Intermittent no-glow on 84SD
 

Daughter's car. Light doesn't come on and plugs don't energize intermittently. Not a huge problem at these temperatures since it will start without the plugs but I need to get it figured out before summer's over. Anyone else experience this? I'm thinking a problem internal to the relay?? Hard to do diagnostics since most of the time it works fine. I did replace the mechanical end of the ignition switch recently due to a failure in the locking mechanism.

Look closely at the big strap fuse under the glow relay cover. Sometimes they'll develop a hairline crack that will open up and cause an open circuit and no glow. That fuse is something that probably should be replaced on a regular basis anyway.

OR, you could just change out the glow relay...methinks they're all the same.

Yup, I've always just replaced the relays when they become unreliable. I would check the fuse first.

Pull the big plug on the GP relay. Measure resistance from each pin to ground. You should see about 0.7 ohms on each of the five pins. If you see significantly more (1 ohm would be a bit high, but not bad, 2 ohms would be stretching it, but more than likely you'll see 20 to 100 ohms or even an open) then the plug is bad and requires replacing. If you don't see any bad plugs, check the voltage at the plugs during a glow cycle. You should see upwards of 10 volts. If not, then you may have a bad connection in the line feeding your glow plugs or bad ground between the engine and the chassis. Check the voltage between the engine block and negative battery post during a glow cycle and you should have zero volts difference. This means your ground is good. These are the first steps in troubleshooting your GPs. If these all check out and you need to go further than this let us know.

Plugs are good. The problem is, that most of those diagnostic procedures are virtually useless at the moment since the problem is intermittent. Most of the time the system works fine.

Kerry,
I though you were an old hand when it came to GP problems. You would need both hands & both feet to count the number of people you have solved GP problems for!!!
This one is interesting though.
It would be worth cleaning the contacts on the relay. I once had an ant get fried in the contacts, the relay worked 95% of the time. Cleaned it out & all was good.
If the relay box is the same on your TD, you could try a swap & see if the problem goes away.

I have a bad contact in the inhibitor switch of my TD at the moment. About 1 time in 10 it does not let it start, there now is a couple of wires coming into the cabin that lets me bypass it if needed. I will fix it properly next time it is on the ramps.

I'll try cleaning the contacts on the relay. I have considered just swapping in a new one but I'd prefer to try to figure out what's going on with this one.
It's the whole intermittent thing that's making diagnosis difficult. I did check the resistance on the plugs but I know the plugs are not the problem anyway since when the glowlight doesn't come on, the interior cabin light does not dim so I know the relay is not sending power to the plugs when it happens. I did examine the fuse closely and could find no evidence of a hairline crack. I am considering an ignition switch issue but I can't remember another instance on the forum when a failing ignition switch caused the glow relay not to function. The only remaining source of the problem is either the wiring to the relay or something internal to the relay itself. I've never had one apart to examine the internals but now seems like a good time.

Urban legened unless it was not a Mercedes relay since there is no way an ant can get into a Mercedes relay.

Is the lower contact on the points in the relay supposed to have a little nipple in the middle of it or is that a burnt spot? I'm filing down the contacts a bit with an emery board and will reassemble it.

Nope. Relay contacts are flat. Draw fine sandpaper through closed contacts to shape them flat to each other for best results.

You say it's intermittent. Does it stay 'bad' for a while or does the issue clear up with the next glow cycle?

On my partners car the light would not come ,and there was issues with other dash lights but the fault was a alternator regulator gone bad.

One brush made intermittent contact,replacing the regulator sorted the problem out,I did have the ignition barrel electrics out and they checked out ok.

Can you hear the relay energize but still get no light? How often does it happen?

When it happens, nothing happens. The cabin light does not dim. It might possibly happen two attempts in a row but usually not and it has been happening randomly. No issues with other dash lights.

I filed down the points with an emery board. We'll see if that makes the problem go away.

funola,
Not an "Urban legend" It happened to my old 300d. It was the type for the old wire GP's. The outer case had a mall crack in it.
Best you not slur others like that.
It gives the impression that one is trying to hide their own ignorance. :rolleyes:

Quote:
Originally Posted by kerry
Is the lower contact on the points in the relay supposed to have a little nipple in the middle of it or is that a burnt spot? I'm filing down the contacts a bit with an emery board and will reassemble it.




X2
sounds like a problem has been found. That little nipple is probably preventing good electrical contact some of the time.

Those Autralian ants must be really dumb.:rolleyes: In other parts of the world ants only go where there is food, not between the jaws of death of heavy duty relay contacts.:D

So the case had a crack in it? How did that happen? Mercedes is not known to use cheap plastic. Were you drunk when you worked on it and smashed the case and drooled beer all over it?:eek:

Hey Kerry, what did you discover with this problem? My 83 SD has just developed the same issue, although it seems to be occurring with more consistency. Typically the glow plug light does not come on, but if I bump the starter, recycle the ignition and bump it again, it seems like they always come on for the third attempt. Glow plugs are new and at last check (3 months ago when all glow plugs were replaced) the fusable link had no cracks.

Remove the smaller connector from the top of the timer. Check for battery voltage at the terminal that connects to pin 15 on the timer, with the key in the RUN position.

One of the white hot glow plugs that he tested fell from his grasp, dropped on the case, and melted a hole in the top.

That's how the ants got in.

Worked ok for a while then the problem began recurring. I got a new relay and it solved the problem.

Thanks Tango, I'll check it tomorrow morning and report back.

Sounds like too much of the contact metal had burned away.
you end up with only the bracket material left.

Crack was there long before the car was mine.
Your slur about drinking is well off the mark.

Full battery voltage at pin 15. 12.45v, same as across battery terminals

Then the problem is not the key switch. Also check for continuity to ground on the brown wire (pin 31) and continuity across the 80 amp fuse.

Have you confirmed that the timer is not starting its sequence (ie: relay does not close) concurrent with the "no light" condition?

Continuity on brown wire (31) and across the fusable link. Removed and examined link, no sign of cracks.

When I turn the key and get a no light condition, I am not hearing anything from the relay.

If there were a faulty condition in the key switch, something loose, bad connection etc, then I would not be seeing voltage on pin 15? Possibly I need to engage an assistant who can turn the key back and forth while I watch for voltage...

Correct. Sounds like you have a faulty preglow unit.