dead 240 speedometer

the car: '89 240 wagon, auto transmission. About 233K miles

the problem: Speedometer stopped working this morning

the story: about 3 weeks ago, the odometer stopped working, although the
speedometer continued to function. I've had this problem before, and knew I
needed the 25-tooth gear replaced. After exhausting the local junk yards
without success, I finally ordered a new gear from Odometer Gears Ltd for
$29. Arrived in 2 days, installed without any problem, and both the odometer
and speedometer worked fine. After driving about 50 miles this morning, I
noticed that the speedometer needle had dropped to zero, and stayed there
the rest of the trip, although the odometer continued to work. Since then,
I've been checking and swapping things, without any success. (I have the
parts from the junkyard speedometer head I picked up a couple years ago when
the original gear broke).

I have swapped:
1. the two speedometer mechanisms
2. the two stepper-motor boards
3. the voltage regulator board in the gauge cluster

I also tried touching the two wires from the speedometer mechanism to an old
watch battery, and found that both of them jumped to around 20MPH, so I
think the actual mechanisms are OK.

I checked the resistance of the little flexible plastic "fuse" that arcs
over the speedometer, and it read zero.

I'm now thinking the problem may be outside of the gauge cluster, although
I'd think that if it was the speed sensor (on the differential?), neither
the speedometer nor the odometer would be working.

Any suggestions???

m9876c at yahoo dot com

 

Indeed. The problem could be elsewhere if neither would work. It is
much easier to troubleshoot the speedometer / odometer if you get it
running, say, on your kitchen table. A signal generator of some
kind (I used an AVR microcontroller that I happened to have around)
and a multimeter are very handy. It's enough to give square wave
between 0 and 5 V to "gauge" connector.

<URL:http://cleanflametrap.com/speedo.html>

I had cracked soldering on odometer step motor. It took a while to
figure that out...

So, you have replaced the circuit board, still only the odometer
works? The voltage regulator probably doesn't have anything to do with
this; the speedometer/odometer unit gets its power from elsewhere.

 

Thanks for the information! The link to the website was most helpful, and I
now have a somewhat better understanding of how the speedometer-odometer
actually works. Apparently, the two functions are tied together much more
closely than I had thought, and it's a mystery why the odometer keeps
working but the speedometer has suddenly failed. As I mentioned in my
original post, I can get both of the speedometers to jump up to around 20mph
by touching a watch battery to the wires (disconnected from the stepper
motor board), so I think the actual meter movements are OK. It looks like
the flexible fuse supplies power to operate both the odometer stepper motor
and the speedometer analog movement, so it's probably OK too, although I may
try replacing it with a glass fuse next time I have the instrument cluster
out of the dash... And, since the stepper motor is working, it would seem
that the signal from the sensor is intact.

All of the above pretty much eliminates everything but the actual on-board
chip and a couple capacitors that deal with the speedometer as being
suspect. Hard to believe that both of my stepper-motor boards would develop
the same problem at the same time, but what else is there??? Guess I'll have
to hunt for one in the junk yard and see if that fixes the speedometer.

The other thing I could try is to disconnect the speedometer movement from
the board again, and connect two long wires to the board, leading them out
from behind the instruments so I can attach a digital voltmeter to the
board. If I get a reading while driving, and it varies with my speed, that
would seem to suggest that the board is OK, and the problem is with the
movement somehow, although I think it's unlikely that both of my speedometer
movements would go bad at the same time!

A very confusing situation, but the information you provided helps a lot!

 

The problem is likely just solder joints on the speedo board, it's
pretty common.