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Car Forum / Mercedes-Benz Cars / November 2005Question on repairing 300SD glo-plugs
Re: its a 1985 300 SD. The temp went down to about 45 overnight, and in the morning it took a long time to start... when in warm weather it starts instantly. Just last year I think I replaced 4 of the five glo-plugs myself, and paid a mechanic to replace the one in the rear. . So they shouldn't be burned out already!(On this car, the glo-plugs are wired in parallel, BTW) I put a volt meter on each one....they all have 12 volts power. My DC amp-clamp is too big to get onto each glo-plug wire, but I did get it around the bundle just ahead of the first glo-plug, so I was measuring the total current. It read only .2, or 200 ma. So the question is, how much current should a glow-plug draw? Should I be able to feel heat on the tops of the glo-pugs after a few cycles? The relay appears OK, they stay on for around 1 minute. I guess tomorrow I will have to read resistance to ground at each terminal, to confirm if open/bad. Come to think of it, from the resistance, I should get an idea of about how much current each should draw. If the glo-plugs are all working, does this mean the compression of the engine is way down? It is not burning very much oil. Smoke billows out of tail pipe only for the first few seconds after starting, then there is very little smoke, and it does'nt appear bluish. Thanks all. By measuring the voltage at each GP you have established that the leads are OK and voltage is getting to the GPs. GP resistance is much less than one ohm so each will draw over 12 amps, I=E/R. IIRC the main fuse is 80 amps and whatever you measured is way off. Each GP should measure close to a short or not quite zero ohms and depending on your ohm meter you should see approximately 0.5 ohm depending on the accuracy of the meter, resistance of the test leads and how good a contact you make with the plug. If a plug is shorted, the meter will read pretty close to what you read when you short the leads. If its open it'll read very high. Just like new light bulbs failing prematurely, GPs can too.  Signature73 Hank WD5JFR > Re: its a 1985 300 SD. The temp went down to about 45 overnight, and > in the morning it took a long time to start... when in warm weather it [quoted text clipped - 17 lines] > starting, then there is very little smoke, and it does'nt appear > bluish. Thanks all. Check the 80 amp fusable link under the cover of the glow plug relay. The cover slides up. Such links are available from dealer and on-line for about $1 each. Buy a spare. With 12 volts on the GPs the fuse has to be OK, unless it has sagged and cracked open and then there wouldn't be 12 volts on the plugs. Hank > Check the 80 amp fusable link under the cover of the glow plug relay. The > cover slides up. > > Such links are available from dealer and on-line for about $1 each. Buy a > spare. > Re: its a 1985 300 SD. The temp went down to about 45 overnight, and > in the morning it took a long time to start... when in warm weather it [quoted text clipped - 17 lines] > starting, then there is very little smoke, and it does'nt appear > bluish. Thanks all. What about the fuel? Did you put any additives in it? How old is it? Marty Greetings all, The reading of "amp draw" in a bundle of wires is not a good practice. Any current flow "TO" a device on one wire, is "RETURNED" on the other wire. The net is "ZERO". Separate the wires and you will read current to the device in question. The value of 200ma is probably from a device that does not have a return wire in that bundle. Hope that is understandable. Also DC Clamp on Amp meters have polarity associated with the position of the clamp head. So the polarity value might change depending on the direction that the meter is "clamped". Bill > Re: its a 1985 300 SD. The temp went down to about 45 overnight, and > in the morning it took a long time to start... when in warm weather it [quoted text clipped - 17 lines] > starting, then there is very little smoke, and it does'nt appear > bluish. Thanks all. Greetings all, The reading of "amp draw" in a bundle of wires is not a good practice. Any current flow "TO" a device on one wire, is "RETURNED" on the other wire. The net is "ZERO". That is true in the general case. But for glow plugs, as for many loads in autos, the return path is through the metal of the car itself, not back through a cable bundle. For glow plugs, the return path is through the engine block, isn't it? So it would seem to me a clamp on DC current tester used over the entire bundle of glow plug wires as the OP stated should read the total draw. Exactly....the return path is through the block on all plugs, so that is why I should get a valid reading of amperage by clamping around the whole bundle before it branches to individual plugs. Didn't think about this yesterday when checking for power with my hi-impedance multimeter, but it is possible that the main power wire or glo plug relay is not completely open, but resistive. I have seen this before on aircraft electrical systems. Checks good with a hi-Z DMM, but when you put a test light on it, which gives the wire a load, the wire cannot supply any appreciable current because of abnormal resistance, so test light does not illuminate as it should. Will have to recheck with a test light. The gas is fresh diesel, BTW. The temperature overnight never got below about 50 deg., yet I had to crank for a long time before it would start this morning. Now that I know what amount of amperage I should have, I should be able to figure