unique glow plug problem? - PeachParts Mercedes-Benz Forum

bill.schweig · #1 12-16-2006, 01:10 AM

as much as i have scoured the threads here, i still can't seem to locate someone with the same GP situation. ok, to start out...

background:
1980 300CD 281k
replaced glow plugs and battery Jan '06
anti-gel formula in diesel tank
have not run veggie oil in 2 weeks

problem:
about a week ago, i began to start the car. temp was cold but not bad (40). the glow plug light came on like normal. i heated the GP 2 or 3 times as i usually do in colder weather. cranked. turned over, fuel being delivered, etc, yet car wouldn't run. when i released the ignition, the GP light flashed on and off indicating some kind of problem with the GP (i don't think it indicates anything in particular, just that there is a GP problem?).
heated, cranked, hooked up to battery multiple times. checked visible wires. no luck!
my car was about to get towed so i used starter fluid on it and it finally started.
back at my house, i tested the GP resistance, all except one were running 30,000 ohms (registered 'infinity' on the 200 range). rechecked connections w/ multimeter, no problems. i wasn't present, but my friend checked the relay to the best of his (and my) ability and couldn't find any problems indicated by voltage/resistyance, fuse is intact.

replaced all 5 GP, even replaced some wiring and filed down leads on battery to make sure connections were good.

car still won't start, GP light still turns on for 'heating' and flashes on and off after cranking to indicate problem.

questions:
1. why did the GP go bad after only 11 months (14k mi)?
2. what is wrong with it and are the GP themselves even responsible for the starting.

any answers that would spare me from slicing up my knuckles again taking out or installing GPs would be excellent. if not, so be it.

thanks!

phil2810 · #2 12-16-2006, 01:58 AM

If your reading that high of resistance on your GP's, it wouldn't matter if the relay was good, you wouldn't heat at all. It does seem odd they all read approx the same...... could you detail how exactly you measured them (ie, disconnect relay connector and checked the GP itself to ground or the connector to ground). What wiring was replaced?

#3 12-16-2006, 02:08 AM

There must have been an error in your measurement. The glowplugs should read in the 0.5-1.0 ohm range.

bill.schweig · #4 12-16-2006, 02:21 AM

Quote:

Originally Posted by ForcedInduction

There must have been an error in your measurement. The glowplugs should read in the 0.5-1.0 ohm range.

the one good one did read 0.6, one read something like 10k and the rest read up to 30-35k. the last 4 read infinity on the 200 ohm range, i just checked it at higher range b/c i was curious. hayes manual says if the meter reads infinity they are bad.

i tested them by disconnecting the relay and testing plugs themselves. also checked them after taking them out and got same readings. i just re-did the wire terminals where they attach to the GP b/c the insulation was frayed...i doubt it had much to do with the problem but i don't know a whole lot.

t walgamuth · #5 12-16-2006, 06:34 AM

what brand of glows did you use?

if not bosch then i suggest just getting bosch and replacing all of them. this is one place where the brand matters.

bosch is best, imho

tom w

running-snail · #6 12-16-2006, 07:16 AM

Did you ream the glow plug cavity before inserting new plugs?

I've never glowed more than once when starting, certainly not at 40 degrees. - is this typical when running WVO?

Palangi · #7 12-16-2006, 12:31 PM

Quote:

Originally Posted by t walgamuth

what brand of glows did you use?

if not bosch then i suggest just getting bosch and replacing all of them. this is one place where the brand matters.

bosch is best, imho

tom w

I agree with Tom. This sounds like Autolite or some other mystery brand of glow plugs. Get a set of the proper Bosch plugs and this problem will be gone.

Brian Carlton · #8 12-16-2006, 03:30 PM

Quote:

Originally Posted by bill.schweig

i wasn't present, but my friend checked the relay to the best of his (and my) ability and couldn't find any problems indicated by voltage/resistyance, fuse is intact.

