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

[kerry]

It's either in the coolant circuit or in the relay box.

[iandiam]

Originally Posted by kerry

It's either in the coolant circuit or in the relay box.

Great.

How?

Can't find any electrical evidence to support that. What am I missing? SERIOUSLY, WHAT?

[kerry]

I put my FSM away but when I was looking at it, there were two kinds of diagrams, one with the sensor outside the relay box and one with sensor inside. I don't know what years that FSM covers and whether it applies to your car or not. I also looked at the FSM for the 207/307d vans. It showed no temperature sensors in those circuits at all, so there is obviously some variation in the 617 preglow systems.

[iandiam]

Thanks for the interest, folks, especially for an issue that we all thought we had tackled many times before.

Originally Posted by kerry

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.

This is the closest explaination as to what has happened (that makes any kind of electrical or mechanical sense.)

Let me recap (again):

1.) I have installed a duraterm post-glow unit.

2.) The plugs in my E.D. w126.120 OM617.952 glow every time

3.) the LIGHT does not come on every time. In FACT only after a full cycle or when the engine is above a certain temperature, seemingly 95F, give or take.

There does not appear, in the diagrams for the 952, to be a temp sensor inside the STOCK relay, but I have not seen a schematic for the updated, duraterm post-glow unit I have installed. Previous to the installation, the STOCK unit would illuminate the indicator, but would vary time-wise. When hot, it might have been only a second or two. Cold, sometimes 20 or more. Please see Kerry's quoted text above.

There seem to be varying opinions here, but the facts are these:

A.) the light will not illuminate for the first cycle if the block is cold

B.) the light WILL illuminate for the second cycle if the block is cold, but only immediately after the first cycle

C.) there is a VISIBLE temperature sensor in the block, between and above GP 2 & 3. It attatches to the multiplug above and rear of the relay, inside LF fender. God...I'm repeating myself...a lot

I am having an EXTREMELY hard time letting go of the possibility that this sensor somehow has an effect on the INDICATOR light, and based upon recap item #3 and facts A, B, and C, I can't see why I shouldn't replace that sensor.

NOW, maybe I will be "...wasting [my] time..," as one member has so delicately put it, but in the meanwhile I am waiting for ANYONE to explain to me how this is NOT a temperature sender issue. If it's not the one that is most obvious, I feel that there is one somewhere that needs a replacin'.

And I don't like wasting my time. That's why I like to find out what the problem is caused by before I start throwing cash at it, hoping it will surrender and leave.

[funola]

Here are some possible explanations:
1. Your Duraterm controller is flaky?
2. There is an intermiitent connection to the the dash glow light bulb.

Time is money. Go ahead and waste it. Replace the block temp sensor and report back.

[kerry]

I have an 85 TD and a 79SD. Both of them have two coolant temp sensors in the head 'between' the glow plugs near the front of the engine. I've assumed that the 85 had the later model relay with the glow light controlled by the temp sensor in the relay itself and the the 79 controlled by the block temp sensor. I know the relays are physically of different shape in each car and the glow light stays on much longer in the 79SD than the 85TD at the same ambient temperature So, I am puzzled at the existence of both temp sensors in the 85. Perhaps someone else can explain.

[iandiam]

Originally Posted by funola

Here are some possible explanations:
1. Your Duraterm controller is flaky?
2. There is an intermiitent connection to the the dash glow light bulb.

Time is money. Go ahead and waste it. Replace the block temp sensor and report back.

Perhaps both are true, but #2 is definitely true. It's just that it's predictably intermittent and I was hoping someone could tell me why.

Does ANYONE know where to find the schematic for the duraterm relay? There is a temperature sensor, thermostat, thermistor somewhere.

If heat=resistance, then the light should light when it's cold as it is designed to do, right? so it's not doing that. I'd like to know why.

[SD Blue]

I'm not sure where you can find a W126 schematic on-line. I can fax you a copy of what I have, just PM me with a number. (Sorry, my scanner isn't talking to my PC at the moment.)

However, It's a pretty simple schematic to explain. Pre-Glow MY83/MY84/MY85

C137
1-5 Glow plugs (internal relay connection to current monitor)
C130
Red 8ga. to fuse (internal relay connection main power contacts & current monitor)
Pin 1 RD/BK Ignition switch Main pwr input START/RUN
Pin 2 VI Ignition switch Glow plug cut-out START
Pin 3 BU/WT Pre-glow Indicator
Pin 4 BR Ground

When I checked my engine, there are two temp. sensors. However, one isn't connected to anything. The main sensor (between #2 and #3 glow plugs) is for the instr. cluster guage. (Note: It appears to be in the same harness as the glow plugs but isn't connected to the relay.) Yours may be a little different.

