OH NO! Not another glow plug relay thread...HELP!

[SD Blue]

Hmmm........There isn't a block temp sensor in the glow plug diagram. Originally, I thought the same thing, but someone here pointed out that there wasn't one.

From the schematic, it appears the comparison is between the internal temp. sensor and a glow plug current monitor. With this scenario, you are looking at grounding of the glow plugs (eng-chassis strap) and grounding of the glow plug relay. Also, condition of the glow plugs, of course, and supply voltage.

Another factor might be an instrument panel ground. Especially since you state that the relay cycles every time but not the glow plug light itself. Personally, I rarely even notice the light and always listen for the very audible click of the relay before starting.

[kerry]

It depends on the year. Early models had a glow plug temp sensor in the block, later models only had a temp sensor in the relay itself. Not sure when the change occurred. I know my 79 SD has it in the block.

[funola]

Indiam, your link http://wagoneers.com/DieselBenz/TECH/PDFs/Engine_617_15-710.pdf is for 617.950 up to 1980 glow system. Do you have a link to a schematic for an 617.952? I am also having intermittent problems with my glow system in my 83 300DT. Sometimes I get the glow plug light, sometimes I don't, somtimes I get glow plug power, sometimes I don't. It is not a major PITA now that it's warmer, but I want to fix it before next winter. I don't think my controller looks at block temp (I ohm-ed out the connector on the glow controller and none of the wires go to the block temp sensor). Problems that come and go are the hardest to troubleshoot. I think I'm going to rig up 4 of the instrument cluster lights to monitor the glow controller signals: ignition on signal, start signal, glow plug power, glow plug light power, so that I can see at a glance what's happening every time I start the engine

[iandiam]

Originally Posted by kerry

No need to drain. Act swiftly to reinstall the new one. It does sound like a problem temp sensor.

Now you're speaking my language.

I've got a spare in the parts car (the engine with abysmal compression) that I'll try. Don't mind draining that one...Now I just need the time...

[SD Blue]

Originally Posted by kerry

It depends on the year. Early models had a glow plug temp sensor in the block, later models only had a temp sensor in the relay itself. Not sure when the change occurred. I know my 79 SD has it in the block.

Your talking about a W116 series and that may be the difference. The schematic I'm using is for W126 series, '82 specifically, and my car, '84 doesn't have a block temp sensor.

I'm not sure when the change occurred either, and this is only a guess, but it may have been with the W116 to W126 transition. Do you have a schematic for your car? If so, what pin of the Glow Plug Relay is the block temp. sensor connected to?

[kerry]

Originally Posted by SD Blue

Your talking about a W116 series and that may be the difference. The schematic I'm using is for W126 series, '82 specifically, and my car, '84 doesn't have a block temp sensor.

I'm not sure when the change occurred either, and this is only a guess, but it may have been with the W116 to W126 transition. Do you have a schematic for your car? If so, what pin of the Glow Plug Relay is the block temp. sensor connected to?

I don't have a schematic but it wouldn't surprise me if the change occurred with 116--126. The same change occurred in 123's too.

[iandiam]

Originally Posted by kerry

I don't have a schematic but it wouldn't surprise me if the change occurred with 116--126. The same change occurred in 123's too.

SO, there is a temp sensor in the head, water jacket. I can see it in the head, above and between the 2 & 3 GPs, shown in the engine cutaways of the 617.952. Green wire enters the GP harness; goes to the 'IN' / MALE side of the multiplug just above the relay, inner fender. 'Out' side of this multiplug feeds into the cabin harness, presumably to the glow indicator light, among other things.

I have installed the DURATERM POST-GLOW unit. However:

According to the schematic of the STOCK relay, there does not seem to be an integral temp sensor. It would make sense to me, by the way it is behaving, that the temp sensor is a variable Open-On-Rise unit that has outlived its mechanical and thermal limits at cold block temperatures, as the light illuminates for the second cycle after "pretending" my light works for one cycle of about 20 seconds. It seems likely that the variability of the sensor is reliant on more than one OOR component, perhaps wired in paralell? Without a schematic of the sensor itself, we'll never know...

Still leaning towards the temp sensor in the head, above and between the 2 & 3 GPs, shown in the engine cutaways of the 617.952.

After "pretending" my light works, the light will illuminate the second time = next minute I have, the old sensor goes; gets replaced with new.

I will report back dutifully.

[funola]

Originally Posted by iandiam

Still leaning towards the temp sensor in the head, above and between the 2 & 3 GPs, shown in the engine cutaways of the 617.952.

After "pretending" my light works, the light will illuminate the second time = next minute I have, the old sensor goes; gets replaced with new.

I will report back dutifully.

You'll be wasting your time. The glow controller in the 617.952 has no connection to the block temp sensor. It is easy enough to verify that by ohming it out. The controller can possibly determine block temp by other means (by looking at glow plug resistance or magnitude of current flow through them).

[kerry]

Originally Posted by funola

You'll be wasting your time. The glow controller in the 617.952 has no connection to the block temp sensor. It is easy enough to verify that by ohming it out. The controller can possibly determine block temp by other means (by looking at glow plug resistance or magnitude of current flow through them).

But the light does, right?

[funola]

Originally Posted by kerry

But the light does, right?

kerry, I don't understand your question.

[iandiam]

Originally Posted by funola

kerry, I don't understand your question.

You guys are cracking me up.

If there is no temperature sensor in the block or somehow inherent in the water jacket, how does the glow plug relay know how much time is needed to glow?

Funola - The relay becomes energized in the II position of the ignition. Sometimes when the block temperature (water temperature) is high, the light will come on for just a moment, long enough for the relay to move from closed to open. If there is no temp sensor, resistance is a great idea...However, with plugs that age and provide resistance by being old and wearing, this speaks nothing to the temperature inside the water jacket. We all know, here, that old glow plugs will glow longer than new ones. How does the relay make that distinction on resistance alone?

Kerry - I understand your question, but funola makes a good point regarding resistance. If the light glows regardless of temperature (which it does) why does it not eliminate the glow light entirely - in other words, is the light just an "idiot light" when the water jacket registers, say, 120F or more? Is the duration of illumination based on the relay switching time?

Regardless, there is an issue here that supercedes the responses I have had so far - UNLESS - it is a bad temp sensor.

Allow me to rephrase:

If the temperature in the water jacket (obviously important to engine function because there is A TEMPERATURE SENSOR THERE,) is isolated from the glow plug circuit and operation, what is the logic associated with the glow plug relay? Does it just glow because you tell it to? If it did, it would glow for the same period of time, EVERY TIME. If you time the glow period based upon the water jacket temperature, wouldn't it follow that the glow period is reduced (as it is) when engine temperature is HIGH?

Maybe I'm high (I'm not.)

[kerry]

It's my understanding that the glow time is fixed, being the same every time regardless of block temperature. The temperature sender controls the length of time the glowplug light operates.
That's clearly the case in my 116 1979 SD as confirmed by timing with a watch.

[babymog]

The glow time is constant, the light is timed. No engine temp reference (just air temp of the relay module).

IIRC the engine (water) temperature connection was added in '87 to the 602 and in '90 to the 603 for the "afterglow" duration.

[kerry]

Originally Posted by babymog

The glow time is constant, the light is timed. No engine temp reference (just air temp of the relay module).

IIRC the engine (water) temperature connection was added in '87 to the 602 and in '90 to the 603 for the "afterglow" duration.

I'm looking at the FSM for 617.91. It clearly states that preglow light time is determined by ambient temperature and engine coolant temperature and the diagram shows a temperature sensor outside of the relay. I quote "A temperature sensor in the coolant circuit changes its resistance in dependence of coolant temperature and activates the electronic component."
It also states that in later models the coolant circuit temperature sensor is no longer installed. Don't know when that change was made.

[iandiam]

Originally Posted by babymog

The glow time is constant, the light is timed. No engine temp reference (just air temp of the relay module).

IIRC the engine (water) temperature connection was added in '87 to the 602 and in '90 to the 603 for the "afterglow" duration.

SO, TELL ME

What is the malfunction here, if it is not a temperature sensor?

Why do I 'pretend' the light is on for 20ish seconds and have the car start seamlessly (with one bad injector) and glow once then have the second glow illuminate the light for less than 5 sec?

Let's recap:

I turn the IGN to II for 20 seconds, No light, but she starts fine.

I turn the key to "start," she starts rough.

I turn the key to II for 15-20 sec (no less, no light) then off, then II again, the light comes on, starts smooth.

I turn the key to II when the engine is HOT, like 120+F HOT, and the light comes on for about 2 sec, and she starts fine.

Where is this leading? Where is the control that tells the relay to light the light for only 2 sec?

Tell me why something is funtioning COMPLETELY or PARTIALLY or NOT at ALL?

PLEASE include some schematics or logic. There is not a case for any part in this thread.