My 240D has been stranded in a parking lot for a couple days since my son couldn't start it during a recent cold snap. It had warmed up a bit yesterday so I tried to start it with a little jumper assist. I ran it through a couple glow cycles, then tried to start, with no success. I think I tried that one more time. Then the third time I tried to cycle the glow plugs, they didn't glow. No starting success, so I left for the night.

It was warmer today, so I went back again and no glow. I hoped maybe it was the glow plug fuse, but I checked that and the fuse seemed intact. The pre-glow light on the dash does not come on.

What do I do next to diagnose the problem? (These are old style glow plugs wired in series)

Also, can a 240D with auto trans be towed a short distance (a couple blocks) at slow speeds with the wheels down? I need to get the car out of the lot pretty soon.

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-Scott J
1978 240D

Those fuses can crack on the end and appear to be good when they're not.

The best way is to check with a voltmeter. First make sure that there is about 12 volts on the end where the lead from the relay connects. Use black lead on ground and red lead on that wire from the relay or fuse block. If you don't have 12 volts there, then check it on both sides of the fuse. If nothing on either side, go back to the relay. If there is 12 volts on the positive end of the glow plug string, then move along checking voltage on each plug. Each plug should drop about 2.5 volts. So the one electrically closest to ground should read out 2.5 volts referenced to ground, the next one abou 5 volts and so forth.

If it is no more than two blocks you probably could tow it that far in neutral, but I personally would not do that unless absolutely necessary.

Hope all this helps.

This is a link to a plethora of info on diagnosing glow plug issues. As I am doing presently.


Glow plugs link thread

When one Glow Plug goes bad on your year and model it creates an open circuit and all of them do not work.

Some helpful info:

Bosch made in India glow plugs 220D

When you have old Cars being a member of one of the Auto Clubs that has some free towing and other services can save your butt.

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84 300D, 82 Volvo 244Gl Diesel

Yes, using an ohmeter across the whole string can quickly tell you if there is an open circuit or not.

It's been a long time since I messed with loop plugs, but I remember having trouble with crud building up around them. Even though they would glow they couldn't transfer enough heat. I made a reamer that fit in place of the removed glow plug and cleaned out the goo.

Hope this helps.

Thanks, gentlemen. That is very helpful. I had it towed on a flatbed today. Flatbed dropped it right into my garage. I'll get to work diagnosing tonight or tomorrow.

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-Scott J
1978 240D

Started diagnosing. There is continuity across the four plugs. There is no power at the fuse holder. I'm thinking it must be the relay.

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-Scott J
1978 240D

Does anyone know a good alternative to the $250 new MB relay?

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-Scott J
1978 240D

Many years ago I swapped an engine from an early 123 parts car for temporary use. I didn't want to change GP relay. I mounted a Ford starter relay on the fenderwell near the battery. I used large cable from positive post of the battery to one large post of the relay and connected the other large post to the supply side of the fuse block. I then got a momentary rocker switch that fit in one of the rectangle holes in the dash. I wired one side of that switch to a hot and the other side to the primary post on the starter relay. This gave me a way to supply the high current to the glow plugs without frying something.

I had no fancy indicator to tell me how long to glow with the rocker switch, but I got the feel for it based and temp after a few days. Under glow at first and if it doesn't start, glow a little more.

Hope this helps. PM me if you want more details or don't understand how to do it.

The old 240D controllers are hard to find, especially a good working one. You can send it to me to convert to manual control (with a push button switch). It will be a lot less $$. I converted one for the Don's 78 or 79? 240D. I also convert the glow light control so that it is on as long as the switch is pressed.

Ps, this is a much more elegant solution than a Ford starter relay since you keep the original wiring except adding a wire to the stock relay for the push button switch. You do not need to hack up the OE wiring to the glow plugs.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks. I may go that route. I also just discovered to my great surprise that my local u pull it actually has a '78 240d. I'm going to stop in and see if they have the relay.

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-Scott J
1978 240D

There was no hacking of the wiring. One wire was simply temporarily disconnected and the wire from the relay connected on that side of the fuse holder. When I went back to the original the only trace of the temporary solution was two holes left when the starter relay was removed.

The man was asking for help and I was trying to help. No need to accuse me of hacking.

No worries guys. I consider both good advice. I'm going to see if the one in a yard has a relay and if that doesn't work out I'll move on to one of those solutions.

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-Scott J
1978 240D

OK, I picked up a glow relay for $5 at the pull a part. Turns out that wasn't the problem. When I was doing my previous diagnosing, I somehow goofed up. I thought I didn't have any power at the fuse block. When I checked again today, I DO have 12V at the entrance to the fuse block. I also have 12V at the exit from the fuse block and at the first glow plug in line. However, I have 0 volts at any of the other three plugs. Does that mean my first glow plug (#4 cyl) is bad?

Also, any tips on removing the plugs?

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-Scott J
1978 240D

Trying to post a pic of what the Ford type Starter Relay/Solenoid looks like.

I used that type of Relay on my Volvo Diesel and have not had an issue since about 1992.

I used a separate Switch but there is no reason that someone could not locate the Wire from the Ignition Switch that would activate the stock relay and use that.

The Volvo also has an expensive aluminum box type relay on it.

PIcture and description in post #3.

Diesel Discussion - PeachParts Mercedes ShopForum

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84 300D, 82 Volvo 244Gl Diesel