I've got an 82 300TD and the glow plug system has worked fine: when you turned the key to accessory, the glow plug dash illuminated for about 10 seconds and then extinguished.

But went on a business trip and the car sat for a week. When I returned, it is doing just the opposite. When you turn the key, the relay does nothing at all. Then after 10 seconds, the relay enables (thunking noise) and remains on permanently.

I've checked the voltage at one of the glow plug terminals to confirm that, in fact, the relay is behaving this way. When you turn the key, the glow plug has 0 vdc to ground. Then after 10 seconds, it has around 12 vdc and remains at 12 vdc.

As a temporary precaution, I'm unplugging the relay coil after each start, but this is a super pain in the butt. Does anyone know why the relay would start reset for the timed period and then set?

Weird, huh?

That is strange. Are you sure its staying on forever? How long does it stay on? Does the light come on too after it starts? If its not staying on for too long after its just telling you your glow plugs are out.

I've got a relay i'll ship you for 25.00 including shipping. But i recommend the newer afterglow relays. Better burn at after startup.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

check this out if you haven't ever before:

http://www.peachparts.com/shopforum/attachments/diesel-discussion/97665d1322684886-92-300d-glow-plug-screenhunter_06-nov.-30-15.31.jpg

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

Thanks, Biopete. The dash light was working and now isn't coming on at all. But irrespective of the light's illumination, the pattern at the relay is always the same. No vdc at the glow plug for 7-10 seconds. Then the relay thunks, and there's 12 vdc.

I drove it for a mile and checked the voltage at the glow plug, and it was still 12 vdc. I'm going to try ohming out the glow plugs. What's the expected resistance on a good one (I'll detach the relay output plug first).

Less than 1.2 ohm, usually around .3 or .4 You'll be able to tell when you measure them, most will be almost 0 and a bad one will be way high.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

Glow plug resistance (or lack thereof) has no bearing on the operation of the preglow timer. Most probably, the solid state portion of the timer unit has failed.

tangofox007, would the timer unit be the relay box I'm unplugging? Right there on the wheel well? If so I'm guessing that it would be easy to switch out with a disused sedan I've got sitting in the drive.

Also, I've had a ton of experience with relays (was in the Air Force back in the 80's), and I've never in my life heard of one working backwards without some knucklehead having wired it wrong (reset for 10 seconds, then set for good when last week it set for 10 seconds and then reset for good). Weird. Must be some East German communist influence.

That's it. Disconnect the battery before you do the swap; the big cable is hot at all times.

I'm guessing the mount is pretty easy to work with, right?

Two bolts.

Thanks, both!

I switched the relay box out and it fixed the problem with it working backwards. it's not exactly right: i.e. if you have the key in the number 2 position, the glowplug light remains on indefinitely and 11 VDC at the plugs. But as soon as you start and the key comes to rest at the number 3 position, the relay resets, glow plug indicator on the dash goes out, and there's no voltage at the glow plugs.

That's a LOT better than opening the hood every time to plug and unplug the relay. So thanks a million. BTW, how much does one of those suckers go for new?

I had some problems with my ignition switch,when I replaced lock and switch all the wire came out of the plug.I then had to search for info on wiring it back again.One of my problems was accesory did not do anything.
There are 2 red wires on the plug in,they can be in wrong posistion.
Don't atempt to repair without right info.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Did you install the car battery backwards?

















sorry couldn't resist

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Cheers!
Scott McPhee

1987 300D

Makes me wonder if the problem is in the key switch and not in the relay.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13