I have read in a couple places that it is not advisable to bypass the preglow relay switch. My relay is fine (better than the non working replacement I just bought) except for a burned off contact point. I spent a couple of hours trying to figure this out. Would have been a lot harder without this forum.

What about this idea: steal the voltage from the circuit board that engages the solenoid that makes the contact points connect. Use this voltage to power something like this.

http://www.oznium.com/relay80

Return the voltage to the copper plate that feeds the 5 pins that feed power to the Glow plugs. This could be a little trickier but I think I could figure it out. It would sure be a lot cheaper to replace the after market 12 volt relay than to purchase a whole new preglow relay from MB.

Any Flaws in my idea.

My manual GP system uses a Ford truck relay and the indicator light is wired into the system which uses the 80 amp strip fuse. When the relay dies in my "85' I will use this system in that car also where I am in control of GP timing

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Here is mine. My Diesel volvo has/had a different type of Glow Plug Relay system. I replaced it with a starter solenoid as Stevo did and you are saying but used a spring loaded push button switch. Battery (+) wires are on the top of the pic as well as the switch (+) wire and the wire comming back from the switch is connected to the "S" terminal on the solenoid. The wire on the bottom goes to the 6 glow plugs.

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84 300D, 82 Volvo 244Gl Diesel

Remember to get a relay that is rated for continuous duty. Many starter relays are not great quality, *cough* Motorcrapt.

Why would you need a relay or a solenoid for continuous duty?

The duty cycle is approximately .1% (10 seconds in 24 hours).

The Ford solenoid..........cheap as dirt.........would be more than adequate for that task and probably last forever.

[quote=Diesel911;1922054] I replaced it with a starter solenoid as Stevo did and you are saying but used a spring loaded push button switch./quote]

I thought about doing the same thing. I was hoping that the original guts of the somewhat expensive starter relay from MB could still be utilized. It would be kind of nice to have the original relay tell the the plugs how long to stay on depending on engine temp etc.

I guess it would be no big deal if the glow plugs were as easy to change as the ones on my Jetta. I just want the the plugs to last as long as possible.

So that's the big question. Will manually operating the glow plugs with an external switch and starter realy shorten their life. If not, probably not worth trying to salvage the smarts in the preglow relay.

Manually operating the plugs with an external switch and starter relay will not shorten the life of the glow plugs, provided that you use a momentary switch or button. If you use a traditional single throw switch, it's a certainty that your one moment of forgefulness will kill all five plugs in about 15 minutes.

Excellent point Brian...a fixed throw toggle is a bad idea ...from the (FORGOT TO SWITCH IT OFF) situation that will surely happen at some point....

My (MOMENTARY) push button switch actually about doubles my glow plug life since the OEM relay always glows much longer than I need to start the 300D here in 100* F central Mississippi...kevin

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)

You haven't been around a fleet full of fords have you?

Not true. The glowplugs can be left on for hours without burning out. You can let them drain the battery down to nothing and they will still work fine for months afterward. Yes, their life expectancy isn't nearly as long but they do not burn out nearly as quickly as above.

I've owned enough of them before you were born. The solenoids rarely give issues.

I'll leave you to try that for yourself...........I'm sticking with the 15 minute rule with regard to continuous glow plug power.............if you run them for hours, you'll definitely shorten their life.

I have, twice.

[quote=cici235;1922173]I think that the newer Plug Relay with the afterglow or whatever it is called is a good thing to have if you live someplace cold. If I had a bad relay in my Mercedes and I lived where it was cold I would consider that.
The situation with my Volvo was that there was no place else to get the Glow Plug Relay from but the dealer and I did not have a computer meaning no internet access to search for parts.
There was also no forum to help me trouble shoot the Relay so it was just easier and cheaper to bypass it and use the starter relay.
A person should be able to get a relay from a junk yard really cheap if they want one to experiment with.

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84 300D, 82 Volvo 244Gl Diesel

I am sort of going to add something about the Autolite starter relays.
I do not think that those Ford starters with the big heavy piece of metal on an arm that gets pulled down to engage the starter gear to the flywheel are as efficient as what I call Delco type starters used on Chevys (my Father has owned a bunch of Fords over the years).
The type I call the Delco type are the same setups as the Mercedes starter.
I think that the Ford type starters are slower to engage and require more current to work. I am sure that this must put a lot of stress on those Autolite solenoids.
Another comment is when someone mentions fleets of vehicles used for business I occurs to me that these are vehicles that will be starting and stopping a lot more during the day than the family car and will cause more of a beating to the starting system.
In the above situations it might be that with fleet vehicles the starter solenoids on Fords bite the dust sooner then Chevys or Dodges solenoids do and that for your family vehicle use the Ford solenoid hold up fine.

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84 300D, 82 Volvo 244Gl Diesel

It is again reasonable to assume a lot of glow plug relays deteriorate with time and use. They are conducting substantial current in fact the whole glow plug load is through their internal relay contacts. As the contact resistance increases with oxidation, current pits from opening etc. Even arcing through lite frost buldup on cold mornings initially. From the moment they are new they are going slowly downhill in my opinion.

There is the resultant contact damage plus the heat transfer from the higher resistance of the contacts. It is probably slowly cooking the coil that controls the contacts as well. . This then represents a lower resistance for the output transistor to work with so the current increases through the transistor. This is a true vicious circle. The additional current through the coil is further cooking it and reducing still its designed resistance. Eventually a resultant failure occrs.

I can almost visualise the older glow cycle controllers under the dash being scarce as hens teeth at wrecking yards or really expensive in the not too distant future. If you used the controllers contacts to control a cheap type ford solinoid the glow relays life would in most cases be vastly extended. Your internal timer function will still operate as intended of course and your old glow relay would think it is on virtual holiday. You may even see greater voltage across your plugs as well.

If you used this approach on the newer fender mounted glow cycle timers you would loose functions I believe. So it is not really as easy an option. Still that might be worked around.

In a way a glow plug relay is almost a maintenance part. I do not think it is designed nor intended to last the life of a mercedes diesel. With the modification I mention that could result in a different outcome. Very easy to wire in as well.

So are you guys just clipping off the connector that leads to the GPs and connecting those wires to the output of the ford solenoid? I haven't looked at it yet, but my car is in another state where my step son left it with it smoking the wire bridge on the GP relay. He left it at Walmart so this might be an easy way to get the car home where I can address it. My question would be are you cutting that connector out and putting terminals on to use the ford solenoid? It is unclear why it would be smoking as I haven't heard of GPs shorting (they normally go open when they fail) so I am guessing the relay has just failed or shorted a wire somewhere. Any thoughts on that would be appreciated as well. I am headed out there this evening to get the car.