OH NO! Not another glow plug relay thread...HELP!

[iandiam]

I've been experiencing some rather odd glow relay troubles that do not seem to have been documented here in the past.

Mid-summer, I replaced the stock glow relay with the Bosch 'Duraterm' post-glow unit. At first (I mean the first couple times I cycled it!) it glowed every turn to the ignition. Slowly, the light stopped going on every time. I thought, as outlined in a previous post "[I] turn the key again, hoping it's a glitch..."

I then reasoned (in the summer) that the light depended upon block temperature and let it go to german design. In the fall, I let it sit cold overnight, and glow was a no go. Back to the drawing board.

Well, it seemed to be a glitch. The second turn it would go on and cycle properly. Then it turned out to be the third, then fourth, then I lost count of how many times I had to key the switch. I proceeded to test the glow plugs, harness, relay; no faults found. Re-connected everthing and what do you know? it works again - for a couple starts. Then it starts increasing the number of keys to the switch just as before.

When the number increases again, I do the same tests as laid out in the FSM and other locations. I'm scraping now. No faults found. Re-assemble: Same as before. It works for a few starts, then the number of keys to the switch increases again.

Today I did the whole test again. Re-assembled, NOTHING. No light until I keyed the switch about 8 times (guessing,) then it lit every time. If I let it sit, I have to key it a few times to get it to glow again.

It clearly is NOT glowing when the light does not light, as it runs rough for the first few seconds; Compression seems to be pretty good, but I know I have one sub-par or out-of-spec injector - starts first time even down in the 30s.

Here are my theories, and feel free to comment.

1.) The Duraterm unit sat on a warehouse shelf for a while and collected some corrosion on the relay arm contacts that are inhibiting it from releasing from the at-rest position until enough resistance heat has been built up in the conductors and contacts. This condition is listed in the FSM as being correctable with the stock unit by keying the relay 10-20 sucessful times repeatedly. Has not worked for me.

2.) the contact in my key switch is dirty or corroded enough to prevent positive condictivity until it is dislodged or heated by resistance and use, see theory #1

3.) The relay is total crap and I should go back to the stock unit, of which I have at least 2.

[Doktor Bert]

In my experience, the #1 cause of glow plug system issues is a bad ground. This was a big issue on my 116.120 until I found the group of grounds near the A/C compressor, on the frame rail, that were loose...Robert

[iandiam]

Originally Posted by Doktor Bert

In my experience, the #1 cause of glow plug system issues is a bad ground. This was a big issue on my 116.120 until I found the group of grounds near the A/C compressor, on the frame rail, that were loose...Robert

I appreciate the help.

I can't find any resistance between the gnd point, terminal connector between the relay and block temp sensor, or any other grounded points. That is, I get no more resistance than the wires of the tester, about .3ohm. any resistance above the probe wires is so fractional...

Even loosened up, cleaned and re-torqued the gnd point near where you mention, the one that grounds the above terminal block, relay, and runs a 10ga to the compartment harness.

Batt voltage is good, even tested with the 2a charger hooked up...Quite a bizzare scenario...

Any other ideas?

[iandiam]

really...anyone at all?

[funola]

Take three 12V test lights, hook one across the glow plugs, 2nd one to the ignition trigger, 3rd one to the purple wire. Report back what you see.

[pizzachef]

Yes. The test lights are a good idea. It sounds like the key switch isn't turning on the relay. One of those pins in the wiring harness gets 12v from the key switch. You'll have to look in the wiring diagram to figure out which one, but connect a light or a multimeter to that and see if you get 12v every time you turn the key to glow.

[iandiam]

Originally Posted by funola

Take three 12V test lights, hook one across the glow plugs, 2nd one to the ignition trigger, 3rd one to the purple wire. Report back what you see.

When you say 'across' glow plugs I assume that you mean from a working + plug ring to gnd to test the glow cycle and rule out a bad light / bulb / switch channel? Been there.no glow unless the light goes on.

I'll get back to you on the purple wire and IGN pins.

[funola]

It does not matter if the light is connected across a good or bad glow plug since all 5 leads are tied together internally in the controller. The 3 lights allow you to easily see the timing sequence and whether any are intermittent.

[SD Blue]

Ran across a similar problem myself a few weeks ago. After disconnecting the relay plug and spraying with contact cleaner, everything started working every time.

With regards to the grounding, a simple VOM multimeter isn't sufficient enough to determine much about the quality of the connection. You would need a special megohm meter (~$400). If there is any visible corrosion in the area, I would suspect a compromised connection. After cleaning connections, such as this, I like to apply a coat of undiluted varnish.