Please provide the exact details of how your friend checked the relay to be sure the plugs are getting 12V when the key is in the #2 position.

bill.schweig · #9 12-16-2006, 04:48 PM

Quote:

Originally Posted by Brian Carlton

Please provide the exact details of how your friend checked the relay to be sure the plugs are getting 12V when the key is in the #2 position.

ok, because my friend didn't do so hot a job explaining it over the phone, i retested the relay myself to the best of my ability and i have a couple questions.

first i tested resistance thru each socket in the GP relay connector. There are 8 sockets. What does each number next to the socket stand for? shouldn't there only be 5?

my meter reads .3 in its own wires so i subtracted that from the reading...

AFTER replacing with new bosch glow plugs, #2, 3, 5, 7 all tested .5 or .6 - good right?
# 8, 6, 1 all tested infinity - bad, right?
# 4 didn't have any conducting material in the socket. what is the deal with that?

tested large guage wire and each side of 80a fuse for voltage, all tested same as battery 12.6-12.7

then, with ignition on, GP light on, tested each pin in relay for voltage.

each pin tested battery voltage 12.6-12.7 except for #6 and #8. (#6=7.1v and #8=5.6v). again, i don't know if this is even significant since, i'm assuming, only 5 of these pins are supplying power to the GPs, what are the rest for?

this is all i knwo how to do, and i'm not even so sure what all these values are indicating...

OMEGAMAN · #10 12-16-2006, 07:09 PM

Since you have replaced all 5 glow plugs you can rule those out.
Get a decent test light. Not one that uses LED's Just a simple one that uses a nice bright bulb.
Turn on the key and see if you get power to all five glow plugs and report back.
It sounds like you have a broke wire to one or more of your glow plugs and this will tell us if so and which ones

whunter · #11 12-16-2006, 07:45 PM

The glow plug harness can go bad, it is cheap to replace.

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Quote:

Originally Posted by bill.schweig

ok, because my friend didn't do so hot a job explaining it over the phone, i retested the relay myself to the best of my ability and i have a couple questions.

first i tested resistance thru each socket in the GP relay connector. There are 8 sockets. What does each number next to the socket stand for? shouldn't there only be 5?

my meter reads .3 in its own wires so i subtracted that from the reading...

AFTER replacing with new bosch glow plugs, #2, 3, 5, 7 all tested .5 or .6 - good right?
# 8, 6, 1 all tested infinity - bad, right?
# 4 didn't have any conducting material in the socket. what is the deal with that?

tested large guage wire and each side of 80a fuse for voltage, all tested same as battery 12.6-12.7

then, with ignition on, GP light on, tested each pin in relay for voltage.

each pin tested battery voltage 12.6-12.7 except for #6 and #8. (#6=7.1v and #8=5.6v). again, i don't know if this is even significant since, i'm assuming, only 5 of these pins are supplying power to the GPs, what are the rest for?

this is all i knwo how to do, and i'm not even so sure what all these values are indicating...

bill.schweig · #12 12-16-2006, 08:09 PM

Quote:

Originally Posted by whunter

The glow plug harness can go bad, it is cheap to replace.

what is the gp harness and can it affect only a few gp as my problem seems to indicate?

864Lightning · #13 12-16-2006, 08:59 PM

I will add a little ,, I just bought my first Mercedes to tinker with 1979 300SD. Just got thru with a valve adjust today.. I went thru the glow plugs last weekend. Here is what I found..

I pulled the plug off the relay and checked the resistance to each plug , .7 ohms was the average. Use the ohm meter to find which pin goes to which plug..
Then checked the voltage at each plug while the relay was heating.. 11.2 volts to each plug.
Next step was to use a regulated Hewlett Packard 50 amp power supply ,set at 11 volts ,and test each plug at the relay plug harness.
Each plug pulled about 24 amps to start with and then dropped to 14 amps after about 5 seconds.

Since I was doing a compression check thru the glowplug holes, I was going to change the plugs anyway.
With the old plugs out I hooked them to the HP supply to observe the effects compared to the new ones.
The new ones instantly glowed red on their tips. The old ones, pulling the same amperage as the new ones, only smoked the carbon on the tip but never glowed red like the new ones.. Both the old and new were Bosch,, the new ones had a different tip,, the last quarter inch was smaller.
With the new plugs installed the startup was much quicker and smoother,, I could tell a huge difference.