I just switched my tachometer/clock for one in better condition. I had new capacitors installed in the clock. Also installed a better used fuel/temp/oil gauge. Upon testing the gauges went awry. The hazard warning switch causes the fuel needle move left and right in rhythm with the ticking and the fuel gauge goes to full when the headlights are turned on. Am I attaching the muti-pin electrical harness incorrectly or might the soldering be bad on my replacement gauges? Your help would be most appreciated! 1985 300D.

You have one or more bad grounds. The fuel gauge, which is normally grounded through the electrical harness, has lost its ground and is finding an alternate ground through the flasher bulbs. When they are "on" (illuminated), the fuel gauge loses its ground. Every time the flashers blink "off," the fuel gauge has a ground again. That's why it moves with the flashers. Similarly, turning on the headlights changes the voltage that the fuel gauge uses to indicate how full the tank is.

You should take the cluster apart again and make sure that all of the metal-to-metal connections, which are not only physical connections but in some cases also electrical ground connections, are good. Also check for cold solder joints on the PC board where the gauges and the multi-pin connector mount.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

There are wiring differences between years. The pins on the back of the fuel gauge will match up with the plug but the wiring isn't necessarily the same across the range of 123 gauges. So after you make sure you grounds aren't faulty, make sure your old and new gauges are the same. It make take some circuit board trace following.

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Thanks for your responses. Both replacement gauges are the correct year. I noticed that if I move the entire housing it corrects the situation. Mainly with the right side of the housing slightly forward. However flush back the problem continues. I even reinstalled the old fuel/temp/oil gauge to see what would happen. Same. Must me some minor connection issue.

notorious issue with these clusters is the plastic the screws go into shatter, and you loose ground. I take them apart, and glue all the screw mounts together prior to re-installation.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I had a feeling it might be with the fastening. Some of the screws did not go straight in smoothly when I reinstalled. Checking that out now. Truly like "clock work"! What a pain!

Just one of the joys of using an antique car as a daily driver.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

So I should glue the gauges down instead?

We after all that BS I managed to fix it by moving around the insulation. I want to thank all of you for taking the time to read my post. You are the nicest people!

If by insulation you mean the heavy felt pad stuff, a technique I found to be helpful when reinstalling is to use a cereal box cardboard "shim" between the cluster and the insulation to provide a smooth surface for the cluster to slide against. Sort of like a shoehorn. When the cluster is almost all the way in, you slide the cardboard out.

It reduced most of the slide-and-move due to the sagging/unglued insulation and made for an easier and snugger reinstall.