Odometers Not Agreeing - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 09-20-2017, 08:44 PM

So this pertains to a 1986 300SDL and I haven't found any info with the same symptoms I'm having.

Last year I replaced the odometer gears due to the famous crumbled gear issue and the odometer has been working normally as far as I can tell ever since. I've always noticed very poor fuel economy in this car and chalked it up to being poorly tuned, however I'm not so sure that's the full story anymore.

Recently I did several fuel economy runs with short fills in the tank. Typically 75 miles or so before refilling to see if my tuning was doing any good. I recorded anywhere from 24.5-26.5mpg over 4 fillups, telling me that the fuel consumption is relatively normal.

Now here's where it gets weird. Once the trip odometer crosses 100 miles, it becomes slow. Really slow. Normally I wouldn't have noticed this, but I've been paying attention to the fuel economy like a hawk since doing the tuning. Now I have 135 miles shown on the trip odometer, while using over 1/3 tank of fuel. The problem with this reading: There's actually closer to 180 miles done at this point according to the main odometer! The discrepancy would explain the typical poor economy I log since I tend to fill up around 1/4 tank (usually around 325 miles on the trip odometer)

To confirm the difference, I ran a trip that I know is exactly 6.8 miles. The main odometer showed I did somewhere between 6 and 7 miles. The trip odometer showed I did 3.4 miles.

What on earth? Aren't the odometers geared together? How is this even possible?! I reset the trip odometer and ran the same trip backwards and came up with 6.6 miles. That's quite a difference from 3.4 or the known 6.8!

Has this ever happened to anyone else? Is something in there loose that can be tightened up? I can understand the entire odometer system being slow or failing, but having the main odometer working and the trip odometer slowing as the miles increase is weird!

Mxfrank · #2 09-20-2017, 10:21 PM

The odometers are separate instruments, as is the speedometer. They're all packed into the same case because they're all driven off the cable. The trip odometer probably has a slipping gear.

Diseasel300 · #3 09-20-2017, 10:52 PM

The speedometer/odometer in the W126 is all electronic, there is no cable involved. The odometer portion is driven off a stepper motor.

I took the dash cluster out this evening to see if there is anything obvious going on. The main odometer and trip odometer are geared together and tight. There is no possible way that the trip odometer can slip while the main odometer doesn't. All of the number wheels are good and tight on the shafts and there is no slop in the mechanism. What I did find was a binding reset button. I can only assume that this was rattling in the cluster or holding the reset mechanism just so that the numbers would increment, but not reliably. I resolved the sticking reset button with a dab of silicone grease and now it operates smoothly.

In addition to the binding reset button, I noticed an unusually large amount of play in the odometer stepper-motor shaft. The amount of slop was such that the drive pinion could be ~85% disengaged from the driven gear. With the wiggle room allowed in the driven gear, it could become fully disengaged and indeed had been doing so as evidenced by a beveled wear pattern on the ends of the teeth. I remember reading somewhere on this forum (I can't find the thread to give the OP credit!) about inserting an O-ring behind the plastic bearing on the stepper motor to eliminate the runout. Apparently at one time there was a rubber bushing installed that has since gone missing. I used a 5.5mm x 1.5mm O-ring which fit perfectly and gave a firm fit without causing binding.

Took the car on a test-drive (the same 6.8 mile trip as earlier) and it read 6.8 miles. I then took it on a couple of other known-distances and the distances agreed with the known-values. I will let the mileage accumulate and see if I still have problems at >100 miles.

As an interesting sidenote, installing the O-ring completely eliminated the "Quartz-Clock" noise that the stepper motor made as it indexed the odometer. The odometer drive is now completely silent.

If anyone reading this thread remembers the thread I'm referencing above (or if you're the OP in that thread!) please chime in to give credit where due!

ykobayashi · #4 09-20-2017, 11:29 PM

Maybe this will help?

Fixed 1982 300SD odometer slip at high speed

Diseasel300 · #5 09-20-2017, 11:34 PM

That's the one! A big thank you to posting that, I remembered reading it, but couldn't find the thread when I wanted to find it. You hoped it would help someone, now you know!

ykobayashi · #6 09-20-2017, 11:41 PM

Glad to give something back to this great forum.