The temp. sensor on my schematic shows it to be an internal component of the Glow Plug Control Relay. Evidently it only senses ambient air temp. but note there is also a current monitor.

For the Pre-Glow Indicator Light, my schematic shows connected to pin 3 of the Glow Plug Relay and G102 an instrument cluster ground.

[kerry]

Originally Posted by SD Blue

When I checked my engine, there are two temp. sensors. However, one isn't connected to anything. The main sensor (between #2 and #3 glow plugs) is for the instr. cluster guage. (Note: It appears to be in the same harness as the glow plugs but isn't connected to the relay.) Yours may be a little different.
.

Does that second temp sensor have a wire connected to it on yours? The one on my 85 TD does. That's why I was puzzled since I thought it should have the sensor in the relay box.

[SD Blue]

Originally Posted by kerry

Does that second temp sensor have a wire connected to it on yours? The one on my 85 TD does. That's why I was puzzled since I thought it should have the sensor in the relay box.

No, there is nothing connected to the second sensor (between #1 & #2 glow plugs). I've looked around to see if something had been disconnected but there doesn't seem to be. Your '85 probably has more in emissions regulation than the '84. I noticed the original question was about a Euro delivery and that may come into play as well. (Some folks have told me that mine is Euro delivery, due to velour interior, but I haven't been able to confirm that.) It seems there may be a few variables we're dealing with.

[iandiam]

Originally Posted by SD Blue

The temp. sensor on my schematic shows it to be an internal component of the Glow Plug Control Relay. Evidently it only senses ambient air temp. but note there is also a current monitor.

For the Pre-Glow Indicator Light, my schematic shows connected to pin 3 of the Glow Plug Relay and G102 an instrument cluster ground.

I am appreciative of the help.

Unfortunately I have never taken apart an AFTERMARKET, UPDATED, POST GLOW UNIT.

Bearing this in mind, the FSM for the model year and information about the STOCK unit do me little good but to speculate.

Even so, if it is an ambient air sensor, and GLOW time is determined by resistance, I still have a strange issue to deal with regarding the light. Next step is to unsnap all the connectors and inspect the connections. While there is continuity, one of the conncetors may be what is making it all wonky.

A loose connection would, however, likely be more aparrent at higher heat and therefore higher resistance, no?

Most peculiar.

[funola]

Originally Posted by iandiam

Unfortunately I have never taken apart an AFTERMARKET, UPDATED, POST GLOW UNIT.

What's stopping you from doing that? If you do, take good pics inside out and post them!

[moruzzi]

So did you ever resolve this??? From the last message in the thread you have not indicated any resolution.

I ask because I have a very similar situation.
1983 300SD replaced the glow plugs and relay with the updated Bosch kit. The relay kicks on and off correctly when checking voltages at all 5 of the glow plug pins. No problems starting.

I never get a glow light when cold (did not try your repeat procedure yet). When shut down hot the glow light will glow on putting the key in position II (as you experienced).
A while after shut down - maybe 15 minutes or so - no glow light again.
Glow light will illuminate fully with the connector to the plugs unplugged.

I may pull the instrument cluster and check grounds there as the indicator lights on the dash have varying brightness - battery is fully bright, next two are fairly dim, and glow plug light is flakey as described in this thread. I have not yet pulled each glow plug and apply voltage to see that it gets red, but I plan on checking that as well.

Paul

[pyrojoe22]

This un-named diagram I found would indicate that there is an internal sensor in this unit (I'm assuming stock unit based off the old looking 300SD text). The brown wire goes to ground, and it appears that an internal component measures the resistance of that circuit as the current goes to ground.

[barry12345]

Just a guess. The current flows the first time you activate it. Something inside the unit although it does not totally function warms up. Then on the next try the unit totally functions.

A simple test to establish this might be to monitor the input circuit at the controller for the on voltage. If the trigger or on voltage is present both times I would suspect the controller.

I guess it could be a really high resistance connection as well that improves once warmed up but think this unlikely outside the controller. The only one outside that would not produce smoke in my mind would be the ignition switch contacts.

But they get ruled out with the monitoing of the input to the controller. Either a meter or a light bulb should be adequate. A meter being preferable.