Thread: Glow plug relay

I have a 1995 chevy 6.5 diesel. It stopped running had taken it to a shop and after about 2200 buck fuel pump and so on and so on. Well it ran fine for about a month. Then i stopping running again. The wait light would just flash when you turned the key. So i did glow plugs didnt fix it. So i put a glow plug relay on it and it ran great for a day and a half. Well now it s doing it again. I cleaned up all the ground wires and still no fix. There where no blown fuses but i replaced them anyways. Still doesnt run. Someone told me to jump the relay. when i did the light wait light came on on the dash but still wont start even with it jumped. I have no ideal what would keep making the relay go bad if that is the problem but the only one i can find. And no codes in the computer either. I M LOST WAY LOST. dont know what to do next

I did not read where you or the shop checked the glow plugs. The need to be checked individually for resistance of 0.8 to 1.2 Ohms cold. Remove wire leading to spade lug on glow plug and measure with one lead on the spade lug and the other on a good, clean ground.

Just one or two bad glow plugs can cause a no start condition. The OEM glow plugs are 6 Volt and will burn out and swell. Replacements can be 10.5 or 11 Volts and will not work well with some glow plug controllers.

That's why I have manual glow plug systems on ALL of my 6.2 diesels including the 99 Burb.

Originally Posted by HJacobson

I did not read where you or the shop checked the glow plugs. The need to be checked individually for resistance of 0.8 to 1.2 Ohms cold. Remove wire leading to spade lug on glow plug and measure with one lead on the spade lug and the other on a good, clean ground.

Just one or two bad glow plugs can cause a no start condition. The OEM glow plugs are 6 Volt and will burn out and swell. Replacements can be 10.5 or 11 Volts and will not work well with some glow plug controllers.

That's why I have manual glow plug systems on ALL of my 6.2 diesels including the 99 Burb.

Second sentence Harry, "so i did glow plugs didn't fix it." Should've said "So I did glow plugs, didn't fix it." But he sorta did get the idea across. Mike

Start with the basics.

-Check for DTC's, <"How To" LINK List them if any.
-Check battery(s) condition and clean all connections.
-Check/clean all accessible grounds.
-Check all fuses, etc etc etc.
-Check for air/fuel leak(s). Check all the rubber fuel hoses for deterioration/dry rot and/or lose hose clamps. Repair/replace as necessary.
-Check the Lift Pump (LP) and LP circuit...

Note: On OBD-1 models, the LP is only energized during START (starter engaged) via LP relay (LPR), and while the engine is running via oil pressure sending unit (OPSU). The LP is NOT monitor'd, controlled, or activated by the ECM as with later OBD-2 models...

Check LP Relay- Set parking brake. Unlock the ignition. Firmly place your foot on the brake and put the gear selector in drive (activates safety neutral switch ie disables starter), then turn and hold the ignition in the start position. With ignition in the START position, the LP circuit should energize so you should be able to hear the LP activate... This would indicate the LPR is good. If LP is not activating, see "Check Lift Pump" below before ruling LPR faulty..

Check OPSU- With engine idling, simply open the WIF petcock (valve). You should observe a steady steam of fuel flowing from the drain hose....
This would indicate the OPSU is energizing the LP as it should.
If little or no fuel is present, open the WIF petcock (valve) fully and wait a minute. If the engine dies, chances are the LP is inop (via faulty OPSU) or defective. If no discharge, see "Check Lift Pump" before ruling the OPSU faulty.

Check Lift Pump- Install a pressure gauge into the open end of the WIF drain hose and open WIF petcock (valve) fully. Start the engine (OR use LPR check above to activate LP). At idle, check for a minimum fuel pressure of 4PSI (with engine off, 6psi min). If so, shut the engine off, close the WIF petcock (valve) and remove the pressure gauge.
Again with engine idling, open the WIF petcock (valve) while directing the overflow into a measureable container. You want a minimum flow rate of 1/2 pint in 15 seconds..
If the LP isn't working, be sure to check for power at the LP connection during normal operation(s) (see note above) before ruling the LP defective.

Could be the ECM thinks the engine is already warmed up via defective CTS???? Disconnecting the CTS as to fool the ECM into thinking the engine is cold, then attempt to start the engine.
If it starts normally and/or unless someone else wants to post how to thoroughly test the CTS, I'd suggest replacing the coolant temp sensor (CTS) located next to the thermostat housing, it should have 2 wires going to it, cost aprox $25.00 and might take 15 minutes to R&R. That particular CTS informs the ECM how hot or cold the engine is which then determines duration and frequency the glow plugs cycle on and off..
Note- the single wire CTS located on the side of the drivers head is for the temp gauge..

Also worth noting. If the ECM, for what ever reason, DEFAULTs to "Back Up Fuel," the GP and Boost circuits become disabled. However when this happens, most often there will be DTC(s) present.

Glows can read good & still be weak check with meter to see if open (wholly bad) spade lug to gnd as Dr Harry says, if those all check good go to the glow controller itself (center rear of engine beside the fuel filter mgr) see if during glow cycle you are getting 12V on the glow solenoids output terminal leg to the glow harness. I've had a few that click but don't provide trons to the glows.

Transfer contactor bar in the glow relay itself either spitted bad or melts plastic across tips so no xfer of tron juice from battery to the glows.

If that is all good, then pull glows 1 by 1 wrap a #12ga wire around glow body (wearing gloves or hold with pliers) connect other end of wire to battery negative, then touch center lug to battery positive, within 4 sec or so you should get hot cherry red tip to center on glow, if it takes longer than that or is more grey than red, glow is just meeting glow oomph, one or 2 you get by if all of em that way, you may have a no start condition especially in winter.

