Ok, so I've been posting here and there on the forum that my silly odometer "slips" at high speeds. It seems to click off miles okay while I'm going under 60mph but when I hit 75 it is clearly slipping.

I was getting really mad because my MPG is all messed up. I checked my odometer with an iphone app and it is 20% off. It dang near stalls when I hit 75mph so the error must be crazy high on long trips. I got frustrated because my highway mpg is lower than my around town.

So I finally solved my problem. A lot of folks wouldn't notice this. But I calculate mpg and something just didn't make sense. I've known for awhile it slipped at high speeds after seeing my trip from San Francisco to LA was only 200 miles. Go figure. Lots of high speed.

Around town it acts normal. This is sad because I thought this was a low mile 300SD when I bought it. It prolly has way more than the OD says. I even checked to see if it spun when I bought it ten years ago and clearly it clicked off miles...but not accurately. So I set about to fix it for good this week.

First I installed the garagistic gear set. When I opened the thing it had rotten gears. Soft and crumbly. Cogs all over the place. I was sure I'd solved the skipping problem. I was missing a tooth here and there and I figured that was it. Put in the gears, no problem. Drove the car. The odometer stopped making so much noise and it clicked off miles. I was pretty happy till I hit 75 mph on the highway and it just stopped. I backed off to 60 and it started clicking off tenths again. ARRRRGGGHH!!!

So I took it apart again. I found a JamesDean post with the electronic pinouts for the unit so I could drive it on the bench. This is the electromechanical deal and it cannot be driven with a drill. You need a signal generator and a twelve volt power supply. The board is clearly marked G+ is speedo signal which I set at 5 V p-p sine wave 100 to 200 Hz. 15 which is 12V in. 31 which is ground.

Once I got it up and running I swept the frequency and the speedo needle went from 0 to 80mph smoothly. No problem. But I could clearly see that the odometer was not reliably clicking off tenths from 60 to 80mph. It got worse as I sped it up. It wasn't really slipping. It was skipping. As if it was missing a pulse.

So I changed the caps.





This did nothing for the skipping. I put in some good tantalum replacements I had lying around. I looked up the data sheet for the speedometer chip that drives the odometer motor on the little board. ITT UAF0115. I found data on a similar UAF2115. The caps are simply power supply decoupling caps. Yeah, if they are bad it will mess things up but apparently that wasn't my issue.



I checked out the electrical signals coming from the chip that drives the motor using an o-scope. They were ugly pulses but they were all there. They appear to be a clock type signal...one pulse to advance the mechanism and another to hold it in place. The motor seemed to have two phase coils to acheive this. pulse - hold - pulse - hold. I think. Anyhow signals looked okay. No skips. But I could audibly hear missing clicks. The gear train in the odometer looked all herky jerkey. Something was still wrong. Something mechanical.

So I started pressing my finger on the drive shaft between the odometer motor and the drive pinion. When I loaded it with some friction it seemed to behave. Less skipping and the noise became regular. I noticed the magnet in the motor had a heck of a lot of end play. So I decided to put a little o-ring in with a dab of silicone grease in the little end cap.

Little o-ring



endcap



Works real well now. Reminds me of the cigarrette butt fix for the tachometer amp. I'm not sure if it is the friction or positioning the magnet on the extreme end of the excitation coils but hey, it no longer skips.

Hope this helps somebody. I really have wondered about this thing for years. I think I've finally licked it. Well, it works on the bench. Checked the odometer with a stopwatch and it matches the needle at 80mph. I'll check using the iPhone odometer app and get back.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles