I have a 82 240D which I’m restoring. To remedy the 3 watt dim dash lights I’m replacing these with LED’s lights to increase illumination. Plus to enhance the general appearance. After replacing the bulbs on the left and right hand side with the cluster un-mounted from the dash and with enough room to get your fingers in and around the light socket, I carefully removed the old bulbs and inserted the T-10 Wedge led’s and reconnected them into the cluster. As I turned on the dash light, I experienced a small puff of smoke coming from the back side of the cluster as well as the normal electrical burn smell. This I figured was not a good thing!


I sensed that I either drew to much current out of the circuit and/or burned out a component or blew the rheostat. After this little episode, I started to check out other things like, fuses, and did the car still start. After determining that all systems still seemed to work, I noticed that the lights on the center console where my manual climate control / fan speed and the horizontal section where the shifter is, the lights for the door switches no longer illuminated. I’ve read on other forums where the light rheostat does not control these lights, but the fact remains that when I smoked what ever component that was in the dash, it affected the other lights also.

I strongly believe that it’s the rheostat that was cooked. I know this can be bypassed on the 300d’s for I have one from ebay I bought to learn on and to sample another repair which needs to be done to a broken odometer. I’m using this to test prior to committing to the real car.

I don’t have a problem with buying a pricey new dimmer switch, but for testing purposes, can the rheostat on the 240D be bypassed ? Also, if other MB enthusiasts have replaced those dim w123 dash lights with led’s, what type where used?


Thanks in advance for any help.
Greg

There's a specific LED thread for the dash lights.

w123 LED light upgrade

"Cooking" the rheostat may be less likely than burning a circuit trace on the panel. You can bypass the rheostat pretty easily.

The console lights are on the same circuit and downstream of the dash, so yes, they're related. Maybe the shifter light specifically is not, but the CCU lights and the radio illumination are.

Understanding the prism design and the light "channel" and getting everything cleaned of 25+ years of haze can improve the lights without going to the LEDs.

If you really want/need 'great' illumination there is a Canadian company that eliminates the prisms and installs "direct illumination" LEDs, put it's pricey.

Thanks Yak, for the response. Going down the road of the burned electrical track and not the rheostat, could this be fixed by bridging over the burned out conductor. I believe that they make a fluid for this?

W123: Instrument cluster Printed Circuit Board repair

I looking and studying the donor instrument tri-gauge portion of the cluster which the rheostat mount to, I noticed the same burnt wire-trace which is highlighted in the supplied links. Thanks.
This brings up another issue! MB according to other forums has had problems with the dash gauges and had replacement circuit boards available at one time. This look like a design flaw. In tracing with a meter, you can see where the wire trace gets thin leading back to the big plug connector in the circuit board. My best guess is that an excess current flow caused the trace burn. A burn makes sense! This trace provides power to the rheostat only. If a burn occurs before 12vdc is supplied or after being supplied to the rheostat this would explain the dash lights being out. My calculations (ohms law) I'm sure are off, but either 1/4 to 1.4 amps would be supplied for the dash light circuit. I think it should be able to handle a little more than that. Maybe a modification is in order with a removable fuse.
I need to use the car for the next few days, so I'll remove the panel after that for other things need to be done also.

LEDs draw less current, burnt traces are cause by "shorts" to the circuit.

I just soldered a piece of wire to bridge the trace & added a external fuse.

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Thanks cypress. That's exactly what I had in mind. What's your recommendation for wattage on the fuse ?

I converted my whole interior to LEDs, after I pulled the external wires for the fuse...I connected a multimeter in series & turned all lights on to see what the current draw was.. it was much less than 1amp...so I used a 1amp fuse.

In your case, you'll still have incandescent lighting...so your current draw will be much higher, so I suggest you do the same to choose your fuse size.

welcome to the Forum!
this is a diesel related question, so I'll move this to the place dedicated to them.
(my diesel discussion subfourm!)

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My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
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1987 300TD
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Cypress, research indicates that a fiber-optic octopus cable is used to supply lighting to those controls, but I haven’t gotten down to that level yet. I bought the car 2 months ago for it’s relatively rust free except for the front fenders, which will be dealt with shortly. Interesting for the PO lived in Harrisburg PA and they have allot of salt in the winter. Not to say we don't around Baltimore. The motor has blow-by but that’s ok. I used to rebuild car and boat engines in a former life at a machine shop when I in my 20’s. My main objective was to make the car mechanically sound as possible and safe to drive. That’s done. The interior is in good shape for the age and the only thing I wanted to do was increase the visability on the dash panel at night.
Having possibly shorted a wire trace didn’t sit good with me, when I couldn’t justify why this happened. As you mentioned, a short probably burned the wiring trace. By definition a short circuit has infinite current flow. A basic circuit must have a resistive load to prevent a short. Studying the original bulbs last night, I noticed that one conductor is on one side of the bulb, and the second conductor is on the other side. This indicates the MB light sockets are serial devices. Comparing this with the T-10, I discovered they where designed to fit into a socket wired in parallel (conductors on both sides of the connector). That’s when it dawned on me. I wired parallel lighting devices into a serial circuit. The research over the net indicates that a user can just plug these into the MB light connectors and they will work fine. That is wrong! . Rerouting or bending the conductors on the T-10 led’s so that they form a series connection, rather than a parallel made all the difference. I’ve did some testing with the T-10 leds last night with test leads, meter and a battery. The led’s lit as normal. It’s been a few decades since I’ve dealt with basic electrical theory so I’m a little rusty!

Yes, these older cars do need a lot of tlc. You do need to be a DIYer some what to make sense in owning one of these older cars.

They are connected in parallel, not in series. These T10 leds are meant to be pnp. You do know leds are polarized, right?

The bulbs that lights the instrument cluster you are trying to replace with T10 leds is also the light source for the fiber optic cables. Your cluster will be brighter, but your dash switches will be dimmer than stock. My solution was to hard wire a led to each of those switches to solve it.

Interesting about the fiber optic cable switches being dimmer than stock. Any idea as to why this would occur? As for the Led’s being polarized, no I didn’t know this. I found sites which sell polarized / non-polarized bulbs but no explanation as to what they are other than a polarized is a narrower beam of light with less defraction’s of light saving energy.
Ok. Using your solution, what kind of led’s did you use in hard-wiring the switches?

Polarized, meaning..LEDs can be connected one way only (unlike incandescent). Connect "Anode" to +12V, "Cathode" to ground.

I guess the led's light spectrum just doesn't transmit too well over fiber optic. At 1st I thought it was directional issue (LEDs are directional). I even tried pointing a led directly at the fiber optic cable, still very dim.

I found this high intensity miniature led locally here. Find a led that is same size as the fiber optic cable connector.

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All the complaints about 240d and 300d dash lights are easily solved with 10 WATT Xenon bulbs from Bulbs.com or 1000bulbs.com - put them in a friend's 240d and the cluster is lit up like a modern one - the factory used a ridiculous 3 watt bulb and the most you can get in a parts store is 4.6 watts in a 168 bulb
Bulbrite 715510 - 10 Watt - T3.25 base

I would be very concerned about the heat generated by a 10 watt halogen bulb. You would likely melt the instrument cluster plastic, etc.

I would stay with the stock bulbs or go LED.

Sincerely,

PE