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Discussion Starter #1
I have a 1980 240D and the Glow Plug Indicator lamp is not working. I checked the bulb but the bulb is okay. The car starts just fine but I am always guessing when to start the car because the indicator lamp is not working. Do you think that it is the glow plug relay causing the problem?

Thanks,

~Chris
 

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Discussion Starter #4
Yes. When the relay dies, it leaves the glowplugs on at all times. You should replace it before your plugs burn out.
Good to know... How can I determine if the glow plugs are on all the time? Should I just quickly touch the plugs? Do you think that I should buy a glow plug relay that is used or new?

I have another question... Do you know of a distributor that sells a 5 speed transmission or do I need to search on eBay?

Thanks, Chris
 

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1959 220S cabriolet, 1983 240D original owner, 1999 E300 turbo diesel, 1988 560SL, 2003 SLK320
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Test the glow plugs with an ohm meter. Good plugs will read about 0.7 ohms.
 

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If one or more glow plugs burn out, the light should flash, so doubt that is your problem. To check to see if they stay on all the time, just take a volt meter and check for voltage at one or more of the plugs after the car starts. I would wait a few minutes after starting as the relay may keep the plugs powered after the engine starts depending on the temperature of the engine.
 

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Discussion Starter #7
If one or more glow plugs burn out, the light should flash, so doubt that is your problem. To check to see if they stay on all the time, just take a volt meter and check for voltage at one or more of the plugs after the car starts. I would wait a few minutes after starting as the relay may keep the plugs powered after the engine starts depending on the temperature of the engine.
I spoke with someone else and they indicated that 99% of the time it is the glow plugs but checking the voltage is easy enough to do. By the way, what voltage should it read?

Thanks...
 

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2001 Volvo V40
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If one or more glow plugs burn out, the light should flash, so doubt that is your problem. To check to see if they stay on all the time, just take a volt meter and check for voltage at one or more of the plugs after the car starts. I would wait a few minutes after starting as the relay may keep the plugs powered after the engine starts depending on the temperature of the engine.
If one or more glow plugs are not working, the glow plug does not light up, it does not blink. See the FSM:



The original relay does not keep the plugs powered after start up, again see the FSM:

 

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Not 100% sure, but I think each glow plug gets 12VDC. Been a while since I messed with them. You will likely have voltage or not. My owner's manual says the glow plugs will stay powered for 1 minute after starting.

One thing you didn't mention is whether your car has the pencil parallel plugs or the larger loop series styles. Early 1980 models supposedly had the older loop style, which is set up different. The 12VDC refers to the pencil style, but the loop style has a lower voltage, more like 2-3VDC due to the series arrangement.


If one or more glow plugs are not working, the glow plug does not light up, it does not blink. See the FSM:



The original relay does not keep the plugs powered after start up, again see the FSM:

I respectfully disagree. I'm not sure which system/year the FSM excerpt is referring to (as there are at least 3 versions on the W123), but on the late 1980 models with the pencil plugs and engine temperature sensor, the light flashes when there is a problem with the glow plug system. My 1980 owner's manual specifically states that the glow plug light will flash when there is an error in the system, and that they stay powered up for 1 minute after start up. I can personally atest to this as I had a glow plug burn out and the light did indeed flash.
 

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Discussion Starter #10
Not 100% sure, but I think each glow plug gets 12VDC. Been a while since I messed with them. You will likely have voltage or not. My owner's manual says the glow plugs will stay powered for 1 minute after starting.

One thing you didn't mention is whether your car has the pencil parallel plugs or the larger loop series styles. Early 1980 models supposedly had the older loop style, which is set up different. The 12VDC refers to the pencil style, but the loop style has a lower voltage, more like 2-3VDC due to the series arrangement.




I respectfully disagree. I'm not sure which system/year the FSM excerpt is referring to (as there are at least 3 versions on the W123), but on the late 1980 models with the pencil plugs and engine temperature sensor, the light flashes when there is a problem with the glow plug system. My 1980 owner's manual specifically states that the glow plug light will flash when there is an error in the system, and that they stay powered up for 1 minute after start up. I can personally atest to this as I had a glow plug burn out and the light did indeed flash.
My 1980 240D was built in February of 1980 so I guess I must have the loop style glow plugs. And perhaps the fact that it does not flash is indicative of glow plug problems... Perhaps not. I'll check the plugs out and let you know this weekend.

Thanks,

~Chris
 

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It's pretty easy to tell which ones you have, other than the newer pencil takes a smaller wrench. If the plugs have a bare steel wire going from one to the other, they are the loop style. If each plug has it's own insullated wire and there is no connection between adjacent plugs, they are the pencil style.

Another quick way to check the plugs to see if they are good is to disconnect the wires to them, then run a wire from the positive battery terminal and touch the plugs momentarily. If a spark is created, they are pulling amps and are likely good. If there is no spark, they are bad.
 

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1985 300TD
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If replacing glow plugs doesn't work don't forget to check the relay. Opened my relay up and had about an inch of water in there along with the affiliated corrosion. New relay needed.
 
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