This..... Is the source of all (or most) ground issues in a w123 instrument cluster....



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1983 123.133 California
- GreaseCar Veg System

That metal tap is grotty too ~ I always polish connectors that are not shiny .

DO NOT abrade them with sand paper etc. ! . use metal polish and clean rags .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Yep, I reflowed that some time ago and definitely helped. The classic ground wire to the chassis did nothing to stop my bouncing gauges. It was soldering the wire to the shell that finally did the trick... unfortunately it also introduced some new, erratic behavior. Will give reattaching the spool a look in the morning and report back.

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1981 240D 4sp manual. Ivory White.

Are these gauges and issues common to other models of 81-85?

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

No real effect... the previous soldering job from rivet to temp gauge shell had come loose. I soldered it again but doesn’t seem to be doing much. For what it’s worth, the issue only occurs around operating temp. If I slap the cluster while she’s warming up, nothing happens.

Think I may have reached the end.

Who’s got a spare 14 pin fuel/temp/oil gauge they’d like to sell me?

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1981 240D 4sp manual. Ivory White.

Okay, I went beast mode and solved this.

Pulled the cluster once more. I’ve long suspected whatever alloy the temp gauge shell is rolled from is unfriendly to solder. I wrapped a thick copper wire (speaker wire) around the shell and twisted it tight till I couldn’t move it with pliers. Soldered a wire from this ring to the spot on the board. It’s hideous. Shameful in fact.

That said, it’s 82 degrees and cool.

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1981 240D 4sp manual. Ivory White.

See above.

This was likely true of the fuel gauge -The golden shell sanded down to a dull silver. The temp gauge however was silver already, and while it may have had a coating, I gave it a fairly good sanding to no avail. In any event, i’m sure you’re right. There must’ve been something preventing a good connection.

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1981 240D 4sp manual. Ivory White.

Maybe not enough heat. I use a Weller 100 watt gun for something like that.

That sounds about right.
My iron (my first to be fair, bought for fixing the clock two years ago) was $5 off a dusty shelf in Napa. It's about the heat of a 100 watt lightbulb. Perhaps time to upgrade.

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1981 240D 4sp manual. Ivory White.

Just wanated to add my anecdote.
My 1979 300D had a bout of bouncing temp gauge when it reached 150F.

A parts schematic I had from "Performance parts" shows a "thermal vacuum valve" for certain models located on the passenger side of engine block.

Curiously, this bouncing gauge of mine prefaced my door locks beginning to fail.

Then my engine would not shut off periodically.

I had to block off the door lock vacuum lines at the firewall and coincidentally, the temp gauge became steady over a period of 3 days.


I had my mechanic, David Pham, repair the entire vacuum system.
Door locks work, engine shuts off and yes, the temp gauge works fine.

No bouncing.

So I did not bother to check the ground solder point.

(Yes that point does crack, as my spare temp gauge displays the fine crack as the previous photo shows.)

So, perhaps a vac problem can contribute to the temp gauge funkiness via the thermo vacuum valve inefficiency. (?)

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

Your 'Benz is clearly haunted .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

http://www.peachparts.com/shopforum/members/rick76.html

I would like to extend a sincere thank you for your pictures and explanation of where to find the broken solder joint. I was able to get it diagnosed very quickly and back to normal operation after reflowing the joint and another ground trace.

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Turbo300Mercede
87 300D W124
83 240D W123
80 300 TD Wagon W123, 4 Speed from 79 240D, SLS Rear Suspension

How timely indeed! My gauge just started the erratic +10-15'C high readings last week. I've been wanting to pull the cluster to replace some bulbs and do the odometer fix for a while now. Good to know there is a good 'while I'm in there' quick fix that can be done!

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1985 300D
The rest:
1957 MGA (comatose)
1965 Falcon (sleeping)
1966 E-100 (rust test in progress)
1976 Ford 3400 D Tractor (workhorse)
1978 Mercury Zephyer (5L playtoy)
1995 Isuzu NPR D (fetcher)
1998 Subaru Legacy (Spare)
2000 Toyota Sienna (School bus)
2008 Toyota Prius (Commuter)

I occasionally get totally lost in the "Might As Wells" when tinkering with one of my jalopies .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better