Okay to start this is a inconsistent issue not one of normal glow plug problems.

Car is a 1984 300cd om617 turbo.

Some times the glows work as they should and other times we get nothing.
The times when they do work we get the glow light on the dash and you can here the relay click if you wait for it.

What has been done so far.

I have confirmed the relay has power to the input terminal.
Strip fuse has been checked and found good.
With key "on" power is going to the relay.
All glows we're replaced two winters ago.
All glow plugs test good and wires ohm out good.


This car does sit around in the salty months and sat for months on end when the previous owner had it. My thought is the glow plug relay. Obviously I don't want to throw parts at this since a glow plug relay is not cheap. The problem is I can't replicate this issue when I want. Every time you turn the key it's a 50/50 shot wether it will work or not.

Any input or additional test would be helpful.

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1980 300CD - 130,000 Daily Driver
1984 300CD - 325,000 Dad's summer cruiser

1977 K10 - The Grinch - lifted, locked, winch, 37's. Beadlocks.. 379 t Detroit Diesel
1956 Willys Wagon - buick 225 oddfire v6
1982 SJ 410

I personally eliminated the glow plug relay in favor of a completely manual system involving the use of a momentary switch and a Ford starter solenoid in my car. This provides total control in a system that is dirt simple and extremely reliable. I actually gutted the old solenoid keeping just the case, fuse and plug so it looks virtually stock. Since I never even attempted to use the stock glow relay, I can't comment on troubleshooting.

I'm not necessarily recommending this as a strategy, but if the relay is bad it might be a very economical alternative to consider over a new relay.

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Neutral safety switch funky?

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83 SD

84 CD

A faulty neutral safety switch would not allow the engine to crank at all. From the description it seems that the engine is cranking but will not start when the problem occurs.

Junkyard relay. Chances are always high the salvage yard car was not scrapped because the glow plug relay was bad. Or in the parts wanted section asking for one should produce results. Technically you could have bad contacts in the ignition switch but usually it is the old well worn relay.

Thanks for all the comments!

I do like the ford starter solinoid solution. I have this on my 77 chevy and it works great. My father wants his car to be all stock so it's not an option for the 84.

The safety switch is fine and the car cranks fine. The engine is very fresh and will start with out the glows but not near as quick as it does with the glows.

Junkyard relay is an option. But it's normally the first part taken when a 123 or 126 diesel gets in the yard. I have yet to find one. I will try the wanted section on here.
Ignition switch could be possible the car does have 340,000 miles on it.


Thanks again guys! I guess the thing to do is try another relay and if that cures it look for a quality new one. I want to get it sorted out for him as I am moving away and won't be able to work on the car much anymore. I see some screws in the relay case can it be opened up and cleaned/ greased?

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1980 300CD - 130,000 Daily Driver
1984 300CD - 325,000 Dad's summer cruiser

1977 K10 - The Grinch - lifted, locked, winch, 37's. Beadlocks.. 379 t Detroit Diesel
1956 Willys Wagon - buick 225 oddfire v6
1982 SJ 410

When I had the same symptoms I found the Strip Fues was Cracked. The first time I loked at the Strip Fuse I was only looking to see if it was burned so I did not take a really close look at it. Also in My caes I have some visionissues.

Others have had the Screws that hold the Strip Fuse loose.

It could simply be the Glow Plug Realay is malfunctioning and needs to be replaced.

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84 300D, 82 Volvo 244Gl Diesel

Hey Ian,

I have a spare GP relay that I had bought for my '84. If you want to borrow it for a little bit to see if it cures the problem, feel free to come over & get it.

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"Senior Luna, your sense of humor is still loco... but we love it, anyway." -rickymay ____ "Your sense of humor is still loco... " -MBeige ____ "Señor Luna, your sense of humor is quite järjetön" -Delibes

1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

Take out the strip fuse and check with a magnifier for cracks. If no cracks put in back. Take the connectors off and check all contacts for green corrosion. If there's corrosion, burnish them away with a wooden dowel and WD-40. Take the connectors apart and gently squeeze the female pins with pliers to close the gap a little and restore tension. Do not open the relay, not much you can do unless you are electronics savvy. Just shake it with pins facing down against a newspaper. If you see water droplets coming out of the breather hole, get a new relay.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked