Bosch glow plug relay: working? broken? stupid? (pick 2)
 

1985 Merc 300TD Wagon.

I could write a multi-volume history of me troubleshooting why I keep burning up alternators. I'm still not sure if it is crappy alternators or (da da DAH) the Bosch glow plug relay.

I just upgraded from O'Reilly's crappy "Ultima" (yeah whatever) 55A rebuilt to the crappy 65A rebuilt. And I'm sure a bunch of you will tell me that's the problem. And you might be right. But enough on that. Let's talk Bosch relay.

This character in question is the Bosch 0 281 003 096 glow plug relay. And my what a character.

I've found that unlike the glow plug relays in my other 2 W123s this Bosch relay stays on for awhile after the car starts. "Ah HAH!" I think. "This MUST" be the problem." But after a bit of googling I discover 2 forums threads (can't remember the links) that say this part number is a Bosch Duraterm relay which is supposed to stay on after the car starts and that this change is a design improvement. Ok whatever.

So I rewired the glow plug light in my car to show when the plugs are actually ON rather than showing the silly suggested-time-to-try-to-start the car. And I am finding that the plugs are on for not 1 minute, not 2 minutes, but almost 3 full minutes *after* the car starts, even though it's about 60*F out and the car started easily and smoothly after 5 seconds of glow plug on-ness.

Is this really how these Bosch Duraterm relays are supposed to work? And if so, is this sane? Or is mine broken?

I opened up the relay box and suspect that, by just adding a diode in the logic circuitry, it seems like it would be pretty easy to modify the circuit to shut off the plugs when the starter starts. That isn't what the relays in my other cars do. They shut off when the engine starts i.e. when the starter stops. But I'm tempted to try this easy mod anyway, although I thought I'd check in and get some advice first.

Advice?

yup, that's how it works.
keep the relay you may need it in the winter, but if your motor has no starting issues, it wouldn't hurt to swap it to an older normal relay and see if your alternator issues go away.

My volt meter is only showing 12.2 volts measured at the battery with the car warmed up (20 minute drive at highway speeds) and engine running. Voltage goes up to 13.7 when I disconnect the glow plug at the relay. Apparently the relay which is relatively new is messed up.

The parts store jockey tried to sell me an alternator. I'm sure he would have been happy to then sell a relay.

Is there an easy way to know what cars had the same GP relay as the 85SD? I may use this as an excuse to go yarding.

ok so doing a little math here:

Looking at the Diesel Giant page:
http://dieselgiant.com/glowplugrepair.htm

We see that glow plugs should read something under 1 Ohm. Let's say they should be .5 Ohm.

We got 5 of these plugs in parallel. The math for parallel resistors brings us down to .1 Ohm now. V = IR so we got 120A draw from the 5 plugs.

120A for 3 minutes? Really?

Ok so I got a 65A alternator. Let's be generous and say it is generating 60A most of the time (which is totally unlikely. 60A when idling?)

So it would take a minimum of 6 minutes of running the car to charge the battery back up from the drain on the plugs.

And that's assuming 1) 100% efficiency of charge/discharge 2) ignores starter motor drain 3) no other electrical appliances used

Given all the generous assumptions, it seems like it would take a minimum of 10 minutes to charge the battery back up every time you start it.

Strange.

Don't forget the the glow plugs (or any circuit for that matter) will draw more amps as they get hotter. The hotter they get, the more amps they will draw. Heat builds resistance in the circuit. Go junkin, swap the relay, move on to the next issue. Don't worry, there will be one eventually! :D

That is an interesting theory. Perhaps you could explain in greater detail how increased resistance translates to an increase in current, assuming that voltage is constant.

The fact that the circuit is protected by an 80 amp fuse suggests that the answer is "not really."

Are the glow plugs in a true parallel circuit? Or are there five separate circuits with a common power source?

The Duraterms do stay on after the car is running, but I don't know for how long. It's part of the after glow set up used on most MB's from the early 80's.

Does yours have an MB part #? A 10 digit affair, such as 001 545 98 32, that is the one for your car.

The above is not ture when it comes to the Glow Plugs.

I connected one Glow Plug in series with an automotive Amp Meter.

When I connected it to the Battery the Needle on the Gauge went to 60 amps (this is as high as that Gauge would read). It stayed at 60 amps for about 2 seconds and I observed that as the Glow Plug Element got hot the the Amperage it was drawing started to drop.

The Glow Pulg heated up the Amperage dropped down from 60 amps to 16 amps and stayed there.

So the as the Glow Plug heated up it drew less amps.

The opposite is true of course. Another great example of dyslexia at work. I was unaware of my condition until now. Perhaps more sleep would help? :o

As far as I can determine, the term "Duraterm" applies only to a type of Bosch glowplug. Their basic function is not going to differ from any other glow plug.

I don't believe that the OE timers worked that way.

That is precisely how a Bosch afterglow timer should function.

There is a thread from several years ago where I said also said that they drew more amps when they were hot.
And, of course people told me that is not so.
That may have been why I did the test with the Amp Meter.

Help me understand what you're trying to get across, the Bosch afterglow timer is different from a duraterm relay, how? Is it OE?

The pic is of a new unit. The plug boxes say duraterm also. Would duraterm be a copy-write term for after glow?

here's a page out of the FSM that explains glow plug current as it relates to GP temp.

Our 617 glow plugs are in true parallel.

All 5 gp's connect to a single copper plate that is connected to the relay contacts.
Each gp has its own wire that connects it. Each gp then has its own connection to
ground.