to locate a circuit that is hot with the key ON to be the power source for your LP relay. NOTE: the reason the OEM configuration runs through the OPS is for a safety shutdown of the fuel pump after an accident so fuel does not keep pumping. The relay mod will circumvent that. ~FH
It will put a lot of extra work on the IP to run without the Lp, yes. And it can contribute to early IP failure. On a '94 truck that may not be so far away.
There's a forum Stickie on the LP relay mod somewhere. Do a search. Essentially, what you want is to locate a circuit that is hot with the key ON to be the power source for your LP relay. NOTE: the reason the OEM configuration runs through the OPS is for a safety shutdown of the fuel pump after an accident so fuel does not keep pumping. The relay mod will circumvent that. ~FH
I did the double lift pump relay mod on the 94 & 95's.
The lift pump runs with the glow plugs and then with OPS. Buddy has an excellant schematic.
I prefer using a relay socket as apposed to plugging the wires to the relay.
You can read 12 volts from the OPS with the truck running and still have a bad OPS. It will sometimes put forth readable voltage but not enough amps to run the lift pump.
If you have voltage and do the lift pump relay upgrade you can often times get away without changing the OPS. After the update the OPS only has to put out enough to trigger the relay.
The object of the relay mod is to take the operation of the LP away from the OPS, so you will not get LP shut down when the OPS LP circuit fails, as it surely will at some point. Following an accident with enough impact to stop the engine, lack of oil pressure is OEM designed to shut off the LP. With the independent relay mod as long as the key is in the ON position the LP is powered.
Putting an additional key ON relay in place of the pump-while-cranking function still puts you at the mercy of the high OPS failure rate since 12v still goes to the LP through the OPS after engine start. ~FH
The relay wiring in the faq uses the OPS to activate the relay and as such retains the LP shutdown in the event of lost oil pressure.
Since the OPS circuit is only for activation it sees very little load and should last for many years,provided you start with an AC Delco OPS.
just a random question, how would you test to see if its the lift pump that is bad? Ive heard to open the t valve and see if you get fuel out and if it dies then your pump isnt working. Its hard to tell on the 94 since you cant listen to it prime.
just a random question, how would you test to see if its the lift pump that is bad? Ive heard to open the t valve and see if you get fuel out and if it dies then your pump isnt working. Its hard to tell on the 94 since you cant listen to it prime.
To test the lift pump - hot wire it. You can try at the lift pump fuse first. One side of the fuse goes directly to the lift pump.
If that doesn't work, try to jumper directly to the lift pump. It's always possible that there is a broken wire.
I usuall run about 1/2 to 3/4 gallon out the drain into a gallpn pckle jar.
I leave it sit for about 20 minutes or so and look for water bubbles and crud on the bottom.
One bad tank of fuel can negate the good of a recently changed fuel filter.
Never trust a recently changed filter to be good.
I also use a 12 volt inline fuel pump to suck the crud out of the boottom of the fuel filter housing. If necessary I use Sea Foam to clean it up before reinstalling a filter
ok, well I replaced the ops and fuel pump relay. I checked the voltage at the fuse and it only reads while engine is running. I also went under truck and used a multimeter on the 2 terminals going to the lp and even with the glow plugs on it didn't read anything, not even while cranking. It did read 12.6 or so when engine started. So the lp does run but not before or during cranking process just after it starts.
ok, well I replaced the ops and fuel pump relay. I checked the voltage at the fuse and it only reads while engine is running. I also went under truck and used a multimeter on the 2 terminals going to the lp and even with the glow plugs on it didn't read anything, not even while cranking. It did read 12.6 or so when engine started. So the lp does run but not before or during cranking process just after it starts.
From what I have read that is normal for a 94.
The dual relay method posted by 94C1500 is able to address that and give you a prime to start from the glow relay and the relay while running.
Ok thanks guys. It starts and runs fine. Time will tell I guess if replacing the relay and ops fixed it or not. ANY EXPLANATION WHY THE CRK fuse or bottom right corner fuse 5 amp keeps blowing.???
Remove the inner fender flap on the starter side and follow the purple wire from the starter solenoid up the firewall. See if it touched/rubbed/burned a bare spot on the exhaust manifold or other body parts. ~FH
I had an issue with a 95 6.5 where it would run rough at an idle and stall at low rpm's. I measured the voltage at the pump while the motor was running and it read 9.6 volts.
I read where it should be at full voltage and could very well explain the running issues.
The truck has a new injector pump, PMD box, lift pump and lift pump relay.
The oil pressure gauge on the dash works.
I dismantled the power box and on the back side of the fuel pump relay prong 30 (the one that feeds the lift pump) and while cranking it only gets about 10.3 volts but jumps up when the motor tried to start. This was with the pump disconnected.
Shouldn't I be getting full voltage at prong 30 while cranking?
AND, what's the common opinion on whether the low voltage at the pump could make the motor run rough and stall?
I'm hearing once the motor is running the pump doesn't even have to run at all, not good for the IP but will run pretty much normally.
I'm concerned that I could possibly have gotten a bad rebuilt IP or possibly a PMD box.
This thing has tied up my shop WAY too long. It's killing me. I've GOT to get the heap out of there.
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.
Fuel Pump Relay Circuit Diagnosis for '95 C/K Trucks. NOTE:Applies to '94 C/K Trucks also with one noteable exception, the '94's DLC "Fuel Pump Test Terminal" is "G"..
