Ok, so after I replaced seal washers and o-rings in the IP valves last night the engine required quite a bit of cranking to get it going, as expected. At any rate, the glow plug light came on and during the first cranking sequence I let it cycle completely but on subsequent cranking attempts, perhaps two more 5+ second cranking attempts, I didn't wait for it to go out.

I didn't think too much about it and I drove it home and parked it. I plugged it in last night, -7F, so that it would be warm in the morning. When I went to go to work this am the glow light did not illuminate as usual even with block heater.

I tested today and found one glow plug out of spec, high ohms but not enough that it should have faulted the whole system so I checked voltage output at the controller with ignition on and found each plug was only getting 8V, not the 12V+ that they should be receiving.

I have to assume that the glow controller is toast although since I don't know what other inputs the glow controller sees (water temp?) I can't be certain.

Am I correct in that the glow controller is shot or is there something else I should check?

Thanks
Steve A

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

A weak Battery does require especial Charging.

1.Proper DISTILLED water fill of each cell.
2.Nice OVERNIGHT 1.5 to 2 Amp charge with a 3 step Charger.
(May be as long as 24-48 hrs.)
3.After Full Charge cycle turn on Headlights for a 10 second period to burn off
"Surface Charge". (Or wait 2 Hrs for Battery to equalize)
4.Measure resting battery BOTH with a DMM (for Voltage)and a Hydrometer
(For Specific Gravity).The Specific Gravity Measurement is the Ne Plus Ultra,
or True Determinant.

http://www.schumacherproducts.com/applications/?id=0007

Check ALL your grounds!(and connections "Clean,Shiny and Bright")
The Pre-Glow Relay has it's own ground to Chassis.
the Engine to Chassis strap Ground must also be good.

The fact that your GP relay is delivering 8 Volts tell you it's working.
"Somezing" is interfering with delivery of Full Voltage to the GP relay.
(Or delivery of Full Voltage from the GP relay to the Glow Plugs)
[AKA WIRING from GP relay to the Gps]

__________________
'84 300SD sold
124.128

Thanks for the tips, the battery is nearly new and delivering 12.9 to the relay with the ignition off.

At any rate, I've checked plug resistance, two GP's were replaced earlier this year, all plugs save for one are at about .5 ohm resistance. The other is at 18 ohm resistance but that's not bad enough to cause the system to fault completely. I'm almost certain that the relay has gone bad.

The clincher is that the relay is only allocating 8V to the gp harness connector at the relay...to every plug and hence I really think it's gone bad. That is unless there's another outside influence on the GP relay, like a temp sensor that is falsely telling the relay that glow is unnecessary.

Regards
Steve A

__________________
Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

It's ALMOST impossible for them to fail in a "PARTIAL" mode.
you either get Voltage @ the Pins or you do not.

You might be able to make a case for "Dirty" contactors inside the Relay,
Some have taken the cover off and gently cleaned the "Dirty" contactors
with superfine Emery.

The One GP that measures 18 Ohm is Really Bad.

When you're measuring the Voltage to the Glow Plug Harness Relay Pins ,is the
Harness plug disconnected from the ReLay?
If the Harness is still plugged into the ReLay, that 18 Ohm GP could be pulling
down the System.

What's the Voltage @ the Power supply connector (The RED ,HOT @ all times)
cable at the Glow relay when the system is in active "Glow".
That 12.9 volts with the Key in your hand is nice...BUT tells you nothing
about what's going on in operation.

I'm still hesitant to condemn your GP relay.

[connections,and grounds grounds grounds]

__________________
'84 300SD sold
124.128

I'm not debating whether this plug is bad, I agree it's bad but it's not nearly as bad as some I've dealt with before on other cars. I've never had a plug that tested at this level of resistance terminate system operation.

Yes, plug disconnected and V taken directly from relay pins.

agreed, but it's not connected so the downstream components didn't impact.

You know tbh I didn't check that reading because my understanding of the ignition switch contribution to the GP relay doesn't impact voltage reading at the non-switched 12v+ connector. In other words, non-switched power, i.e. direct connection, shouldn't change.

Well I can pull it out and take it apart, I've taken them apart before, just not this variety.

Thanks for helping me try and diagnose this problem. I'll post back if I can get out there and take a reading on the 12V constant power with the ign switch on.

Steve A

__________________
Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

Ok, the Simpsons was delayed so I had time to run and take the reading. I had to use a different meter so that introduces an element I'm not happy about but I can take another reading tomorrow with the same meter I used for the prior readings.

With the key on I read 12 to 12.1 V at the constant power lead to the relay. the reading with this meter without the key on was 12.4-12.5 so it seems inconsistent with the other meter to some degree.

Thanks again
Steve A

__________________
Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

I agree with you Compress. The low voltage suggests that its not a straight forward "something is toast" problem. More like a bad connection. Relay contacts are just another connection. I once had an insect get in to the contacts & got fried, it gave a similar result. Supply voltage measurement under load to the GP control will reduce the possible causes list.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving