I decided to purchase a ’77 300D about six months ago – my first venture into a diesel model. It had been garaged for the past 6-7 years without an attempted start. Once started, she runs well – operative phrase being “once started”. I initially changed all 5 plugs (series, loop style) the inline 50-amp fuse, and the ground (which looked ratty). Started fine for about 4-weeks – then one morning the indicator lamp did not turn on and the car refused to start. After testing the plugs, found out that #5 was toast. Changed the plug, car started fine.

Went another 3-4 weeks, same thing. This time #4 and #5 were toast – changed the two plugs – this time nothing. Proceeded to change out the remaining three plugs along with another fuse – nothing. The relay seemed to work well (indicator light duration comparable to ambient temp). Plus all five plugs registered about 2.5 – 3.0 on a voltage test which should mean the relay is working okay. I will check the timer just for the hell of it, but I really don’t want to buy another part that is not the problem.

At this point, the only thing I can think of is that the pre-chamber is full of deposit which may be shorting out the plugs. Would this cause enough of a problem where it causes a break in the series? Besides using a reamer (which I don’t have the tools or the skill) to clean out the bores, is there any other method to blow this crap out? Should I consider looking at the timer relay even though voltage is reaching the plugs? Any info would be appreciated!!!

Usually it is the GP that are the problem. Most times Bosch are the best, but even they have given us all fits at times. If you use others, like Champions glow plugs or Autolite, they are seldom found working after a few weeks.
Doubtfull the prechambers are the problem. Car will start with a gooked up prechamber.
Use your voltohmeter to check for 12 v at the GP's. If all is good, crack an injector line and see that fuel comes out when the engine turned over. If not, then you may have a fuel problem, look at the filters first.

If it was a fuel problem would that cause the glow indicator light to shut off??

I would just want to check the actual voltage to the plugs is being turned off at end of relay cycle. If they are being left on for most of the time would expect a higher rate of burnout.I agree you should not be going through glow plugs at the rate you are. The thing that is kind of bothering me a little is your voltage drop of 2,5-3.5 across each plug. The wires between the plugs are resistance elements themselves I thought. Actual voltage drop across plug should be in the order of 1.2 volts. Not really sure as you may be reading them somehow differently. A far better test is to do a current draw but you have to borrow, beg, or steal an ampmeter with at least a 50 amp dc range or even more perhaps. Have a 77 300d myself with the old loop plugs but no trouble with the setup so far. Since the loop end of the plug is electically hot I think your thought of grounding through the carbon has some possible merit as well. You could take an old burnt out plug and weld something a little bigger than the loop on the end. Then thread this modified plug into each hole and that would tend to ream out any carbon deposits in the imediate area of the loop but be careful. Just a thought and I would do it if felt it was required but cannot recomend this ideal to others as have not done it.

Krueger, thank god at least the fuel system and glow plug system are totally separate on these cars and any type of overlap virtually impossible. Does not exclude the possibility of him having fuel problems as well just that to the best of my limited knowledge they cannot be interrelated with the glow plug circuit as they have no common element to the best of my limited knowledge. Of course I make errors sometimes so allow for that please.

In regards to cleaning out the pre-chamber - I read in an old thread that someone recomended cranking the engine once the plugs were out. This apparently would blow the crap out. Would anybody else recomend this?

Certainly would cause no harm and who knows? Actually a good ideal to try. Just keep your head out of the engine bay while cranking I suppose. Certainly would be a good thing to do after you loosened up some stuff in there before installing the glow plugs.

I'll try cleaning the pre-chamber tommorrow and see if that will make a difference. Otherwise I'll have to check the plugs individually - what would be the best way to test this? Could I bypass the system and hook the plug up to the battery to see if it heats up? (Doesn't sound safe)...

It's a 77' model,

I believe you can buy a pencil style update kit for that engine. That puts the later (ie start in colder weather ) plugs in that engine.

The carbon should burn itself out with hard driving.

Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E

Krueger, you can't test the series plugs that easily with a 12 volt system unless you hooked them all together outside the car, using all of the interconnecting wiring. I had exactly the same problem that you have, and the only solution was to upgrade to the parallel plugs. They are totally trouble-free in my case.

The series (loop style) plugs each use up about 2 or 3 volts in the series chain which is 12 volts. Your suggestion would be kinda like plugging a 12 volt bulb into 110 volts. Not enough resistance and the series plug loop would melt very fast. Go for the upgrade to parallel (pencil tip) plugs. That is a huge improvement!

i would be reluctant to try the welding of a larger loop on the old plug. it it breaks off int here it would prob be a pita.

also i think removing all the glow plugs in an effort to blow stuff out would not be very effective and a lot of work and also a pita.

also i am not sold on the fast glow plug conversions for the old loop style plug equipped engines. i did this on my old 115 and had problems with short longivity. before i came to this conclusion i bought extras in anticipation of them becomming unavailable. so i have a set in my garage free to anyone brave enough to use them. again i dont recommend it.

best setup is usually factory. imho.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Thanks for all the suggestions guys.

So over the weekend I pulled all the glow plugs out and attempted to clean out the bores with a makeshift reamer. Then I cranked the engine for about 20 seconds which produced a nice soot cloud. After cleaning up a bit, it seemed like the bores looked pretty good. Then performed a fool-proof test to see if the plugs were working. I connected the plugs in series (same as they are in the engine) attached a ground on one end and then a power cable from the battery to the other end. All plugs heated up - so that problem was solved. Re-installed the plugs and connections, turned the key in the ingnition - still nothing. At this point I believe it has to be the timer relay. However I never had a problem with the relay - it was solid. Could it just blow while driving?? I also still can't find the thing under the dash. I've found every other relay / amplifier / connection under there - any suggestions on the exact location???

mine is under the hood on the driver side....my car starts and runs find but i keep having problem with starting because the battery would drain....needle on my instrument panel would jump all over the place. after pulling the dash and check for bad grounds, i opened up the preglow relay and found parts inside were burned...

here is what the relay looks like

http://img.eautopartscatalog.com/live/P205812031BER.JPG

Krueger, have you connected a voltmeter to the glow plugs and checked the voltage? Your plugs seem to be OK since you got them all to glow with a separate wire from the battery. You should have about 10v and decreasing over time at number 5 when you first turn on the glow plugs. I wonder if you are getting any power there at all using the fuse and relay circuit.

A glow plug shorting out will just eliminate the ones downstream toward the ground side, thereby spreading out the current over fewer plugs. If this happened at number 1, then you would have 4 plugs consuming the currrent that 5 normally do and likely no plugs would burn out. If however number 2 shorted to ground, you would have plug 1 consuming all of the current and it would quickly burn out.

With your glow plugs on, using the fuse and glow plug relay source, check out the voltage as you go from one plug to the next. You should have a decreasing voltage as you probe from 5 to 4 to 3, etc. to 1. This voltage drop will be proportional to the current consumed at each plug. I don't think you need to crank the engine, just get the glow plugs to go on.

It seems that you could have a couple of things going on here. First plugs shorting out and burning out, leading to an open circuit. Then possibly a dead cirucuit. I could be wrong, just a hunch.

Krueger,

Not sure about the relay location. Mine is uner the drivers side dash, but mine is '79 that used to have the default series plugs. Check your fuse as well, its on the firewall near plug 5, just follow the wire.