Hi, I am new to the forum so, hello to evrbdy.
Sorry for posting a problem already known, but I can't find answer in other posts. I just got a 1992 300SD, when I turn the key in position 2 (when normally the preglow indicator stays ON for a while), the indicator stays off, but after about 40 seconds, I hear a "click" and... the preglow indicator turns on for a long long time, then turns off.
Weird, no?
First I ask if anybody knows what this behavior means, then if somebody can explain me where can I find the preglow relay, fuse and connector, so that I can perform the ohm test described in many other posts.
Thank you in advance.
MB

Welcome to the forum.

The 603, like all other M/B diesels, uses an idiot light to tell you when to start the engine. This light actually has no relationship to when you should actually turn the key to "start" and, it also has no relationship to how long the plugs will remain energized.

With regard to the former, this engine won't start cleanly unless you let the plugs warm for at least 15 seconds at any temperature. Colder temps required close to 30 seconds.

With regard to the latter, the light goes off but the plugs remain energized for approx. 35 seconds, as you have observed. At that point, the relay opens and you hear a noticeable click.

It's a bit strange that the light illuminates after the relay disengages. This may be a sign of a bad glow plug, but, I've never heard of such behavior in the past.

Do you have any difficulties in starting it, and, once started, will it run cleanly with all cylinders firing.......or will it stumble?

Hi Brian, the car usually starts in 1 second cranking, now starts in 3/4 seconds, has 3/4 seconds stumbling (pretty the same then before), and then fires fine. I'm talking about cold start, of course.
Tomorrow I have a date with Bosch dealer to check the injectors (you know, new car so I have in budget some fixing, like new brakes, new tires, new wipers, cleeneng injectors, etc.), so I can tell him to put new glow plugs one time, and see if the problem disappear.
I will post the result.

I tested with an ohmmeter the 6 plugs, by the connector on the relay, five are OK ( 0.5 ohm), and one gives 1, let's say is open circuit.
I read tha bad plugs give 000 reading, let's say no resistence. Here looks like no current flows trough that plug.

Mario,

Usually bad GP will read open (infinite ohms) which may give different readings on different ohmeters. In almost 40 years of MB diesels, I have only seen one shorted GP and that was the series type. If U had a shorted GP, it would blow out the fuse in the GP relay.

It sounds like U have one open GP, which is similar to a burned out light bulb. This will not read 0 but infinity Digital ohmeters dislpay this different ways.

Check what the readings are on your ohmeter with the leads connected and disconnected. Infinite resistance will be displayed with the leads disconnected. My Radio Shack meter reads 0.F for an open circuit (infinite ohms). Harbor Freight meter reads 1 for open circuit.

P E H

Thanks PEH, yes you are right, the reading is infinite ohms, open circuit.
So, this put me in a "normal" situation of a bad G.P., I guess. I feel better.

I tested the pins of the relay side of the connector (key in pos. 2), and the reading was around 12 V., then when the relay clicks, after around 40 sec., the reading was around 7.6 V.: is that normal? I would say that at the end of the glowing process, the relay would switch off to zero Volts. Am I wrong again? (I hope)

Mario,

With a open circuiut, strange voltages can appear. Since U are using a high impedance meter, there is not enough conductvity to connect the the pins to ground, so they may float. If U put an electrical load on the pins, they would read 0 volts.

P E H

You make me feel so good...
If the problem is only a bad glow plug, by tomorrow this time it should be fixed...

The glow plug light in my '87 wouldn't come on at all. Turned out the #1 glow plug was bad. Replaced the plug, light now works normally. As Brian says, give it a good 15 seconds of glow, even in a warm climate like yours.

__________________

"Buster" in the '95

Our all-Diesel family
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I will.