I posted this on the mbz.org diesel list earlier today:

As many of you are probably aware, if you snip the violet wire at the GP relay, you can get your GP's to glow for an extended period after the engine is running. This should theoretically reduce smoke, emissions, and coking issues. The problem is that snipping the wire also forces the GP's to run 'afterglow' even when the engine is warm, which will diminish the life expectancy of the plugs. Many folks have wired in a manual switch to turn off this feature, but I wanted a more 'hands-free' operation.

I installed a thermally switched relay into the GP relay's violet wire circuit. I used an NC (normally closed) relay from a Volvo 700 series fusebox. These are a 5 terminal relay, similar in look to the familiar 4 terminal NO (normally open) versions used in Euro headlite conversions and countless other little projects. These NC relays have an additional 87a terminal to which one side of the violet wire is connected, opposite the 30 common for the other side. http://www.the12volt.com/relays/relays.asp Be careful of other similar 5 terminal relays with an 87b terminal--these are just NO relays with two bridged terminals--these won't work.

I piggy-backed onto the 50 C thermal switch above the thermostat housing on my '87 300 td OM 603, which is apparently designated to assist and smooth cold shifting in the transmission.

When the relay is energized, the violet wire is snipped--afterglow enabled. In this phase, the 50 C thermo-switch has continuity with ground.

When the relay is not energized, the violet wire is bridged--afterglow is disabled. In this phase the thermo-switch has no continuity with ground.

I'll let you know how well this works in real world situations.

That's ingenious! I will investigate this further and will wire up a switch to mine. That is if my relay HAS a purple wire.

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I believe all MB GP relays for the OM 616/617 and OM 601/602/603 series engines have a purple wire. All of the later OM 606 engines, and possibly some '90's era OM 603's had an afterglow relay installed at the factory. Try the switch first, then if you like the results, go for the relay option.

Good luck.

I'll just stick with the switch. That way I can keep her glowin when I want. I have no problem with a switch.

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1984 190D 2.2 Auto 220k
2001 Jeep Cherokee Sport 70K
2004 Lexus RX-330 ??K
2005 Chrylser Crossfire LTD 6K

Play guitar? Go to www.cyberfret.com for free online lessons!

This is a great way to add "afterglow" cheaply, although plug life will be less than normal (duh). At next plug replacement time you could get the special afterglow-rated plugs which would help. A full factory conversion (for the 603) is expensive- you need the late relay, plugs, temp switch for the cylinder head, and some custom wiring... $200+ last time I checked. The relay+50C switch trick costs about ten bucks, tops, for a new relay. I've been meaning to do this on my car and haven't gotten around to it yet. Hmmm... maybe this weekend....

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Bump! I finally did the install last month, after discovering two dead plugs (only 20kmi old - don't buy Monark brand, stick with Bosch or Beru.) Anyway the mod works GREAT. It's 100x better than simply yanking the purple wire, as it only afterglows when cold. Wish I had done this a long time ago...

Cool, need to look at my car and see about it. Would help cold weather starts a lot.

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I suggest more folks give this mod a try, as it really does smooth out those cool early morning startups.

Thanks - my 603 engine is particularly cold natured. Starts up instantly and runs smoothly, then within about three seconds when the glow plugs cool down it smokes and rough idles. Less of an effect when I am patient enough to run an extended glow cycle. But what it really needs is the afterglow system that MB went with in the 1990s.

I want more direct control so I'll probably go with a toggle switch and indicator light. This is a low current-carrying wire, right?

Ken300D

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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

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andy t
'78 300d
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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.

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Rick Miley
2014 Tesla Model S
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Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
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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.

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

Well, it's that time of year again. This modification still works quite nicely, even after all these years.