740 Speedo oddity

  • Thread starter Thread starter jimb
  • Start date Start date
J

jimb

Good day,

See if you can figure this out.

The speedometer/odometer in my 85 740 has been flaky for some time.
Sometimes works sometimes doesn't. Mostly works.

No particular rhyme or reason but it tends to fail intermittently.

I could make a case for heat, as it seems to fail mainly in warm
weather.

Checked the pickup connection on the rear end. Seems OK.

Pulled cluster, nothing obvious there.

Have not checked ground in trunk (boot).

Recently, I had some welding done on the car. At the time speedo was
not working. Ground was disconnected from the battery to prevent
damage to electricals during welding (SOP?)

Reconnected battery and voila, speedo works!

A bit later, speedo stopped. (Of course it was a warm day and I was
going rather fast).

He say's hmmm, I wonder?

Disconnected battery and sure enough, speedo began to work.

Thoughts?

jimB
 
jimb said:
Good day,

See if you can figure this out.

The speedometer/odometer in my 85 740 has been flaky for some time.
Sometimes works sometimes doesn't. Mostly works.

No particular rhyme or reason but it tends to fail intermittently.

I could make a case for heat, as it seems to fail mainly in warm
weather.

Checked the pickup connection on the rear end. Seems OK.

Pulled cluster, nothing obvious there.

Have not checked ground in trunk (boot).

Recently, I had some welding done on the car. At the time speedo was
not working. Ground was disconnected from the battery to prevent
damage to electricals during welding (SOP?)

Reconnected battery and voila, speedo works!

A bit later, speedo stopped. (Of course it was a warm day and I was
going rather fast).

He say's hmmm, I wonder?

Disconnected battery and sure enough, speedo began to work.

Thoughts?

jimB
Click to expand...

The circuit boards in these use some sort of really poor solder, it
crystalizes and cracks with age. I've had good luck fixing them by removing
as much existing solder from each joint as I can and flowing in some new
quality rosin core tin/lead solder.
 
intermittant odometer = bad gear inside the speedo head. Small teeth break
and eventually it stops working until gear is replaced (about $25 online)

My speedometer went out a while back ('89 240 wagon), and I fixed it by
replacing the two electrolytic caps on the circuit board. I had tried
reflowing the solder, but when that didn't work, I unsoldered the existing
caps and put in two new ones. Works fine now... One of them seems to be a
filter for the power coming into the board, and the other acts as an
integrator for the pulses... at least, that's my take based on the circuit I
found for the board...
 
AFTER PULLING MY WIFES SPEEDO APART I feell the issue could be a poor
connection as in a loose and or corroded connection.My wifes speedo is 4 klm
per hour faster than it was and no more speeding tickets.Here 3 klms over
and its $135.00 au dollars fine even if the police equiptment is untested or
faulty ..THATS 1.8 miles an hour .I noticed poor contacts such as earth and
loose clips and such which could be your problem .
 
The speedo in my 92 740 was intmt. for a couple of years. Installing a
new inst. cluster did not fix it. The fix turned out to be running a
new two-conductor cable all the way from the speedo to the sender
unit. Has worked perfect since.
 
Disconnected battery and sure enough, speedo began to work.
Click to expand...

So, anyone got any ideas why disconnecting/reconnecting the battery
cable fixes the speedo?



jimB
 
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