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Old 03-27-2006, 01:24 PM
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Join Date: Jun 2005
Location: Portland - OR, West-Coast
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mix 'n match glowplug relay...?


I am refurbishing or "rescuing" a vintage 110.110 "finny". The glow plug relay in it no longer works and I have to touch a wire directly to a battery terminal with the hood up any time I'm going to start it up from cold.

Recently a very cheap relay part from an 80's 123 (240D) chasis became available (used) and I'm wondering if I could possibly use this as a replacement for my finny. The biggest potential I can think of is that perhaps the 240D relay is too strong for the 4-cylinder 190D engine? Should I just go with a brand new specifically correct relay part, and if so, how much would one cost brand new?

Your thoughts out there...?

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- Theo
- Fins make 'em fast! (looking..)
- '65 190-Dc
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Old 03-27-2006, 01:54 PM
Cabernet red, actually
Join Date: Jan 2006
Location: Willamette Valley, OR
Posts: 503
I'm not certain, but the relay you need might depend on the style of plugs you have.

You probably have the old circuit-style plugs in your 110. I think MB changed to the pencil-style plugs in '81, so the relay you're talking about may be for those plugs, if in fact the relays are different for the two styles of plugs.

I would think you'd be alright barring that.

1985 300D Turbo, CA model
248,650 miles and counting...
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Old 03-27-2006, 02:32 PM
katja's Avatar
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Join Date: Jul 2005
Posts: 307
If it's a stock 190D or 200D fintail it doesn't even have a relay for glow plugs. You have to hold the start knob out just to the point of resistance where the alternator (generator?) indicator light dims slightly but the starter is not activated. If the glow plugs work, you'll see a glowing filament in the round "salt-shaker" thing to the left of your steering wheel (that's your glow plug light!). If this doesn't glow, either your pull start switch is bad (you can by a new one at **************.com) or one of the series glow plugs has failed, so the whole circuit is dead. The only reason I know this is that both these problems had happened to the 240D I was originally going to buy.

Of course if the car has been upgraded to the new design plugs it should have an aftermarket relay installed. The new plugs would have parallel wiring, so there are no metal "W" shaped things between the plugs but insulated wires instead.

Good luck restoring your fintail...they are eye-catching cars!

'76 240D "Katja"
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Old 03-27-2006, 02:45 PM
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Join Date: Jun 2005
Location: Irvine, CA
Posts: 646
try this


I've just been working on my 240D relay. It should be able to switch up to 80Amps on your plugs so I'm sure even with series deals, this should work just as well as a wire touching on the battery.

This relay sends out 4 leads to the plugs that are wired in parallel. The 240D isn't too strong I'm afraid, it may be too "weak", that is, each circuit only handles 1/4 of the current and the connector may not be spec'd to light up all 4 plugs in series.

Good post on the "salt shaker". That is really cool how they engineered that.

My suggestion is to go over to Harbor Freight and get one of their high current battery cut off switches Item 92688 and use that to light off your system, that way you don't risk smoke and fire with your bare wire. Add a 80A mbz glow plug fuse inline for safety. Oh yeah, and don't leave it on This is a hack and not a good resto job but it sounds like you're just building a driver, not a trailer queen.

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Old 03-27-2006, 04:44 PM
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Join Date: Jun 2005
Location: Portland - OR, West-Coast
Posts: 41
OK, good info to go on everybody...

Thanks all for your comments. I'm pretty certain (from memory, as car is parked away at a friend's right now) that it has the "W" connectors between plugs, making the plugs the original stock in-series kind.

I think I will go with keeping it pretty close to stock original, although the car is no contendor for becoming a trailer-queen. It has some rust issues in the flooring which will mean non-stock welding eventually. It'll just be my eye-catching around-towner some day.

I will have to test the starter pull-switch next time I visit it. Perhaps I'll be lucky and discover that the PO that I bought it from just didn't know how later-model plugs worked and rigged up the wire to touch to battery that I'm using now.

Is there much advantage to having the plugs in parallel BTW, as BMZ transitioned to with later models? One advantage I CAN think of is that if one plug goes out, you still could potentially get the block warm enough for mild-cold starts. Do any others come to mind?
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- Theo
- Fins make 'em fast! (looking..)
- '65 190-Dc
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Old 03-28-2006, 06:59 PM
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Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
No problem with the series plugs just carry a small low voltage bulb with a couple of wires soldered to it plus a small wire with two healthy alligator clips in the trunk. Of course a 12v test light factory made is the same thing. If you suspect you have lost a plug while away from home just test with light and apply the jumper if you find an open plug. Never leave home without mine.
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Old 04-04-2006, 02:44 AM
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Join Date: Mar 2006
Posts: 3
series vs. parallel

*EDIT* sorry Barry, I saw you mention the test light and didn't even notice you also mentioned the bypass wire, sorry for the repaet information. *EDIT*

A parallel electrical circuit is superior in (almost ?) every way to a series circuit. Now, im not saying this alone would merit the time/money for a conversion, but it is just a bit of truth.

The main issue with a series circuit is that the entire circuit has one power source and one ground. If the circuit is broken anywhere along the circuit, the entire circuit goes dead. If you have a faulty glow plug that is open internally, all four glow plugs stop working. In a parallel circuit, only the faulty glow plug would stop working, the other three would work normally.

This is because in a parallel circuit each consumer( in this case each glow plug) has its own power source, and its own ground. I have series glow plugs in my 123, I don't plan on converting to a parallel system, unless somehow the parts end up in my lap for cheap.

I agree with the idea of keeping a test light in the vehicle for diagnosis reasons. Another little piece i keep in the trunk is a bypass wire to allow me to get home, after i have located an open glow plug.

I went and got 2 alligator clips rated at 70 amps at 12 volts from radio shack, connected them together with a strand of 6 guage wire about a foot long. If an open glow plug is located, for ease of description, lets assume the glow plug for cylinder 3 is faulty. if you place one clip between glow plug 3 and 4. and one clip between glow plug 2 and 3, the alligator clip/wire will bypass thew faulty glowplug and allow the other three to heat up the motor.

this is in no way a permanent fix, and should only be used in emergencies to make it back to your destination. Doing this greatly increases the electrical load on the other three glow plugs and may result in premature failure of the other three if done continually. Hope i have managed to help a couple people out with some peace of mind. for 10 bucks, I can't imagine not carrying one in my trunk.

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Old 04-04-2006, 02:59 AM
Posts: n/a
An easy bypass that I use is a heavy-duty relay. I don't know the NAPA model number but it's an 80amp continuous-duty rated relay. All you need is a power source, SPST switch, wire to connect to the glowplugs, and a fuse holder.

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