As far as manual glow override, use spring loaded switch that comes off when you let go, also verify you have AC60G or equivalent self regulating modern glows, 9Gs not self regulating and can swell if glowed too long, 11Gs generally don't swell but aren't self regulating and don't last very long in extended glow mode. Pretty much all 9 & 11Gs are gone off store & dealer shelves, most of what you find are 60G but ask to be sure as part guy a glow is a glow to them, so possibly they will say these are new, yeah been on the shelf but are still old style you don't want.

Originally Posted by Ron Rousseau

Also worth noting. If the ECM, for what ever reason, DEFAULTs to "Back Up Fuel," the GP and Boost circuits become disabled. However when this happens, most often there will be DTC(s) present.

Limp is supposed to be reset with key off so this should not be an issue,

What tech reference said back up fuel would disable GP & boost; disable maybe during engine operation to keep from adding too much boost with a limp issue.

Been a while since I since I looked at that logic function and I'll have to study up on that some, so possibly you are correct here.

BUT as what I call soft limping to 50^3mm max fuel as in say a high IAT no code situation boost is not hampered in any way, so I doubt glows are disabled but if they do disable does not matter as engine is already running, hard limp happens with some of the transmission stuff then you get check engine along with the limp like a temp switch a or B in trans.

Bbut over & above that I'm thinking once you resume "safe" status like engine has been shut off, then the glow & boost disable would go away so you could restart it, most anything that would generally "limp fuel" gets cleared unless that condition is still present at the restart attempt but that also will be accompanied by a hard code check engine illumination if the bulb is still functional.

LP not coming on is generally not a cause of a no start condition unless the IP is weak and incapable of pulling its own fuel thru a dead or weak lift and losing prime to the IP.

It is good to check & know results of a lift test as there has to be free flow to the IP, no blocked filters/socks dead or weak lift will keep engine from making power, but if fuel lines tank to IP inlet have fuel in it IP should be able to draw suction.

Originally Posted by Turbine Doc

Limp is supposed to be reset with key off so this should not be an issue

Yes. PCM should reset when the fault condition(s) no longer exist, and the ignition switch is cycled "OFF" then "ON."

Originally Posted by Turbine Doc

What tech reference said back up fuel would disable GP & boost; disable maybe during engine operation to keep from adding too much boost with a limp issue.

Been a while since I since I looked at that logic function and I'll have to study up on that some, so possibly you are correct here.

Reference Material: 1995 C/K Truck Driveability, Emissions, And Electrical Diagnoses Manual- GMT/95-CK-2F (updated edition).
I only mention'd "Back Up Fuel" aka limp mode because its POSSIBLE for the PCM to default during the start cycle, been there done that. "PCM not communicating."
As you stated, there are different levels to PCM Default, as such not all limp modes affect fuel rate, boost, EGR, glow plugs, trans, and/or other driveability functions. Likely depends on severity and/or combination of fault(s) present. However, if the PCM becomes confused or overwelmed by all the data while in default mode, PCM can take a time out, just turn itself off effectively killing all PCM functions to include GPR. Again "PCM not communicating."

Originally Posted by Turbine Doc

BUT as what I call soft limping to 50^3mm max fuel as in say a high IAT no code situation boost is not hampered in any way, so I doubt glows are disabled but if they do disable does not matter as engine is already running, hard limp happens with some of the transmission stuff then you get check engine along with the limp like a temp switch a or B in trans.

Bbut over & above that I'm thinking once you resume "safe" status like engine has been shut off, then the glow & boost disable would go away so you could restart it, most anything that would generally "limp fuel" gets cleared unless that condition is still present at the restart attempt but that also will be accompanied by a hard code check engine illumination if the bulb is still functional.

Thats why I mentioned it.
An "On-Board Diagnostic (OBD) System Check" must be the first step of any diagnostic procedure.

Originally Posted by jasonduke5

I have a 1995 chevy 6.5 diesel. It stopped running had taken it to a shop and after about 2200 buck fuel pump and so on and so on. Well it ran fine for about a month. Then i stopping running again. The wait light would just flash when you turned the key. So i did glow plugs didnt fix it. So i put a glow plug relay on it and it ran great for a day and a half. Well now it s doing it again. I cleaned up all the ground wires and still no fix. There where no blown fuses but i replaced them anyways. Still doesnt run. Someone told me to jump the relay. when i did the light wait light came on on the dash but still wont start even with it jumped. I have no ideal what would keep making the relay go bad if that is the problem but the only one i can find. And no codes in the computer either. I M LOST WAY LOST. dont know what to do next

The day and a half it appeared to work, what were outside ambient air temps? Above 40 degrees F perhaps?
I do know the fuel heater shares power with the GPR. If the fuel heater shorts out, it will blow the GP fuse. Knowing that, I'd assume if the fuel heater is defective and begins drawing excessive voltage, it might cause the GPR to fail, much like an OPSU does with lift pumps?

As far as what to do next? I'd suggest finding another shop, preferably one with extensive 6.5TD experience. Otherwise you may be better off taking it to your local Vo-Tech..
By chance, did the shop replace all the old rubber fuel lines hidden under the intake and fuel manager when they R&R'd the IP? If not, you might want to get it done or DIY sooner rather than later..

Your '95 being OBD I just like my '94, I have this to offer. My fuel heater shorted out about 8 or 10 years ago and it DID NOT blow any fuses, but it sure drove the instrument cluster crazy with feedback issues when it did. Try disconnecting the fuel heater wiring where it plugs into the main harness right behind the filter housing. It's the plug with three wires in it if I remember right, find the wires that go to the bottom of the filter housing and trace them to the three plugs on the main harness, they're all attached to a metal bracket. See if your problem goes away after the fuel heater is unplugged. If so, then you need to replace the heater, you will also need the o-ring that seals the heater to the filter housing, both are available from the dealer or maybe elsewhere. Mike