I've noticed in the wiring diagram that the ground connection also comes into play with regards to the temp sensor inside of the glow plug relay.

Also, don't forget to physically check the fuse strip inside the relay. I've read here, a few instances where folks have found a hairline crack in the fuse and did not find it until they removed the screws holding it in place.

[iandiam]

Originally Posted by SD Blue

Ran across a similar problem myself a few weeks ago. After disconnecting the relay plug and spraying with contact cleaner, everything started working every time.

With regards to the grounding, a simple VOM multimeter isn't sufficient enough to determine much about the quality of the connection. You would need a special megohm meter (~$400). If there is any visible corrosion in the area, I would suspect a compromised connection. After cleaning connections, such as this, I like to apply a coat of undiluted varnish.

I've noticed in the wiring diagram that the ground connection also comes into play with regards to the temp sensor inside of the glow plug relay.

Also, don't forget to physically check the fuse strip inside the relay. I've read here, a few instances where folks have found a hairline crack in the fuse and did not find it until they removed the screws holding it in place.

Now THAT is some stuff that I was unaware of. I will get on that as soon as it stops raining cats, dogs, and elephants.

80A Fuse is OK, BTW. Had it out in total, just in case

I had similar experiences, but the situation has degraded to complete nonfunction. There's a bad connection somewhere, as I no longer have reason to suspect the relay...

[kerry]

I'd suspect the ignition switch. It's pretty well documented here that dirty connections cause just that kind of problem in the 'start' position so I can see where it would also occur in the glow position. If you can jump from the + terminal on the battery to the 12 volt connector that comes from the ignition switch on the relay and it works every time, I'd say it's the ignition switch.

See the conclusion of this thread:

1980 240d Glow plug relay source...

[iandiam]

Originally Posted by kerry

I'd suspect the ignition switch. It's pretty well documented here that dirty connections cause just that kind of problem in the 'start' position so I can see where it would also occur in the glow position. If you can jump from the + terminal on the battery to the 12 volt connector that comes from the ignition switch on the relay and it works every time, I'd say it's the ignition switch.

See the conclusion of this thread:

1980 240d Glow plug relay source...

AGAIN, that is the kind of information I'm looking for. EXCELLENT suggestion - again, as soon as Noah & menagerie disembARK, being double-parked in my driveway...

Anyone know, offhand: Is the ignition switched from terminal #1 in the four-point connector on the 126.120? I'm not so good with schematics, but I think that's what I find here:http://wagoneers.com/DieselBenz/TECH/PDFs/Engine_617_15-710.pdf

[SD Blue]

The ignition switch comes into play directly to pin #1 of the glow plug Control Relay. Also, indirectly, the ignition switch ties through the starter lockout/backup light switch and to pin #2 of the glow plug control relay.

I noticed an interesting statement on the bottom of page #7 of the pdf file you mentioned. "By preglowing several times one upon the other (5-10 times) the capacitor in the preglow relay is again activated and the specified preglow time is obtained." Evidently, this happens after a long storage time.

[iandiam]

Originally Posted by SD Blue

The ignition switch comes into play directly to pin #1 of the glow plug Control Relay. Also, indirectly, the ignition switch ties through the starter lockout/backup light switch and to pin #2 of the glow plug control relay.

I noticed an interesting statement on the bottom of page #7 of the pdf file you mentioned. "By preglowing several times one upon the other (5-10 times) the capacitor in the preglow relay is again activated and the specified preglow time is obtained." Evidently, this happens after a long storage time.

Thanks, Blue.

I have further developments that have made me think...

IF I key the IGN switch as though I would glow normally (turn to position II and wait about 20 sec,) light still off, then key off and back to II again, the light glows every time. It never lights the first time.

I have taken many cold starts and no light; most hot- or warm-starts, the light comes on. I have established (by listening and test lights) that the GP relay hits the plugs every time I key to pos. II, but the light only comes on when the block temp closest the plugs has been warmed somewhat.

New Theories:

The temperature relay inside the GP relay is bunk;

The block temp sensor is partially cooked and works only when the block is warmed up to a certain point, perhaps to whatever temperature after a full glow cycle.

Any thoughts? I think it's the temp sensor, now. Only problem is that I HATE [read as "HATE"] doing coolant drains, so wasteful and without guarantee, asks for leaks, etc. Can the temp sensor be swapped without draining below grade, or is it an evil necessity?

Ian

[kerry]

No need to drain. Act swiftly to reinstall the new one. It does sound like a problem temp sensor.