tried disconnecting the violet wire from the plug on the gp relay this morning.it was a cold start(0 deg c).definately smoothed out the start and reduced the smoke.i could tell when the relay timed out by a brief period of roughness lasting maybe 2 seconds.so far so good ill leave it like this for awhile see what happens

Usually the signal that the relay has timed out is the headlights (or other lights) suddenly get brighter. I wouldn't expect any roughness unless you have a weak plug or two, or it's REALLY cold out... ;) Glad to hear it helped, though!

:cool:

Can someone clarify the snipping of the violet wire? if I cut it, then the GPs are just always on? Wiring a switch into the wire should be real easy but I am just curious about whether they are permanently on otherwise. Thanks, Andy

And while we're at it..
We are talking about the violet wire that goes from the fuse box to the glow plug relay located (in my '81 SD) above the left front wheel. Is this all correct? I'm headed to the wrecking yard today to get a switch for where my fader wheel is now located, and make this modification.
What exactly does disconnecting this wire do? Remove the timer from the glow plugs entirely? How then does the gp relay ever shut off? I just want to be very clear on what is happening. Simply saying that it causes the relay to power the plugs for an extended amount of time is a little vague for me. Why does it? Thanks!

The relay has a timer built in that runs it for a specific length of time. That time is generally longer than the time it keeps the preglow lamp on. If I remember correctly it cuts off at 1 minute. The violet wire tells the relay that you are starting the engine, so the relay cuts power to the glow plugs. Interrupting the signal on the violet wire by whatever means just makes the relay run for its full time before turning off. No, it doesn't leave the plugs on all the time.

Rick is correct. I thought this was explained earlier in the thread but I guess not - ooops. Note that we are specifically discussing the OM60x engines from 1984-1989, for which there are no aftermarket afterglow retrofit kits. 1990-up has factory afterglow. And you can get a nifty Bosch retrofit kit for the older the OM61x engines complete with correct plugs, afterglow relay, etc.

The OM60x engines (well, the 603 anyway) has a temp switch which makes this mod even better. If you simply disconnect (not cut!) the violet wire, you'll get a full 60 second burn on every hot start too, drastically reducing plug life. A manual switch might work better if you don't want to mess with adding a thermal coolant switch (assuming your engine doesn't have a handy switch to borrow, like the 603 conveniently does. ;) )

To reiterate, the relays usually turn off at 60 seconds if they don't receive an "engine started" signal. Therefore with this mod, if you glow for 12 seconds, then start the car, the plugs will remain on for another 48 seconds since the relays doesn't get the message that the engine is running.

Too bad I missed out on the eBay relay deal from PGA, I would have picked up a couple if they were cheap enough. :o

Great explanation. I checked the schematic out, and it was very clear. Thanks for another great mod guys...

OK, I missed the 603 refference earlier too, there is no afterglow kit availible for my 116 that I have heard of, would it apply to my car too or not?

116 afterglow
 

Good question. Do you have the old "loop style" series plugs, or the newer "pencil style" parallel plugs? I would have sworn there was a conversion kit, maybe not, if the 116 has a weird glow relay. Regardless, I believe the mod will still work, since they should all have a wire that provides a 'start signal' to the relay. Interrupt this signal and you should get afterglow. Then it just depends if you want it all the time, or want to control the afterglow with a manual switch, or an automatic setup like on the 603's...

:thinking2

I believe the '79 300SD uses pencil style glow plugs ( I think anyway)....they are NOT series wired in any case. Part numbers for the 123 , 126 and 116 glow plug relays are not the same, but being as this is the only one I ever had, or even looked at under the hood I couldn't tell you the differences, and why they would be different.

just started having serious cold start problems but to be fair I have other issues right now that may be the cause. I plan on pulling them this weekend to check them.

Definitely check the plugs if you're having starting problems. If they're all good, the next biggest factor to poor cold starting is valve adjustment. I assume you're waiting a good 10-15 seconds after the glow light turns off before you crank the engine, right?

:wacko:

Oh yes....several times....valves were just done about a thousand miles ago.

Hard start could be related to this.....

http://www.peachparts.com/shopforum/diesel-discussion/105386-loss-power-briefly.html

Problem started just as temps dropped. Related? maybe but not certain.

2nd day of testing modification.what ive noticed so far.morning starts have been cold starts both days were below zero the night before.definately much smoother startup alot less smoke.but engine rpm at idle was very low almost like electronic idle was not in effect.this is different than situ before mod but i dont know if its related.does anyone know if the start circuit is somehow tied in to the electronic idle regulation

Last year I was trying to start my 300D (with old loop style plugs) in the cold. Of course it had a weak battery, but the glow circuit really pulled the current. I would think one reason the plugs shut off while cranking is to get all the battery power directed into the starter motor.

One consequence of severing this start signal could be less cranking ability, especially on the loop style plugs, as they are extremely inefficient, putting half their heat outside the engine.

Just a thought I had reading this thread. I'm now driving a 240D with the pencil type plugs and a standard plug relay. Yesterday after work, the temperature had dropped to the high 20s (F), and it fired right up after leaving the plugs on right up to the point where the relay times out. A new battery helps out a lot though.

My battery is less than 2 months old. forget exactly when I got it. But I had to scream at 5 different Exide dealers before any would replace a failed 10 month old battery.