1996 E300D Speedometer/Odometer Broken - PeachParts Mercedes-Benz Forum

briandownunda · #1 07-22-2008, 10:36 AM

I have a 1996 E300D that has an electronic speedometer and odometer that do not work. I have been using the tachometer to estimate my speed, however, my 15 year old daughter now has her driving permit so I figure I'd better fix it soon. Any help in diagnosing this would be most appreciated.

Here are the symptoms:

1) Functionality is intermittant, although recently, they almost never work.
2) In cold weather they seem to work more often.
3) The ABS and ETS (Electronic Traction System ?) lights are on, when not functioning, which is almost always now.
4) Sometimes they will start working after honking the horn, also, sometimes they start working after going over a bump in the road.

I was told by a mercedes mechanic that the engine wiring harness was bad & needed replacement. I replaced that but it made no difference (was just hoping it had something to do with that). In all fairness to the mechanic, his reason for replacing the harness had nothing to do with my speed/odo failure.

Any ideas on how to troubleshoot this problem ? I have a mercedes manual so could probably dig up a wiring diagram for this circuit.

Thanks in advance,

Brian

e300D97 · #2 07-22-2008, 11:51 AM

Could it be the clock spring?? Hitting the horn might be the best clue

TMAllison · #3 07-22-2008, 12:03 PM

Mileage AND speed are electronic?

Guessing SDS could scan that module and provide clues.

Also guessing the K40 relay feeds it power.

briandownunda · #4 07-22-2008, 12:55 PM

Hi, what do you mean by "clock spring" ? Also, how would you relate that to the horn & speedo ?

Yes the both the speedo and the tacho are electronic.

Thanks for the replies !

Brian

Matt L · #5 07-22-2008, 01:55 PM

The ABS and ETS lamps are the clue. Check your ABS sensors. They contain magnets, so fine ferrous particles will cling to the tip, reducing the voltage output. Disconnect each sensor and connect an AC voltmeter. Then spin the wheel. If one sensor puts out significantly less voltage then the rest, you've found the problem.

briandownunda · #6 07-22-2008, 02:10 PM

Thanks for the suggestion Matt, I will try it ! By the way, it seems like a poor design (to me) for the speedometer/odometer to fail because a single sensor out of 4 fails :-(

Thanks again,

Brian

briandownunda · #7 07-22-2008, 02:12 PM

Oh Matt, one question.....when you say "disconnect the sensor" does that mean that, by spinning the wheel, it generates it's own voltage output, like a magneto ?

Matt L · #8 07-22-2008, 03:35 PM

They generate AC voltage. The frequency depends on the wheel rotation speed, which is fed to the ABS/ETS system. That feeds the speedometer.

I don't know that one sensor out of four will kill your speedometer, but checking the sensors is the first obvious step. I would start with the fronts, because when I spin my rear wheels in the snow, the speedometer continues to read zero.

pjc · #9 07-22-2008, 10:20 PM

To clarify Matt L's suggestion: Leave the sensor connected to the wheel, but separate the wiring connector you'll find when you follow the wires away from the sensor. Connect your voltmeter at that point. The sensor generates an AC current based on the interaction between the tip's magnetic field and the gear-like disk attached to the interior side of the hub.

I had similar symptoms in my 96 E300D, and I tried checking the sensors, too. My problem turned out to be the K40 relay, which I tried repairing twice (many thanks to Matt L for his informative posts on that topic) before I bit the bullet and bought a new one from Phil for $107 including shipping. If you've got the hundred bucks to spare, spring for a new K40. It's a 5-minute replacement, and the worst-case scenario is that you end up with a new K40 and a good spare.

Cheers,

Paul

Matt L · #10 07-23-2008, 12:25 AM

I agree with pjc here. If your K40 relay module is original, you are almost certainly due for one. My last solder job has held up so far, but I'm still leery of it.

briandownunda · #11 07-23-2008, 09:43 AM

Many thanks guys for the info on the K40 relay ! Can you tell me where it is located on an E300D ? Also, is there a way to test it to see if it is, indeed, bad. One more thing, can you point me to a description of what this relay does ?

Thanks in advance,

Brian

TMAllison · #12 07-23-2008, 09:50 AM

Its in the right side computer box under the bonnet.

You can remove the bottom of the plastic housing and look with a magnifying glass for minute cracks on the printed circut board.

pjc · #13 07-24-2008, 06:17 PM

Here's two pix to locate K40 relay and prime areas on circuit board for inspection/resoldering. Larger joints on board may crack first, but resoldering them may cause problems in smaller joints.

Decision tree (from experience): Are you broke? If yes: pull K40, inspect, repair, pray. If not, buy new K40, put old one in trunk as emergency spare.

briandownunda · #14 07-25-2008, 02:18 PM

I just went to the dealer this morning and purchased a new K40 relay...it took all of about 2 minutes to install.......and my problems are completely solved !!! No more ETS light, no more ABS light, the speedo/odo work like a charm !! I am ecstatic :-)

You know I went to the dealer about a year ago when this problem first surfaces and had a full diagnostic executed....they told me I needed about $3500 in repairs & none of them included the K40 relay.....shows you how much their diagnostic tools are worth.

Thanks very much to all who have responded to this thread.....I owe you one !!!!!!!

Thanks again,

Brian

pjc · #15 07-26-2008, 05:16 PM

Are you in Oz or the US? What did your dealer charge for the K40?