240 Odometer skipping miles at highway speeds

[504]

1986 246

I have previously repaired my odometer multiple times. It currently has a relatively new set of gears (1 year old) and I have re-soldered multiple connections.

For the most part, it seems to have been working fine the past year. However, I have mostly been driving locally, so I didn't see this problem until just recently.

After a trip of around 140 miles, the odometer had only advanced 70 miles. That seemed odd, but it seemed to be working normally during the next couple weeks of local driving. Yesterday I made a trip of 112 miles, and the odometer only advanced 84 miles. The point in common in both these incidents was that the majority of the trip was at highway speeds.

During a portion of the journey, I was watching the odometer vs. the interstate mileage markers and it definitely seemed to be lagging.

Before I tear into it again, has anyone seen this problem before and found a solution?

 

[Khrrck]

Is the speedometer still accurate, or is this issue affecting both the speedometer and odometer?

You can check with GPS speed on a phone (Google Maps will show speed while in driving directions mode for example)

 

[OVERDRIVE]

My odo has sometimes stopped for a few miles, but hasn't ever slowed consistently and hasn't done it in a while. It's not only the gears that fail, certain electrical components on the daughter board can go out, but I would believe that if you had something like a capacitor fail you would have no odo at all. There may be some old grease that gums it up, I never got to the bottom of the issue. These odometers do weird things. It's common to replace the gears, still have no odometer, and then 3 weeks later the odometer is just working again.

 

[Khrrck]

Another common fault is for the broken teeth of the failed gears to be caught in some other tiny component of the odometer, locking it up.

 

[504]

Is the speedometer still accurate, or is this issue affecting both the speedometer and odometer?

You can check with GPS speed on a phone (Google Maps will show speed while in driving directions mode for example)

Speedometer is working fine.

 

[504]

Another common fault is for the broken teeth of the failed gears to be caught in some other tiny component of the odometer, locking it up.

Yes, I'm familiar with that one. I removed a number of fragments during the last series of odometer repairs.

 

[cleanflametrap]

I'd first suspect you have a cracked solder joint (again) noting you said you reflowed connections before. It becomes electrically intermittent with mechanical vibration. Being only a "first" suspect, I'd go over the connections again with fresh solder and at least eliminate that easy-to-solve possibility.

Just last month I encountered the opposite problem on our daily driver with 350K on it. The speedometer chooses (twice now) to lay at zero while the odometer clicks merrily along. Whacking the dash puts it right again, until I get annoyed enough to pull the cluster.

 

[John242Ti]

Probably wouldn't hurt to replace the capacitors on the board as well. These were made right around the time a few computer companies were using the lower quality electrolytic capacitors in their computers. Wouldn't surprise me if VDO also used them. You can replace them with new electrolytics, go with tantalum, or with the multiple layer ceramic chip capacitors. The latter two won't leak as they age, but the tants do have a tendency to explode if too much current is passed through them. Easiest way to tell is if the capacitors on the daughterboard smell like rotting fish. If they do, time to replace them.