There is no timer. It basically uses one of the glow plugs, number one I think, as a temp sensor by measuring current flow. This determines when to turn on the dash light.

" No Timer "
 

Are you sure about that ?

I ask because I took the time to use a test lamp and stop watch to see how long the glow plugs remained powered after the dash lamp went off....

You're wrong there ~

I have tested every old Mercedes Diesel I've had and all of them continue to glow the plugs after the light goes off .

*VERY* handy when it's cold out , I wait 30 seconds after the light goes out before cranking and it always starts in less than a full revolution .

I'll trust my anecdotal experience over your claims of what Mercedes says .

Dang ~ now I'll have to go test again :rolleyes: .

Do you or does someone else have a pic of the Ford solenoid modification ? My stock glow plug relay stays on for only 6 seconds and when cold the engine stumbles a bit for the first 30 seconds.

make sure you understand what is happening.

the glow LIGHT on the dashboard has no relation to how long the glow plugs are powered.

the light is just an indicator letting you know they have been turned on, and it extinguishes LONG before the glow plugs do.

watch the cabin light when you turn on the key, it dims when you power the plugs, and it does not brighten until the plugs shut off... often around 45 seconds after you turn the key.

stock relay
 

Vstech is correct on all points.

The glow plug relay in an '84 w123 sedan I bought and re-sold did that - stayed on for 6 seconds or less. I replaced it with a used unit I bought off a well known auction site. Once replaced everything was fine. The PO had no idea why it would not start in the Colorado winter cold. I even found cans of starting spray stuff in the trunk. Glad no damage was done! If you listen carefully you can hear the glow plug relay "pop" when it shuts power off to the plugs. In most of my cars it's about 18 - 20 seconds after the seat belt warning buzzer stops buzzing. You can also see a subtle change in the brightness of the "idiot" lights along the bottom of the instrument cluster - the brake and battery indicator lights. They brighten ever so slightly when the power to the glows stops. That's the hottest point in the cycle and my cars like to be started at that point. Some people start the OM617 that way no matter what the engine temp - or outside temp for that matter. I always run the full cycle on cold (engine) start and when starting the engine already at operating temp, I wait until the seat belt warning buzzer stops and then hit it. That's maybe 10 seconds or so. I pay no attention whatsoever to the glow plug light on the dash. Hope this helps!

Kind of late for the OP, but for others I suggest you grab a glow plug relay box and other boxes at the junkyard for the future. Paying $5 now is better than $300 when you must have it. That is the only way to affordably maintain these cars. I have at least one spare g.p. relay. My problem is forgetting what I have. Last time, I grabbed a TemperatureRegular box because so easy, then found I already have 2 spares. The problem with those is they seem to fail predictably the same way (aged tantalum capacitors) so a spare is more for a rebuild. I haven't read that g.p. relay boxes suffer a similar endemic failure.

Update OP
 

Thought it odd this popped up now, seeing as I posted in 2012. I was the orig. poster; I guess I'm 'subscribed'. I did end up picking up 1 (+1 spare) from the parts yard.
I like what DenverW123 was saying about wiring the glow plug light to tell you when they're ON. That way, if the relay gets stuck, like mine did, it won't (possibly{?}) take out the battery/alternator/voltage regulator.
Need to update sig. below, car now has 124,000 miles on it.

I hope you've posted that for others to know how the glow system works; I've been a w123 owner for 14 years. The plugs are all working units but the relay is not operating as designed. My plugs stay on for less than 6 seconds- I've a helper turn the key- while at the relay you can both hear and feel the relay switching on and then off within 6 seconds. I currently do not have a spare relay but will look in the yards here soon. W123 are not showing up like they used to anymore...

So please as I asked- post more info about my previous request. thanks.

Gotcha.

To install a ford relay, you need a switch, and cable to feed it, and the glow plugs. Mach4 has a neat method of connecting the relay to the original harness neatly.

THANK YOU !
 

For having the patience to listen to my error and teach me .

" You are confused with the words used in this discussion."

I certainly was .

It took me until to - day to replace the bulb in my old test light and power test the glow plugs with it as SWMBO was shopping , the glow plugs power is cut as soon as the starter begins to operate .

I'm STILL waiting over ten days for Peach {arts to find a driver's side ATE caliper.....

They've been very good about calling me on the 'phone with up dates (nothing new) but having the car out of service this long is a real PIA .

I may have to go hunt up a good used left front ATE caliper and install it so I can get the car back on the road then replace it whenever the rebuilt one comes in .