I've wanted a permanently-installed voltmeter in my 1995 E300D for some time, to help me keep track of the battery and charging system. After experimenting with a number of options, here's what I chose. [This is not a DIY but I could add some sage advice / instructions if there's interest.]

An analog voltmeter could have gone into a panel replacing the ashtray (I still like the idea) but I decided not to go that direction, as I did on my first 124, with a 3-gauge panel (no voltmeter, though).



For over a year I used a plug-in voltmeter from the local auto parts store. It worked well but clearly wasn't a factory part and required the ashtray be open all the time.



Recently I discovered the large variety of voltmeter modules available from various suppliers on the Internet. I bought a couple to experiment with and tried a number of locations, using adhesive putty to hold the part in place.



I liked the red LED voltmeter but it was too bright at night. An LCD panel with an adjustable backlight would have worked better but all were too large. I also wasn't really happy about irreversibly cutting a hole in my lower console wood panel.

Finally, this forum's Sixto, your friend and mine, suggested building the voltmeter into the dome light assembly and that's what I did. This involved cutting away part of the seat belt warning, a non-reversible step, but replacements for the dome light assembly are inexpensive in the junkyards so I wasn't worried.



The tiny green LED voltmeter is bright enough to be seen in daylight but not so bright that it causes glare at night. It's wired to the power lead and ground for the sunroof so it goes on and off with the ignition switch. It's easy to see yet not where it will interfere with the driver's view of the road. Finally, it makes use of a spot (the seat belt warning) that I wasn't paying attention to anyway, since putting on a seatbelt is pretty much automatic these days. [The 13.2 Volt reading is because I had a trickle charger on the battery when the picture was taken.]

Jeremy

__________________

"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

Jeremy,

I like it. Part number? Vendor? Cost?

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Ditto..

__________________
1983 123.133 California
- GreaseCar Veg System

This was an eBay purchase.
Seller "frentally" in Duluth, Georgia
Title of eBay listing "Green DC 0-30V 2 Wire LED Display Digital Voltage Voltmeter Panel Car Motorcycle"
Category "Business & Industrial > Electrical & Test Equipment > Test Equipment > Meters > Amp & Voltmeters"
eBay item number: 221524041592
Size 2.3 cm long, 1 cm high, 0.6 cm thick

Voltmeter uses two wires, requires at least 3 Volts to operate so it can't really work down to 0 Volts even though the listing says so. My tests show the voltmeter draws about 10 milliamps so you could safely leave it "ON" all the time provided you drive your car regularly or keep the battery on a trickle charger.

Cost $3.99
Shipping free
Payment PayPal

Looks like these parts are still available and in different colors.

Jeremy

__________________

"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