1991 740 error code Speedometer - diagnosis

Our '91 740 (non-turbo wagon) has flashed an error code for the
non-operating speedometer/odometer. This, of course, is not news to
us - the speedo has been working only part of the time for nearly a
year.

I understand that there are 2 repair options before us:
1. Have the dashboard removed - rebuild the speedo head - re-install
the dash (~$300 - $450).

2. Replace the sending unit at the differential (parts, $90)

Question: Does the fact that it's flashing the code help to isolate
either #1 or #2 as the more likely failure?

Thanks.

JS

 

As for option two, if your 91 is like my 1987 740, then you can isolate
the sending unit problem for free.

Jack up the rear of the car, remove the bolt from the pan hard rod and
move it aside, disconnect the sending unit attachment (unscrew it using
channel lock pliers). Check the wiring and if damaged you can cut off
the housing about 1 inch down the wiring, pry out the two wire-ends
inside and rebuild this as follows. Purchase 4 butt end connectors for
12 gauge wiring. Take 2 pieces of 12 gauge wire about 3 inches long and
insert one end of each wire into each of the butt connectors. Re-insert
these two butt connectors and wires back into the housing and seal it
up with something like Goop. Reinstall the connector and connect the
two wires using the other 2 butt connectors.

I am told the sending units rarely, if ever, fail. Sometimes the wiring
deteriorates and shorts against each other.

The method I described above simply provides a new connection to the
sending unit. When I removed my connector, all I discovered was this
housed the two wires securely. I found that by using the cheap butt
connectors and fresh wire, I could easily replace this.

If your odometer still functions, I believe that is the tell-tale sign
you have a signal from the sending unit.