Hunting for an Errant Glow Plug
 

Recently bought an '87 300D, and suspect a faulty glow plug--or maybe two. Between one and six key turns are required to light up the glow plug lamp. All plugs test good with an ohmmeter, and all pins at the relay have 12 volts. Is the next step a current measurement for each plug? Existing plugs have about 90,000 miles and 9 years on them. The holes were reamed when last replaced. The relay was replaced about a year before that, and there appear to have been problems with the GP light from then. I could also just replace them all and see what happens--maybe it's time?

Rob

hows the fuse?

http://www.dieselgiant.com/glowplugrepair.htm
http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm
Second link is for hoop style GP... not on your car but while your discovering the wonders of GP ...
Have you tested the replay timer?
If the relay tests OK and the GP's test ok then you have the blub and ign. switch left to test.
I have not 'hot wired' (actualy a jumper) the GPs but I'm sure someone on this site has so you can verify everything works beside the ign. switch.
You may have a bad ign. switch.
There is a past post with details of bypassing the ign. switch.

Welcome to the forum.

Please provide details of how you performed this test and the results of this test for each glow plug.

Thanks for the ideas. I suspected glow plugs because of the rough starts, with clouds of smoke, when cold--coupled with the pesky GP light. Could the GPs be so sooted up that some have become ineffective? Maybe I've got two issues here: the relay and/or ignition switch, and the plugs themselves. There's a note with one of the maintenance actions that the light could be made to illuminate by tapping the relay, although I've not confirmed that myself. Still doesn't explain the hard starts, though.

Oh, yes. The test consisted merely of checking the resistance for each plug at the relay. Each plug tested the same-- .4 ohms, if my DVM is to be trusted. New plugs measure about the same, though, so the values must be about right. Measured voltage at each pin of the relay socket, and all measured 12 volts. And the fuse is good. My next step was going to be measuring the current to each plug by inserting an ammeter with appropriate shunt between the relay and the plug--one plug at a time. I still have to put together the shunt, and my resistors may not survive the load for long...

Something doesn't sound right. All symptoms point to a bad glow plug.......or more than one........

You've got plugs that have 90K on them and they all test lower than a brand new factory plug (typically .6-.8 ohms).:confused:

Your test with an ammeter might show some different results than one would expect with those resistance readings. The initial inrush current will be about 30 amps per plug for a brief moment if you actually have .4 ohms. But, I'll wager that one, or more than one of them is nowhere near this value.

Each and every plug draws 35 amps. Each and every relay output provides adequate current. Now what? Is it possible that reaming the prechambers would make a difference to the GPs' performance?

Rob,

U should do the voltage test at the GP terminal when the current is going thru the GP. Open circuit voltage tests don't always mean much.

I have a W116 with a 617 engine and the GP light is very intermittent. Some times it lights, sometimes it doesn't. But that doesn't have any effect on the GP and the engine starts easily as long as I wait long enough for the GP to get hot. I didn't think this problem was worth spending the time to fix it.

Try waiting longer before turning the key to the start position and see if this makes any difference in how the engine starts.

P E H

Well, you've clearly proven me wrong. There is nothing wrong with the glow plugs.

So, any starting issues must be related to something else or the operator.

How long do you let it glow before you try to start it? A 603 definitely prefers 20 seconds or so.........even in warmer temperatures.

When I can get it to come on, the light only stays on for about 10 seconds--maybe not long enough?

on the om617 the glow light goes off but the plugs still heat for an additional time IIRC .. perhaps the 603 does the same thing

The light is an idiot light. It's got nothing to do with the time the glow plugs are heated. The plugs will heat for 35-40 seconds..........no matter what the light does. In theory, you wait for the light to go out and then start the engine. In practice, if you do that with the 603, it will stumble and run very rough for nearly one minute at idle.

Replace the glow plugs, replace the strip fuse, report back.

Brian,
You are so right. The plugs light up for 40 seconds my my car, and after 20 or so the car starts much better. 10, or even 15, just wasn't enough. I just found it difficult to accept the seller's statement that the plugs were fine--"it's just a problem with the light," he said. But he was right. Thanks for the help!

Rob