240 Odo drive motor board

[Kjets On a Plane]

Anyone know what actually fails on the 240 ODO board? I have spares for parts, but want to do a better repair and conserve parts, as well as fully understand how this system works electrically? Gear is fine and tested, motor won't step on the "broken" board.

I never punch out the miles and the motor worked one day and now doesn't. Mildly annoying as I always like to monitor the mileage and make sure the car is all working correctly (as correctly as a 90 245 can in 2011).

I have an 89 and 90 K9800 set, and though the wire colors are different for the service light board, they seem compatible. The car is now repaired with used parts and some solder. No matter as the 89 donor in this case was trashed on the speedo side, so using it for parts was a good use this time. I see this with the regularity of the rising sun, however and wish to do better repairs and better understand these.

Art? Anyone?

*posted via mobile device*

 

[James10952001]

I've seen mostly cracked solder joints. The electrolytic capacitors fail too, that usually results in glitchy behavior when cold that improves as it warms up.

The most notorious problem with 240 odometers is the gear cracking, but if you're sure the problem is electrical, resolder the board.

 

[Kjets On a Plane]

Gear is fine. Solder joints look okay under magnification. Caps seem likely, anyone have a definitive test for those in place...id imagine you would want to desolder and remove them to test.

I've resoldered them or replaced gears, both with good results. Haven't had too many where the speedo worked 100% and gear was good. Motor definitely does not step on the "bad" board, however.

 

[cleanflametrap]

The caps are not the problem. One smooths the speedometer gauge and the other decouples the motor drive, but both are probably dried out by now in most of ours. The gauge and odo work reasonably well with no caps at all.

If the speedometer is working, and the mechanics of the gear (seating) are OK, the only thing left is the motor drive. The output transistor can be ruined by an inadvertent connection of the tach lead, but the intermittent, non accidental failure is this:

http://cleanflametrap.com/#links

 

[James10952001]

Bad joints are often difficult to detect visually.

I test capacitors in circuit with an ESR meter. Unless you do a lot of electronics work it's easier to just replace them and see if that helps. Capacitors are cheap.

 

[Kjets On a Plane]

This car was a bit of a challenge to trace out. Ended up having a bad diff sensor. Replaced that, speedo worked all the time, then no odo, but odo gears worked in another sedan unit that I knew worked fine and had already resoldered.

When I pulled the cluster the tach wire was hooked up to what I believe is the cruise control output. I thought I saw the odo twitch, but the speedo worked so seldom with the failing diff sensor, I'm not sure. Switched those and the cruise worked. That is a good clue and I should have mentioned that. Any other damage that can do? All appears okay now.

Working on volvos or any lousy euro car one is doing basically nothing but electrical. Still, what is "expensive?"

How wrong is it to delete the service light? 86 and 87s don't have it...I guess some originality is lost, but most people driving a 20+ year old heap probably are going to neglect it or not by now, service light or not. Im all for original, but most service light pieces are broken on the board by now...kinda a lot of effort to bother repairing for one lousy light. I seldom repair those or upshift lights.

 

[cleanflametrap]

Putting the tach wire on the cruise terminal usually "fries" the 2115 chip. I say usually, only in reference to your case, because in every other case it isn't usual, but certain. The damage resulting doesn't affect the speedometer, only the odo.

Some folks think the service counter adds strain to the odometer gear, but it only loads it at the 100 mile interval. Once you get a new gear in there, feel confident to tell your customers to resume using the trip reset. The problem making the service light work again is the spring breaks the clear plastic pin it is secured by. Most people don't know what the light means or how to reset it (especially pre-airbag).

Here's a time-saving technique for isolating an intermittent sender problem: http://cleanflametrap.com/speedoDiag.html

 

[James10952001]

Interesting stuff, I've never actually reverse engineered the speedo. What is the 2115? Commodity part or custom IC? Shouldn't be too hard to find one either way, lots of speedos out there with broken mechanical parts.

My cars never had service lights. I always found them to be rather useless anyway as I've always been in the habit of putting the mileage on a sticker to remind me when the change the oil.

 

[Kjets On a Plane]

Good to know. I knew the jumper trick from trial and error and tracing it out years ago and hating pulling the speedo from the cluster over and over.

I didn't know the wall transformer trick, I ran the car in the air, which wasn't hard, but not that easy. I did break into the box, but suspected the diff sensor after I hooked the cruise up and the speedo needle would jump and waver like it was losing signal and the speedo would only work at 55 plus mph and would always waver and wash out slowing down much at all. I suppose something else could explain that, and I don't usually replace a lot of diff sensors (sometimes clean them...they ar
e magnets), but had seen that enough to be 90% on the diff sensor. The circuit board failure looks different...usually cold, usually intermittent/sticking/can be broken or fixed by flexing the board a bit without describing it more technically.

I had the plastic peg broken for the service light assy return spring. I don't worry about it ruining the odo like the lambda light or resetting the trip in motion. Ill devise a way to repair the spring connection.