Glow Plug Wiring Question

[bhammontree]

Hello

I have a 1967 200D and noticed that there is a ground strap running from the block over to a location very near the front glow plug on my car. The strap looks like it should connect to the plug but was not connected when I inherited this car a few months ago. The car starts ok but takes a minute or so for the indicator to get orange. I found a wiring diagram online that seemed to show the last plug in the series grounded to the block so I hooked it up to see what happened. The indicator in the cab got hot much quicker, but the car would not start, and the battery terminal started to smoke. I disconnected the strap and the car starts fine now. Just wondering what the ground strap is for?

Thanks

[cleeves]

Originally Posted by bhammontree

Hello

I have a 1967 200D and noticed that there is a ground strap running from the block over to a location very near the front glow plug on my car. The strap looks like it should connect to the plug but was not connected when I inherited this car a few months ago. The car starts ok but takes a minute or so for the indicator to get orange. I found a wiring diagram online that seemed to show the last plug in the series grounded to the block so I hooked it up to see what happened. The indicator in the cab got hot much quicker, but the car would not start, and the battery terminal started to smoke. I disconnected the strap and the car starts fine now. Just wondering what the ground strap is for?

Thanks

I don't have a 200D and am not familiar with your particular car. But it's possible that someone re-wired things (e.g. for pencil type glow plugs or something else) in a way that is different from stock. If it were me, I'd find the schematic see if the existing circuit matches.

Or, perhaps poke around with a voltmeter when its on. If they are wired in series, and you have 4 glow plugs, you should measure 12V - 9V - 6V - 3V down the row, between the positive end and ground, unless there are resistor wires between some of the plugs. It sounds like you connecting the ground strap shorted either all or perhaps all but one of the glow plugs. You might just pull all the plugs and check their resistance anyway to see if they are good.

-Henry

[Sugar Bear]

Does your car have wires to each plug or are they connected through squiggly rigid wire?

[801mbz]

ground wire was for the old slow glow plugs

[bhammontree]

My car has a flexible heavy copper wire ran in series from the resistor on the firewall to the glow plugs. I am assuming these are the old style plugs since it takes about a minute for the interior indicator to glow orange, but maybe it was upgrades at some point before I got it.

[jsb357]

Originally Posted by 801mbz

ground wire was for the old slow glow plugs

^^^ this if someone replaced the loop style plugs with the pencil type which are internally grounded at each plug

[gmog220d]

Originally Posted by bhammontree

My car has a flexible heavy copper wire ran in series from the resistor on the firewall to the glow plugs. I am assuming these are the old style plugs since it takes about a minute for the interior indicator to glow orange, but maybe it was upgrades at some point before I got it.

What connects the glow plugs to one another? A solid bent wire with ceramic insulators like what you can see in the photo below (for loop plugs wired in series)? Or flexible pieces of wiring without the ceramic insulators (for pencil plugs wired parallel)?

[801mbz]

you have the fast glow plugs do not use ground wire

[strelnik]

Originally Posted by 801mbz

ground wire was for the old slow glow plugs

My '67 200 gas car had a woven ground wire under the car. In the old days, the Germans used two ground wires: one from the (-) post to the frame and another from the frame to the engine.


Merkins usually didn't replace the one underneath when it got corroded and complained about the electrical system.


I started to reinforce the ground system when I got my first W123. I kept the ground wire to the body and added a second cable from the (-) post to the rear of the 616 block.


I am curious about slow vs fast older glow plugs I have a 1950 OM636 and a 58 180D that use the big candle stick plugs and the series-connected wire. Are those "fast" or "slow'?

[Frank Reiner]

Originally Posted by 801mbz

ground wire was for the old slow glow plugs

Originally Posted by bhammontree

My car has a flexible heavy copper wire ran in series from the resistor on the firewall to the glow plugs. I am assuming these are the old style plugs since it takes about a minute for the interior indicator to glow orange, but maybe it was upgrades at some point before I got it.

Originally Posted by 801mbz

you have the fast glow plugs do not use ground wire

How has it been determined that the OP's engine is equipped with "fast glow plugs" ?

Do "fast glow plugs" = pencil type?

[Shortsguy1]

Frank-
If they were wired in series, and the ground wire was disconnected, the GPs would never run. So we know they were wired in parallel at some point. When the OP grounded the wire (with parallel plugs), all of the electricity went to that one wire, bypassing the plugs. No glowing occurred which is why he could not start the car.

Generally, yes, I think people say that fast glow plugs and the pencil plugs are synonyms. If the had the old loop plugs and they were wired in parallel, they would all have fried very quickly. The loop plugs are only designed to handle 1.5 volts, so 12 would be way too much.

But this is definitely a situation where a quick photo of the GPs and their wires would easily answer this one for us all.

[bhammontree]

I will take a picture of the wiring tonight and post, but it is wired in series with flexible copper wire daisy chained from the firewall to the plugs. Very possible that this is not the original setup and was modified by the previous owner.

[Shortsguy1]

A wire running between the plugs does not mean they are wired in series. It is probably providing 12 volts across each of the plugs, meaning they are wired in parallel. That copper wire is providing roughly the same voltage to each GP, and then the engine block is taking the electricity away (the ground).

People often remove the salt shaker GP indicator when wiring for parallel, but that didn't occur in your car. So instead of each plug getting 12 V, they are each getting somewhat less than 12 V. That might explain the long warm up time despite having plugs in parallel (the normally faster setup).

[bhammontree]

Here is a shot of the wiring.

[bhammontree]