Diseasel300 · #7 09-22-2017, 09:48 PM

I've done a bit of driving these last couple days so got up to 141.8 miles and decided to do a fuel economy check since that's been my issue. Typically by 125 miles I'm down to 2/3 of a tank, today I was just shy of 3/4 according to the gauge. The 141.8 miles took 5.887 gallons of diesel for a fuel economy of 24.09mpg for mixed driving. Pretty well right where I expected it to be. Looks like the O-ring trick worked for the odometer!

ykobayashi · #8 09-23-2017, 09:39 AM

Hey that's good mpg. I get 21 average after the fix. But I drive a steep mountain road daily. Mine shot up about 15% after the fix. I guess this doesn't count as a fuel economy improvement fix

Funny, my car on long straight highway trips does 30 (done by gps not wonky odometer in past) if I don't run AC. AC kills about 20%. This was my experience on my 240d too. Hills kill another 15%.

I never did understand why the ring stopped the odometer from slipping. What you said about the beveled gear sounds right. When I ran mine on the bench I looked through the clear case and it seemed the motor vibrated but didn't really spin forward at high speeds. It made that rattle sound. I wonder if something just wore out of its specified tolerance like the end play.

Diseasel300 · #9 09-23-2017, 10:11 AM

I'd imagine that the mpg would be more "textbook" if there were actually some flat surfaces to drive on around here, but there aren't. It's all hills, winding roads, and sloping surfaces. The Interstate has several areas with steep enough hills for grade warnings, so highway economy will never be what its supposed to be. For a 31 year old car that sat in a field for dead, I'm very pleased with the mpg I'm getting! The A/C compressor alone is rated at consuming 8hp. Add in the alternator and cooling fan load and you're probably closer to 10-12 all said and done.

The way the end cap on the odometer motor drive end is set up, it looks like it should have had a rubber cup or some sort of bushing to hold the white bearing piece in place. There's no other explainable reason why it would be so oversized, or why they would have a bearing piece that isn't attached to anything. Perhaps it was made of some sort of foam and rotted away over time? Who knows. What it did was allow the shaft to wobble in and out. Without any drag, I could certainly see how it would just vibrate at higher speeds instead of indexing. It was VERY sloppy in there.

ykobayashi · #10 11-26-2017, 09:15 AM

Just an FYI

My odometer started under reporting miles again. The o ring fix may not be all that permanent. It doesn’t make noise but I drove 60 miles and it read 35 miles.

When I’m bored next week I’ll pull out the cluster and bench test it. I tossed my notes about how to drive it with a signal generator. Good thing I documented it on this forum.

Diseasel300 · #11 11-26-2017, 10:56 AM

Mine's still keeping mileage at a reliable rate. I took it on a highway run recently and for the first time ever, it actually reported the correct/known mileage for the trip.

One thing I did differently than you was the O-ring. Yours fitted inside the plastic cover and pressed on the end of the shaft. I didn't have one that small, so I fitted mine over the end of the plastic shaft, so it pressed on the plastic cover. *edit* should have looked up, it was a 5.5 x 1.5mm

optimusprime · #12 11-27-2017, 05:17 AM

Did you remove the shaft that holds all the large gear wheels together ? If they have moved off in any way the clocking up of the miles will be out . I use a piece of tape to hold them all in line ..

Diseasel300 · #13 11-27-2017, 09:49 AM

This thread is on a W126. They have an electronic odometer/speedometer and are very simple inside. They don't use the speedometer cable, they have a stepper motor driving a simple gear-reduction train.

H-townbenzoboy · #14 01-20-2018, 01:29 PM

I'm going to use this fix for my 300SD's slipping odometer. Did you have to unsolder/solder anything to get better access to the stepper motor plastic bearing?

Diseasel300 · #15 01-20-2018, 01:37 PM

No. Once you pull the speedo out of the cluster, the plastic bearing housing is right on the side opposite the gear train. If you're taking the time to go that far in, make sure your gears are good. So far the O-ring is holding up, still keeping accurate mileage.