Question on repairing 300SD glo-plugs

• posted on October 25, 2005, 5:38 pm
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.
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• posted on October 25, 2005, 9:10 pm
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.
--

73
Hank WD5JFR
"Geronimo" < snipped-for-privacy@somewhere.net> wrote in message
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<%-name%>
• posted on October 25, 2005, 10:10 pm
Check the 80 amp fusable link under the cover of the glow plug relay. The cover slides up.
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<%-name%>
• posted on October 26, 2005, 12:41 am
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

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• posted on October 26, 2005, 5:11 am

What about the fuel? Did you put any additives in it? How old is it?
Marty
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<%-name%>
• posted on October 26, 2005, 12:00 pm
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

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• posted on October 26, 2005, 12:51 pm
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.
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• posted on October 26, 2005, 2:22 pm
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!
On 26 Oct 2005 05:51:36 -0700, " snipped-for-privacy@optonline.net"

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• posted on October 28, 2005, 2:05 pm
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!
wrote:

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<%-name%>
• posted on October 29, 2005, 5:54 am
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!
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<%-name%>
• posted on October 29, 2005, 8:59 am
On Fri, 28 Oct 2005 22:54:55 -0700, "T.G. Lambach"

==========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.