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  #1  
Old 11-10-2012, 09:38 AM
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Glow plug timer question.

Qwerty and I had a disagreement the other day over how the glow plug timer worked and I want to inquire the group as to the consensus. The discussion in which we disagreed was over the length of time the glow light stayed lit. I said that was determined by the temperature sensor but the length of time the plugs stayed energized was independent of the length of time the glow light stayed on and was controlled by the timer which always kept the plugs on for the same length of time as long as the key was not moved to start and then released. Qwerty's view is that the timer is also controlled by the temperature sensor and is longer in cold weather.
When I was messing with the plugs/relay/timer in my 79 116 300SD a few years ago which had the pencil plugs but the early temp sensor in the engine, I thought I measured the amount of time the plugs were energized and the length of time was fixed independently of temperature. It was around 42 seconds I think.
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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
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  #2  
Old 11-10-2012, 10:01 AM
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Quote:
Originally Posted by kerry View Post
Qwerty and I had a disagreement the other day over how the glow plug timer worked and I want to inquire the group as to the consensus.

When I was messing with the plugs/relay/timer in my 79 116 300SD a few years ago which had the pencil plugs but the early temp sensor in the engine...
The view that I expressed pertained to a 1985 300TD and should not necessarily be interpreted as having universal applicability. For sure, I was not talking about a 1979 300SD at the time.

With regard to the 1985 300TD, my view is totally supported (complete with graphs of time/temp relationships) in the 617.95 Engine Manual. FWIW, the length of the glow period does not increase materially until the temp is well below freezing.
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  #3  
Old 11-10-2012, 10:24 AM
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Where is the temp sensor for the glow plugs on the 85? Did the 83 have a glow plug temp sensor? This is just for knowledge and not for need since my 83 glow plug relay has been modified to be manually controlled and I will be doing the same to the 85.
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  #4  
Old 11-10-2012, 10:40 AM
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Temp sensor is in the relay on an 85 I think. On earlier models like my 79SD, it was in the head.
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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
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  #5  
Old 11-10-2012, 11:05 AM
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I am looking at a 1982 W123 wiring diagram and it shows a temp sensor inside the glow plug relay. I am wondering how that sensor operates and its exact function. Surely it is not the best place to measure engine temperature.
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  #6  
Old 11-10-2012, 11:32 AM
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It's not intended to measure engine temperature as far as I know, just air temperature.
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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
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  #7  
Old 11-10-2012, 11:41 AM
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Quote:
Originally Posted by kerry View Post
Qwerty and I had a disagreement the other day over how the glow plug timer worked and I want to inquire the group as to the consensus. The discussion in which we disagreed was over the length of time the glow light stayed lit. I said that was determined by the temperature sensor but the length of time the plugs stayed energized was independent of the length of time the glow light stayed on and was controlled by the timer which always kept the plugs on for the same length of time as long as the key was not moved to start and then released. Qwerty's view is that the timer is also controlled by the temperature sensor and is longer in cold weather.
When I was messing with the plugs/relay/timer in my 79 116 300SD a few years ago which had the pencil plugs but the early temp sensor in the engine, I thought I measured the amount of time the plugs were energized and the length of time was fixed independently of temperature. It was around 42 seconds I think.
That is Correct. As long as you keep your Key in the Pre-glow position the Glow Plug Relay stays on until the Timer inside of it times you out and turns off.

If you turn the Key off and back to Pre-glow you can start over with another timed pre-glow cycle.

The total length of time before the timer times out on mine is close to 30 seconds.

The Glow Plug Light is supposed to be controled by a Temp Sensor inside of the Glow Plug Relay (where there is no specific Coolant Temp Sensor for that) and that is supposed to keep the Glow Plug Light on longer in Cold Weather.
However, the function of the GP Light appears to be tailored for an Engine with great like new compression.

This is easy to prove. Have whoever does not believe it stand by the Relay with the Hood open and tell him when the Glow Plug Light goes off and have him count the seconds till you hear the Relay click off.