it out. I have the diagrams in the Haynes manual to go by. How in the world do you make sense out of those German electrical schematics? US/American is so much easier! Thanks all! >Greetings all, > [quoted text clipped - 10 lines] >DC current tester used over the entire bundle of glow plug wires as the >OP stated should read the total draw. I found good power with a test light at all the glo-plugs, so I read resistance of each. All five plugs are burned out! This despite replacing the plugs only about a year ago. I have never had voltage regulator problems where the voltage was going over about 13.5 - 14, so I don't know why all of them burned out so quickly. They are Bosch glo-plugs, made in France. Got them at Autozone....I think they cost 9.99 each....when NAPA wants about $25 each! Maybe I would have done better to buy the $25.00 ea glo-plugs, if they last a lot longer! >Exactly....the return path is through the block on all plugs, so that >is why I should get a valid reading of amperage by clamping around the [quoted text clipped - 31 lines] >>DC current tester used over the entire bundle of glow plug wires as the >>OP stated should read the total draw. Beru also makes an excellent GP. There's something wrong in the glow plug relay that caused all five GPs to fail within a year. The relay isn't shutting off the current and the GPs are glowing and glowing and glowing while you drive. No GP can survive that. New ones will work for a few days or weeks and then will need to be replaced. GPs usually last a long long time. The original GPs in my old '80 300SD lasted about 100K miles - 22 years! >There's something wrong in the glow plug relay that caused all five GPs >to fail within a year. The relay isn't shutting off the current and the [quoted text clipped - 5 lines] >GPs usually last a long long time. The original GPs in my old '80 300SD >lasted about 100K miles - 22 years! =========== Some people think that when the dash glow plug light goes out, the relay de-energizes the glow plugs. Not true! The glow plug relay keeps them on for quite a while after the vehicle starts. I have a voltmeter in my old Panzerwagon and it stays under 12 volts for about 3 minutes or so, it seems, then the voltage returns to the normal 13.8 or so. And there is no relay circuit control tie in to coolant or head temperature, so that the glow plugs go through the cycle again on restarts, even if the engine is warm. So when I go down to my rural post office and take 2 minutes to pick up the mail, I just leave the engine idling so that the plugs do not have to go through another heating cycle unnecessarily. It drives the local environmental militants nuts. TGL, Yea, that must be what was happening to make them all go bad so fast. But when I was troubleshooting them yesterday, I pulled the glo-plug connector off, and put my test light on each pin, then cycled the ign. switch for each one. Yes, they stay on for longer than the inst panel light, but after about one minute the relay deenergizes/ test light goes out. So There is nothing wrong with it now...which does not make much sense. I thnk I will put a switch on the dash to enable/ disable the glo plugs. They aren't needed at all in the summer. Come to think of it, it would be very easy to add an LED on the dash that would indicate when the glo-plugs are actually on! The next owner of the car will be the junkyard probably, so I just mod/rig up the car however I want. My diagram on CD shows that there is at least an ambient air sensor of some sort for the glo-lug timer. Is this not correct? I just hate these post- 1990 cars: highly computerized, with front-wheel drive /transverse mount engines! Only a pro with specialized equipment can work on them. Often you can't even change spark plugs on them yourself, as the back of the engine is right up against the firewall! It s a conspiracy! ;-) Thanks, Jim >>There's something wrong in the glow plug relay that caused all five GPs >>to fail within a year. The relay isn't shutting off the current and the [quoted text clipped - 23 lines] >have to go through another heating cycle unnecessarily. It drives the >local environmental militants nuts. " My diagram on CD shows that there is at least an ambient air sensor of some sort for the glo-lug timer. Is this not correct? " There has to be some kind of temp sensor in the glow plug control because the dash light indicating it's time to start cranking goes off quickly when it's warm out, or if the engine is still warm. In winter, it takes much longer for the glow plug light to go out It sounds like the temp sensor may only control the dash light and not how long the glow plugs actually stay on. And it must be mounted on the engine, measuring that, not the ambient temp, since the light still goes out quickly in winter if the engine is warm. GPs glow for about 30 sec. after the GP dash light goes out - and they should, but that's all. (New CDI diesels burn their GPs for another minute or two after the engine starts to reduce smoke and emissions while the engine is still cold.) GPs are needed to start the engine in all seasons (to avoid long cranking); in '80 there were two temperature sensors associated with the relay - ambient and engine. The '80 GP relay was a one time affair so you car's GP relay may differ in that respect. That all your car's GPs burned out simultaneously leads me to suspect an electrical problem either in the GP relay or the car's charging system. Reference: 1980 M.Y. GPs operate on 11 volts and draw 7.5 - 9.5 Amp after 60 sec. |
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