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Car Forum / Mercedes-Benz Cars / April 2008'85 300D - Glow plugs stopped glowing
So the other day the glow plug light on the dash of the aforementioned '85 123 stopped lighting. The driver asked me to see about replacing the lamp since the car had started the night before, the first time she noticed it out, coming home from a restaurant (so the car had been up to temperature before cooling down for an hour or so). Perhaps unsurprisingly, the car was very difficult to get started the next morning, which pointed me to the thought that it's not the lamp that has failed in this case... :) Anyhow, I've popped the cover off of the relay. The fuse is (or appears to be) good. Putting a voltmeter between the fuse and the block (the oil filter mount was in easy reach) showed me 12v with the key not in the ignition (this ruled out needing to find C105, which I couldn't easily locate in the guide). So if the fuse is good and there's voltage there but the plugs don't glow, is it basically a given that the relay needs to be replaced? Another thought I had is that the temperature sensor might be stuck, but the drawing in the wiring diagram makes it look like it's all part of that same package, as a thermo-resistor deal of some sort. thanks for the help, -tom! -- Tom... first thing first, measure the resistance of each glow plugs. you have to disconnect each glow plug so you can measure each one individually. Otherwise, you will measure all glow plugs. When you measure each one... jot down what you read... each one should put out about 0.5 ohm or less. if you get some in like 20 or 2000, that is bad glow plug. To test the relay, all you have to do is turn on the ignition, and then measure the voltage at the glow plug. When the light stop working, it means that one of your glow plug is bad. > Tom... first thing first, measure the resistance of each glow plugs. you > have to disconnect each glow plug so you can measure each one individually. Ah ok, so is the connector on the glow plug just push/slip-on? I'll go take a peek and see what I see... > Otherwise, you will measure all glow plugs. When you measure each one... jot > down what you read... each one should put out about 0.5 ohm or less. if you > get some in like 20 or 2000, that is bad glow plug. Ok thanks. > To test the relay, all you have to do is turn on the ignition, and then > measure the voltage at the glow plug. When the light stop working, it means > that one of your glow plug is bad. Mm, good point. Thanks. Although, on the '85 the glow plugs are in parallel from the factory, right? There are five leads from the relay. This contrasts against my '79 which (as I understand it) is series-connected. Another mod for this summer... -tom! -- If you got 5 leads, then each is individually powered by the relay. > If you got 5 leads, then each is individually powered by the relay. Right, I realized that I could just pull the plug on the relay to do the resistance test, but have not sat down to do it yet. Will get on it shortly. I have discovered that the relay does work, though; it clicks pretty loud about 15 seconds after turning the key on. After doing three cycles on the relay, I turned the key to start and it started right up. So, it seems that some number of glow plugs are working adequately. I just didn't have the desire to actually disconnect each of the glowplugs from their leads as it was getting dark and my hands are bigger than that. It's too bad I realized that testing from the plug side would yield the same effect... ;) -tom! -- I don't want to hijack this thread, but I am having a similar problem. My glowplug light will not always come on on my 84 300D. Last October or so I was having no problems starting the car, but under the advice of someone on some forum (can't remember who or which one.) I changed out my glowplugs as a preventative measure. It's been all down hill since then. Immediately, the light wouldn't light when I turned the key. If I turned it off and back on, about half the time it would come on. It would pretty much always come on by the third or fourth try. I'd let the plugs glow for two cycles(which sometimes required a lot of turning the key back and forth.) then try to start. It would work about a third of the time, but would always eventually start. I suspected that I didn't tighten a plug or lead enough. So I checked them and found one plug that wasn't as far in. I took it out, ran a 1/4 inch drill bit in there to clean out any carbon, and reinstalled it. After that the light came on with about the same regularity. Since then though, it has become harder to get it to show the light. Sometimes I turn the key back and forth ten or so times before I get a light. The light always stays on for about 7 or 8 seconds when it does come on. The car seems a little easier to start now that the weather is getting warmer, but never fires up on the first try, even when the weather is warm-like the 60s. For a while I thought that maybe fuel starvation had something to do with the hard starting, like a filter clogging up or something. But when the car is warmed up and I shut it off to go in a store, it starts up with no problem within a fraction of a second of turning the key. I can see that fuel is in the filter, so it is getting fuel. I also have big hands, and find it a real pain to get those wires on and off and plugs in and out. It's about a three hour job. Does the intermittent light indicate anything other than a possible loose wire? I'm thinking that the only way I'm going to troubleshoot it is to take each lead off and check the resistance of the plug and the voltage of the lead. I'm just hoping that someone will chime in with some answer that prevents that like, "this relay or inline fuse is likely blown- check that. It's located here." Have a good one, Bill Bill... everything I said here is the right way to do so. I am suspecting you probably have a bad relay unit. When you change out the glow plug, you should change the relay unit. They also wear out too. Another possible cause is fusible link for the glow plug too. I got big hand too. Long handle needle nose plier helps alot. Get a set at Costco. > Tom... first thing first, measure the resistance of each glow plugs. you > have to disconnect each glow plug so you can measure each one [quoted text clipped - 7 lines] > measure the voltage at the glow plug. When the light stop working, it > means that one of your glow plug is bad. Tiger, is it not the case if only one plug goes out then the glow light will flash once the car starts. It will then go out once the relay opens. If 2 or more go out, then I believe the light will not come on. Paul Paul Hoffman Burlington ON phoffman@cogeco.ca or hoffmanp@mcmaster.ca 1980 300D 280,800 + km I am not sure... but it really doesn't matter because something is wrong and it is really easy to tell which glow plug is bad. You don't need that dummy light to tell you that something is wrong.... just by starting up the car, it will tell you if anything is wrong. It really depends on the relay and year... On my 95, when my relay is bad, the light does not come on... and if one of my glow plug is bad... after starting the car, the light will stay on for a while before it goes away. |
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