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  #1  
Old 01-21-2017, 09:00 PM
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Erratic temp gauge

Hi Folks,

For a while now the temp gauge in my 82 240D auto has been screwy.

I believe the gauge is giving false high readings. Everything seems to operate as normal until I have driven for roughly 20-30 minutes. Then the gauge begins to creep up to the 100 degree mark. If i tap on the gauge cluster the needle will drop down to the normal operating temp range right above 80 degrees.

Today on my drive after some long hill climbs I noticed the gauge was reading around 100, if I tapped on it it only moved slightly making me unsure if the motor was actually running too hot or not.

I checked the gauge against my inferred thermometer to test it accuracy and the hottest point on the engine was 78 degrees, lower coolant hose wasn't even hot, leading me to believe the motor wasn't warm enough to warrant the thermostat opening.

Is it possible that higher engine speed means a more intense electrical signal is being sent to the gauge cluster making the gauge go haywire?

I believe the engine is operating within the correct temp range but would like to be able to rely on the temp gauge.

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  #2  
Old 01-21-2017, 09:37 PM
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The temp gauge on the W123 cars are known to go haywire after so many years of use. Changing the gauge (temp, oil,fuel) is a pretty simple job. A good used gauge costs about $40-50.
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  #3  
Old 01-21-2017, 11:49 PM
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Something else to look at; potential ground issues with the instrument cluster. I had issues with my gauges sporadically flicking around, including one time when the whole cluster went dead for a moment, then came back a few seconds later. I performed the repair mentioned below, and viola, no more gauge flickering, save for a bouncing speedo around the 45-55 mph marks, but as I understand it, swapping for a new speedo cable will fix that.

Mercedes 300D 240D w123 Gauge Cluster Short DIY Fix | Up-Fixin | The Motoring Journal

It's kinda hard to see what's going on in this Internet article's picture, but once you get the cluster out, you can clearly see where to attach the new ground wire, both to the cluster and the ground stud (bolt). You'll see what seems to be about 100,000 brown ground wires coming to one location (the ground bolt). That's the chassis ground point; the other can be seen in the pic above.
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  #4  
Old 01-22-2017, 03:38 PM
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Another common cause of erratic gauge needles is caused by a bad ground connection in the gauge itself. They use a long brass rivet to hold it together and to provide a ground. Over time the brass oxidizes and develops a bad connection which causes the needle to bounce.
A fix is to bypass the rivet with a wire going to the gauge shell from the connection on the small circuit board (that is part of the gauge).

A fine tipped soldering pencil (and steady hand) is required for the one connection. The other to the gauge shell will require quite a bit more heat. Also some shells have a gold chromate coating that might need to be filed or sanded off to get a good solder connection. If doing this mod, go ahead and do the fuel gauge as well as it suffers from the same defect.

While you have the cluster apart, resolder the ground connection shown in the 3rd photo. It provides ground to the right side of the cluster and develops cracks as shown. Best to lay a fine piece of wire across to give it extra strength.
Attached Thumbnails
Erratic temp gauge-temp-gauge-1.jpg   Erratic temp gauge-temp-gauge-2.jpg   Erratic temp gauge-cracked-ground.jpg   Erratic temp gauge-ground-repair.jpg  
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  #5  
Old 01-22-2017, 06:25 PM
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I'll re-reiterate...... Almost all weird electrical issues are ground related. I sure would do the ground fixes as detailed above and see if it gets you where you need to go.

Dan

Last edited by Dan Stokes; 01-23-2017 at 10:40 AM.
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  #6  
Old 01-22-2017, 08:50 PM
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Quote:
Originally Posted by Dan Stokes View Post
I'll re-reiterate...... Almost weird electrical issues are ground related. I sure would do the ground fixes as detailed above and see if it gets you where you need to go.

Dan
Ditto.....Check the battery-chassis and the chassis-engine grounds. Also, check the connection on the temp. sensor itself before taking things apart or modification.
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  #7  
Old 01-23-2017, 06:30 AM
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I had to cut damaged wire off the plug,and resolder new wire,I spliced.going to temp guage and glow plug temp sensor on my 617. the plug comes apart carefully.
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  #8  
Old 01-23-2017, 07:07 AM
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Quote:
Originally Posted by mabbonizio View Post
If i tap on the gauge cluster the needle will drop down to the normal operating temp range right above 80 degrees.

This is a pretty good indication that the problem is in the instrument cluster.

Those brass rivets not only oxidize but also become loose with time as they are clamping plastic bits which may deform as they age.
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  #9  
Old 01-24-2017, 05:57 AM
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I'll have to check all the above out this weekend and report back
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  #10  
Old 11-03-2018, 12:46 PM
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Did you get lost? How’d It work out?!
I did all the things in this post about a year ago and still have issues.
After running new wires to the gauge shells, the bouncing stopped but my fuel gauge is generally off by 15% and the temp sensor rises 10 degrees too hot unless I knuckle the cluster
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  #11  
Old 11-03-2018, 04:37 PM
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At the risk of offending Rick, I'm going to borrow one of his photos. The coil that I've circled damps the movement of the meter. If the plastic core is cracked or otherwise compromised, the coil can ground to the metal gauge case. This will produce erratic readings. The quick & dirty is to use electrical tape or heat shrink to insulate the coil. The permanent solution would be to replace the gauge. Just one more thing to check.
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Erratic temp gauge-temp-gauge-2.jpg  
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  #12  
Old 11-03-2018, 05:02 PM
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Originally Posted by Mxfrank View Post
At the risk of offending Rick, I'm going to borrow one of his photos. The coil that I've circled damps the movement of the meter. If the plastic core is cracked or otherwise compromised, the coil can ground to the metal gauge case. This will produce erratic readings. The quick & dirty is to use electrical tape or heat shrink to insulate the coil. The permanent solution would be to replace the gauge. Just one more thing to check.
Now we're talking.

As it should happen, both of those little plastic spools are cracked at the top (or bottom of the spool). So to be clear, am I placing a stripe of electrical tape on the shell so that they can't bounce into it?
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  #13  
Old 11-03-2018, 09:03 PM
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Quote:
Originally Posted by Shern View Post
Now we're talking.

As it should happen, both of those little plastic spools are cracked at the top (or bottom of the spool). So to be clear, am I placing a stripe of electrical tape on the shell so that they can't bounce into it?

That would do it as a temporary fix.
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  #14  
Old 11-03-2018, 11:25 PM
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Thumbs up Gauge Weirdness

Thanx Frank ! that's the sort of good tech tip this board thrives on....

I too am always looking for poor grounds and/or dirty / corroded connections before getting to worried and throwing parts at the problems .

" K.I.S.S. ! " .
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  #15  
Old 11-03-2018, 11:41 PM
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Quote:
Originally Posted by Shern View Post
Now we're talking.

As it should happen, both of those little plastic spools are cracked at the top (or bottom of the spool). So to be clear, am I placing a stripe of electrical tape on the shell so that they can't bounce into it?
Don't mistake the cracks from the actual factory slits on the spool to allow the wire to go through. But if your spool is loose, then just get some super glue and glue it down so it won't flop into anything else.


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