And, then there is those Glow Plug Relays with Afterglow. I don't know what the light does on those.
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  #8  
Old 11-10-2012, 11:48 AM
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Quote:
Originally Posted by kerry View Post
It's not intended to measure engine temperature as far as I know, just air temperature.
Actually some of the Loop/Filiment type and early Pencil type Glow Plugs systems did have a Coolant Temp Sensor tied in to the Glow Plug Light.

I think some of the newer 6 cyl Diesels have them also.
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Last edited by Diesel911; 11-10-2012 at 03:59 PM.
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  #9  
Old 11-10-2012, 11:49 AM
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Originally Posted by Diesel911 View Post
This is easy to prove. Have whoever does not believe it stand by the Relay with the Hood open and tell him when the Glow Plug Light goes off and have him count the seconds till you hear the Relay click off.
How does that prove that the length of the glow period does not vary inversely with temperature? If the service manual is correct, the glow period (and the duration of the light) will be longer at -30*C than it is at +30*C. The only constant is the duration of the glow period after the light extinguishes.
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  #10  
Old 11-10-2012, 04:02 PM
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Originally Posted by qwerty View Post
How does that prove that the length of the glow period does not vary inversely with temperature? If the service manual is correct, the glow period (and the duration of the light) will be longer at -30*C than it is at +30*C. The only constant is the duration of the glow period after the light extinguishes.
My comment was basically that the Timer functions indpendantly from the Glow Plug Light. And, the test in My previous Post shows that the light can go off before the relay times out.

But, I guess you are saying I did not take into acount some sub-artic temperatue where the Glow Plug Light may never go off even though the Glowplug Relay time out and is off.

I felt that the Original Post was aimed at more reasonable temperatures since no specific temp was given.
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Last edited by Diesel911; 11-10-2012 at 04:14 PM.
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  #11  
Old 11-10-2012, 04:23 PM
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Quote:
Originally Posted by qwerty View Post
How does that prove that the length of the glow period does not vary inversely with temperature?
I understand that you have data to support this?

If so, I am quite interested to see it.
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  #12  
Old 11-10-2012, 04:26 PM
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Quote:
Originally Posted by Diesel911 View Post

But, I guess you are saying I did not take into acount some sub-artic temperatue where the Glow Plug Light may never go off even though the Glowplug Relay time out and is off.
I did not say that at all. But the concept does point to the logic of an increased glow period at lower temperatures. If only the light duration increased and the glow period remained constant, there would be little to no time left once the light extinguished.

But don't take my word for any of this. If the detailed description in the 617.95 Engine Manual doesn't make it crystal clear for you, I am certain that I won't be able to do any better.
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  #13  
Old 11-10-2012, 04:29 PM
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Quote:
Originally Posted by Brian Carlton View Post
I understand that you have data to support this?

If so, I am quite interested to see it.
The 617.95 Engine Service Manual describes the glow timer operation in great detail, complete with time/temp graphs.
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  #14  
Old 11-10-2012, 04:29 PM
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Here's how I thought (think) it works. When you turn the key to glow, it starts a timer. That timer is set for a fixed time--something between 30 and 40 seconds which does not vary according to temperature. The glowlight is on a timer which is effected by temperature--the colder it is the longer the light stays on. I'm not aware of any circumstances in which the length of time the glowlight stayed lit would exceed the length of time the plugs glow.
__________________
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
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  #15  
Old 11-10-2012, 04:31 PM
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Quote:
Originally Posted by qwerty View Post
I did not say that at all. But the concept does point to the logic of an increased glow period at lower temperatures. If only the light duration increased and the glow period remained constant, there would be little to no time left once the light extinguished.

.
Have you ever experienced a glowlight stay on anywhere near as long as the plugs are energized by the timer? I've had my 85TD at 35 below zero and the light never stayed on for more than about 20 seconds, if that.
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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
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