My kids 240d burned up another loop GP last week so she ended up calling me to come start the car. (of course) Anyway I decided to upgrade the system to fast glow pencil style plugs. Was able to pull a wire harness from a 300d (for free since I was buying 4 new plugs from the place) and use it on the 240 with slight modification. Works perfect except it seems to take about the same amount of time to glow as before. Does the 240d (77 123 style) relay have a lower voltage or allow less current to the plugs than the 300d? My 300d will heat and be ready to start in about 15 seconds, the 240 still takes about 40 seconds.

Are you going by the glow plug light on the dash to decide when its "ready to start"? If so perhaps the sensor electronics that work the dash light are not operational in your 240D installation.

Maybe you can just count to 10 and the 240D will start OK.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

I am using the dash light and it seems to work on both cars. If you don't wait for it to go out on the 240 the car cranks for about 10 seconds then starts rough. Let it go the full time and it fires right up and runs great. It is not a problem but was just wondering if it is the norm on with the new style plugs I put in the car.

Rebootit,

Check the voltage at the GPs. It should be the same as the battery voltage minus a small IR loss.

You might also remove a GP and visually see how fast it glows to an orange hot. Then compare that time with the GP directly connected to the battery. Be careful not to burn yourself on the hot GP.

P E H

I would think where you are 10 seconds would be sufficient.

Is there a size adapter required to go from loop to pin glow plugs in bolting them into the head? Just asking for future reference.

In reading over your first post, I don't quite see where you said that you changed the relay. Did you pull a glow plug relay from a 300D?

So if you used the original series-style relay, I suppose you had to wire each of the four parallel glow plugs directly to the relay? Perhaps that connection has some resistance to it and is not able to provide sufficient current. In fact, your old 240D relay may not be up to the task. I think you could probably use a newer-style 240D/300D relay with some modifications to the wiring in the car. If you happened to get a 300D relay, just don't use the 5th connection. [ I am sure you are not using the same 240D series-style relay along with the original wiring - that would not work. ]

For my glow plugs, I always run a rather dangerous, but effective test. I hold them with a set of lockjaw pliars and put 12v on them. They need to glow red hot within about 2 seconds. If they don't they are bad. If they are in the car I can get just about the same test accuracy by measuring their current draw, but you need a 20-amp capable ammeter.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

40 seconds sounds way too long. I never glow my 240D more than 5-10 seconds and it starts great.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

I didn't get the 300d relay, just the wiring harness. On the 240d (77) the setup is different than what is on my 83 300d as far as where the relay is located. I think it is in the dash on the 240 as you don't see it. Fuse for the 240 is on the firewall and in in it's own block, nothing else in it.

The new plugs don't require any adapters. They are the same size as the loop plugs just screw them in.

Install is very easy. Remove old plugs and all the solid wire connectors. Remove the ground strap from the last plug to the head. I used a 5 wire 300d harness so I ended up cutting off the plug. I then cut off the shrink wrap to get to all the wires. The two temp sensors on the 240d are not wired into the same harness like the 300d was so pull those off and left the original where it was. Remove one wire not needed on the 4 cyl. Shrinkwrap the thing back together and solder all 4 wires into a heavy copper terminal. Hook each wire to a plug and hook common end to the fuse block.

You don't have to have any special harness. I just wanted the heavy duty wire and soldered connections that MB used. You could make your own with no problem, at least for the 77 model.

System works great so far, just not as fast a glow as I was expecting.

Rebootit,

I would still like a voltage reading directly at the GPs.

P E H

It does sound to me like you wired it the correct way.

But as PEH said, the real test of the overall system is a voltage reading right at one (or each) of the glow plugs. Your original 240D GP relay may not be able to handle enough current to get the things HOT.

I suppose the other thing that can happen is a carboned-up pre-chamber. Don't think that's your problem, but I mention it as a possibility. Much easier to fully check out the glow plug system first.

Just like you need a Mity-Vac to diagnose the MB vacuum system, you need a volt-ohm-amp meter to do the electrical stuff. Otherwise its like trying to fix the car according to Old Wives Tales and Folklore from the Forum.



Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles