Glow plug conversion questions
 

Planning to help my friend (who bought my old '78 300D) upgrade to pencil style glow plugs with a conversion kit from Mercedes Source. I realize the glow light runs on a timer, and therefore loses its purpose after upgrading to faster plugs. I'm wondering if the glow timer can be replaced with a later model unit for better accuracy or if anyone has modified the glow light to run on a heat sensor rather than a timer.

i thought the kits included an appropiate timing device.

I don't believe that either the early loop plug relays or the later pencil plug relays powered the light strictly by a timer. In both cases, temperature is taken into account. On the early models the temperature of the block is sensed and in late models it's the ambient air temperature. The only difference is that the glow plug light will stay on longer because it thinks the engine still has loop plugs. This is if you are still using the original relay. And there's no reason to swap out the original relay. Just install the pencil plugs, removing the ground wire from the front of the engine. Hook up the back plug using the wire from the fuse box on the firewall and make up some jumper wires to go between the each of the plugs.

Good to know. I found a thread on BenzWorld that mentioned installing a new glow plug timer module (part #0 281 003 005) that picks up on the temperature under the hood and pulses the glow plugs until the engine warms up. This unit is a bit pricey online, so I'm wondering what models it was used in (my guess is later model 300 diesels). Would much rather grab one at Pick-N-Pull next time I go.

As far as the glow plugs themselves, is there any better deal than Mercedes Source or Diesel Giant?

The ole' manual GP timer has worked for me and my conversion.

1mississippi2mississippi3mississippi4mississippi5mississippi6mississippi7mississippi8...

START!

That is one way to do it...LOL
I really like the idea of the afterglow this other module offers.

...

I'd just order the plugs themselves from here, or elsewhere, and wire them to your stock timer.

MS kit is a waste of money, as is DG's kit...

If you installed only the Glow Plugs and Wires you did not install the whole system. But, you did install enough for the System to help start the Engine.

Since the core purpose for the Glow Plugs is a starting aid for the Engine they are meeting their purpose if the Engine Starts.
Instead of a Light to act as a guide the user has to count mentally X amount of seconds. After you go through one Winter you the User will learn how long to keep the Glow Plugs on to reliably start the Car.

I think there is a difference between a Fast Glow Plug and a Fast Glow Plug System.
The Plugs themselves get hotter faster and if one goes bad the rest will work (that alone makes it better then the original system).

Also when wired normally you Glow Plugs are on during Starter Cranking; at least on the Stock system.

These glow plug conversion kits seem to only upgrade plugs to pencil style and provide different wiring that still leaves them in a series setup, correct? Auto Haus has pencil style plugs individually...can I just order 5 of these and snag a new relay and wiring harness from a 1980s model diesel at the junkyard?

That is exactly what you should do.
This newer glow plug relays are by far the best and cheapest way to improve the glow system of any pre 1979 mercedes diesel.
I converted 4 of my cars to this system.

I installed the pencil plugs, and adapted the wiring, without changing the relay on my car and it works fine. The light turns off when the plugs have heated up properly. I.E. when the car is cold the light stays on for perhaps 15 seconds and then goes out. When the car is warm, the light only stays on for a couple of seconds. So that part of the relay remains functional.

For what it's worth.

Nothing wrong with doing the full swap of course, just that I didn't find it necessary. Been running like so for going on two years.

I'll second the notion of using a manual system. It's super easy to install and in use you'll only use the amount of glow needed and can easily add any after-glow that you might want.

I'm a believer in the idea that glow plugs essentially last for a fairly constant amount of glow. With a manual system, you only use what you need and you're in complete control. Currently the morning temps are around 50 which requires about 3-4 seconds of glow for a quick start and I usually give it another 1-2 seconds of afterglow. Minimum glow is easier on the battery and the plugs.

Not exactly. The original set up was in series with one ground.

The up grade Glow Plugs are wired in what I think is called Series Parallel.
Since each Glow Plug is grounded through the outer body of the Glow Plug if one plug become open and stops conducting Electricity the other still work.
What would stop all of the from working is if the Fuse burned or the Wire from the Fuse Box to the Glow Plugs broke or somehow burned.

After that if one of the other Wires broke or burned the least amount of Glow Plugs you could have working would be one. How many remain working with one Wire Burned or Broken depends on which wire.
In actual practice the wiring method used in the kit is simple and seems to be for the most part trouble Free.
The only issue with the Kit type of wiring is that in order to use the Ohm Meter to check the Glow Plugs you need to remove the Wires.

But, while the kit comes with wires that jump from Plug to Plug you actually can wire them in Parallel if you want to. Jut run a separate Wire from each Glow Plug and Join all of the Wires just before the Fuse Box.

I don't know if it is true on the Loop Type Glow Plug setups but on mine after the Glow Plug light goes out if you keep the Key in the preglow position the Glow Plugs stay in till the Timer turns them off. I get a total Glow Time of about 30 Seconds.

If you need to preglow again you just cycle the Key and start over.

Since the Glow Plug Light works off of the Coolant Temp or the under the Hood Temp it has no Brain to tell it what condition your Engine is in so it does not know how long you really need to preglow to get your Engine started.

Seems like the slickest way, especially if the timer works correctly. Is there a guide for this type of setup? How does the new relay wire in?