As for testing the lift pump itself... see below..
FUEL SUPPLY SYSTEM CHECKS
If the fuel supply system is not delivering enough fuel, or air is being drawn into the fuel injection system, driveability could be greatly effected or a "Cranks But Will Not Run" symptom could exist. If other diagnostics indicates, or if the fuel supply system is suspected of not delivering enough fuel or drawing air, it should be tested as follows:
!!Important!!
-- Air leaks or restrictions on the suction side of the fuel pump will seriously affect pump output.
-- Make sure there is sufficient fuel in the tank.
-- Check for leaks at ALL fuel connections from the fuel tank to the injection pump.
-- Tighten any loose connections.
-- With engine running, check all hoses and lines for flattening or kinks that would restrict fuel flow.
Lift Pump Flow Check
1. Disconnect the electrical connector for the engine shutoff solenoid at the injection pump.
2. Disconnect the pipe at the lift pump outlet fitting.
3. Install a hose at the lift pump outlet fitting and place a 1 liter/quart container at the hose to collect fuel.
4. Crank the engine and measure the amount of fuel :
-- If more than .24 ltrs (1/2 pint) in 15 seconds, refer to "Lift Pump Pressure Check" in this section.
-- If less than .24 ltrs (1/2 pint) in 15 seconds, refer to "Lift Pump Suction Line Check" in this section.
Lift Pump Suction Line Check
1. Remove the fuel tank cap and repeat the "Lift Pump Flow Check."
-- If more than .24 ltrs (1/2 pint) in 15 seconds, replace the defective fuel cap and refer to the "Lift Pump Pressure Check."
-- If less than .24 ltrs (1/2 pint) in 15 seconds, go to the next step.
2. Separate the lift pump suction line from the fuel sender.
3. Connect the suction line to a source of clean fuel, using an additional hose.
4. Repeat "Lift Pump Flow Check."
-- If flow is more than .24 ltrs (1/2 pint) in 15 seconds, remove the fuel sender and check it for restrictions.
-- If flow is less than .24 ltrs (1/2 pint) in 15 seconds, go to step 5.
5. Check lift pump suction line for restriction:
-- If restriction exist, repair it and recheck the lift pump flow.
-- If no restriction exist, replace the lift pump and recheck lift pump flow.
6. Attach the lift pump suction line to the fuel sender.
Lift Pump Pressure Test
1. Install a tee adaptor at the injection pump fuel inlet connection.
2. Connect a pressure gauge with a dial indication of 0 to 103kPa (0 to 15 psi) to the tee adaptor.
3. Run engine and measure fuel pressure.
-- If pressure is at least 3 psi or 27 kPa go to step 4.
-- If pressure is less than 3 psi or 27kPa, refer to Chart A-5 in SECTION 3 (Fuel Pump Relay Circuit Diagnosis) before replacing lift pump.
4. Remove pressure gauge and tee adaptor.
5. Connect outlet pipe at the lift pump outlet fitting.
6. Clean any fuel spillage.
7. Run the engine to check for fuel leakage.
Fuel System Air Leak Test
1. Install a transparent hose between the fuel manager/filter outlet and injection pump fuel inlet.
2. Start and idle the engine, observing the fuel for air bubbles.
-- If air bubbles are not present, stop the engine and go to step 6.
-- If air bubbles are present, stop the engine and go to step 3.
3. Check the lift pump suction line for air leakage:
-- Disconnect fuel pipe from the fuel sender and plug it.
-- Disconnect the fuel pipe from the lift pump, and install a hand held vacuum pump with gauge.
-- Apply vacuum to the fuel pipe and observe the gauge reading:
--- If vacuum does not drop, connect fuel pipe and go to step 4.
--- If vacuum drops, repair the air leak in the suction line and install the suction line pipe and hose.
4. Check the fuel sender for air leakage:
-- Remove the fuel tank.
Remove the fuel sender from the fuel tank, remove strainer and plug the bottom of the pickup tube.
-- Apply a vacuum to the upper end of the pickup tube, and observe the gauge reading.
--- If vacuum does not drop, install the fuel sender and fuel tank.
--- If vacuum drops, replace the fuel sender, install the fuel tank, connect the fuel pipe and go to step 5.
5. Start and run the engine, observing the fuel for air bubbles:
-- If air bubbles are present, stop engine and recheck steps 3 and 4.
-- If air bubbles are not present, stop the engine and go to step 6.
6. Remove the transparent hose and connect the hose of the fuel manager/filter outlet to the injection pump inlet fitting.
7. Disconnect the return hose from the injection pump.
8. Install a transparent hose between the injection pump and the hose of the return line.
9. Start and run the engine, observing the fuel for air bubbles:
NOTICE: It is OK to see a small stream of air bubbles on snap acceleration
-- If air bubbles are not present, go to step 10.
-- If air bubbles are present, replace the injection pump.
10. Stop the engine.
11. Remove the transparent hose and attach the fuel return hose at the injection pump.
12. Clean any fuel spillage.
13. Run engine and check for fuel leaks.
As is standard operating procedure following any Fuel System Test eg see above and/or routine maint procedure eg fuel filter changes, and/or repair(s) are complete and normal engine operation has been restored. Check for, make note of, but more importantly CLEAR DTC(s).
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 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....
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.
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.
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
Under perfect conditions, the pre-prime feature isnt necessary as I find my recently restored fuel system still pressurized after sitting unused for days.
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.
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 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
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