'75 300D glow system modification: will it work??? - PeachParts Mercedes-Benz Forum

cscmc1 · #1 12-13-2002, 03:57 PM

Hey folks,

I have a 75 300D whose glow plug relay has never quite worked right. I turn the key and no glow, no dash indication. If I jump the plugs straight from the battery, it works fine. Go figure.

I looked at the little caged relay dealy under the chrome trim between the dash and the engine firewall, but can't make sense of it. Once I cleaned it really well by using a lo-pressure sandblaster and it worked for a couple of days, but then went right back to not working.

I am content to just jump the plugs, and have gone so far as to terminate a nice 12 gauge wire that screws onto the first plug (where the wire from the relay would normally go). I am tired of popping the hood to start, though, and wondered whether it would be possible to install a toggle under my dash to glow the plugs manually. I found a 50 amp toggle at the local Rural King store (biggest rating they had): will this work? Will the extra wire to run from battery to dash back to glow plugs add much resistance? I figure it will double the length of wire I have now. Really, though, the darned thing runs to the dash to the relay and then to the plugs on a stock system anyway, so I don't *think* there would be a problem.

Would I be better off trying to tie my feed into the glow plug feed from the fuse box or is it OK to come right off the battery? I was afraid I might cook the 50amp toggle is I come right off the battery.

Any thoughts would be much appreciated...

Thanks!

Chris
cscmc1@eiu.edu

Motorhead · #2 12-13-2002, 04:07 PM

If you are looking to eliminate the glow relay, DON'T use but an intermittent switch..that is one that you have to hold on to activate. There could be a danger of accidently leaving the switch on and forgetting it. This could cause engine damage from the glow plugs melting.
If you go from the Battery, use an inline blade fuse holder right after the battery connection for protection incase of a short. Chris

Wasuchi · #3 12-13-2002, 06:09 PM

The switch should work fine, go with the switch Motorhead suggests. I had a similar setup in a 6.9 Ford Diesel.... the relay always worked on it but would stay on too long and burn the glowplugs out, they could only handle about 10 seconds on that truck. MB plugs are apparently significantly better and can be on a lot longer.

cscmc1 · #4 12-13-2002, 07:40 PM

Thanks for the great advice; a momentary switch WOULD make much more sense. I'll go swap the one I got. 50 amp ought to do it, though?

As for the relay under the chrome grille below the windshield... if you search the mercedeshop catalog for a 75 300D and look at the "Body Electrical," and then choose the "Preglow Time Relay," will that replace the caged relay I have? They look completely different... is this a new revision or something?

Thanka again!

Chris
cscmc1@eiu.edu

cscmc1 · #5 12-13-2002, 06:51 PM

TXBill -- I'm not sure we have the same glow/start system... the 220D parts car I have has the pull start that you describe, but not my 75 300D. It has a glow indicator on the speedo face; it's supposed to glow when the key is in the accessory position (or one click past, just before the start position).

Please let me know if I mistakenly read your reply.

Tks,

Chris

Motorhead · #6 12-13-2002, 10:05 PM

Chris, If your concerned about the switch handling the amperage and to avoid all the heavy wires to and from the glow plugs and battery I have a solution. Buy a Ford starter frame solenoid, the're cheap. Mount the solenoid under the hood somewhere close to the factory glow plug foil fuse. Hook one side of the heavy terminals to the glow plugs and the other side to the glow plug fuse or run a fused line from the battery. Take a small guage wire and hook it to the "S" terminal of the solenoid and run it into the cabin where ever you want it. Hook it to one side of the intermitent switch and the other side to a 12 volt source in the car. The solenoid only takes about 2 amps to close. You may need more than a 50 amp fuse as the 4 cyl VW diesels took a 50 amp one. I looked at my '84 300TD and it uses an 80 amp fuse.
I think the relay would be alot safer. Chris
PM me if you have any questions

Aaron · #7 12-14-2002, 12:13 AM

Bill,

When the '75 300D came out with the key-start system, it switched to having a preglow light in the same binnacle right under the speedo rather than the previous heating element as used on your car.

Chris,

Why not just replace the relay with a factory unit and save yourself the trouble and possible damage to your system? Remember that if/when you go to sell the car, someone might look down on that "modification" as a rig job.

Fisherman · #8 12-14-2002, 11:43 AM

One other thought...
If you have the newer replay then isn't it a common problem to have the blade fuse go bad even though it looks okay?
Have you tried to replace this part?
It shouldn't be more than a dollar and might be the ticket here.
Just a thought.

TXBill, I would have fallen into the same diagnosis... I guess there are more to 115s than I thought!

psfred · #9 12-14-2002, 02:32 PM

If the relay always clicks when you turn the key, the contact are burned and while you can clean them (must disassemble the relay), it is probably better to just replace it. A bit pricey, but a proper clean will often fix them. Lots of crud can get on them, they are open.

The light is operated by the thermo timer screwed into the block under #1 injector. It does NOT indicate the operation of the glowplugs directly, that is controlled by the relay! They will stay on a LONG time if you don't crank the engine.....! Typical failure is for the light to stay on all the time, or never come on, reguardless of whether or not the plugs have power.

Peter

Stevo · #10 12-14-2002, 09:15 PM

Chris I have a system like Motorhead suggested but I used a 15 amp

push button switch from Napa, with a solenoid from my tractor parts box. The switch is located under the dash (5/8 hole) on the left side so I can activate it while cranking. I have it wired into the indicator light so I not only hear the solenoid but can see the light. I did not use the solenoid before I switched to the new style GPs but figured the new ones would fry my 15 amp switch. I love the system and would not consider using a relay as I am in control and there is one less thing to go wrong.

CHAPP · #11 12-15-2002, 07:45 AM

I have a system much like Stevo (300SD)...but my relay works 75% of the time. I have a horn button under the left side of the dash. Mine was only a temp. fix 2 years ago, but a relay is on my short list of improvements...but just bought a 300D that is almost perfect, so who knows? My relay has a jumper to the button...that dims the interior lights when the button is pushed, but doesn't light the glow plug light. It works perfect...but so does MB's system...there is a circuit in the relay that keeps the glow plugs on until the engine is running smooth...just as another poster stated.