1986 240 speedo

My recently acquired '86 240 has it's fair share of problems - gas gauge
that acts as a "full" indicator only, indicating empty after 20 or so miles.
The overdrive does not work reliably EXCEPT when holding the horn switch
closed. Pressing the horn shifts into OD in any gear - very creative wiring
and also, no horn. High beams also will not engage without pressing the
horn. Anyway, my interest is in the speedo. It stopped at 126k over 3
years ago. Both speedo and odo are non functional. Where is the speedo
sender? The trans doesn't seem to have any wires that serve this function.
I have discovered that in 4th gear every 500 rpm is approx. 10 mph so I'm
not too worried about this either but it would be nice to have it working
again. Any insight would be appreciated.
J M

 

The speedo sensor is on the rear axel on the back of the differential
housing. There's a decent chance the wire has chafed or broken somewhere
under the car. Check your rather odd electrical problem first though, first
thing I would suspect is corroded fuses, next make sure the under hood
ground strap between the engine and firewall is intact, weird electrical
gremlins like this are very often ground issues. Since the speedo is also
electrical it's possible that this is related as well.

 

Behide the speedo their is a thing called a voltage ragulater that contolls
the gas tank reading and thats what is the promblam.

 

Maybe, but in this case I would suspect the sender in the fuel tank is
broken, still not hard to replace though.

 

Ok, on that note, could the regulator also cause this symptom?

With a ful tank, guage reads empty.

When level goes down to about 1/4 tank, guage starts to rise towards
"full" reading.

Looked at connection on the instrument cluster, no way to have
connected this backwards.

Checked wires from sending unit on the tank, reversed them, no
difference.

Bad sending unit?

From the manual, (Haynes) it says to connect a 68 OHM resistor in
parallel with the 2 wires from the sender, while it's disconnected
from the sending unit. (My next try) Guafge should read 3/4 full whith
ignition turned on with this setup.

Another strange one from my "new" 84 760 Turbo, although I do notice
that occasionaly the battery indicator goes to 0 volts occasionally,
and stays there for the duration of the trip; even though the gas
guage still does it 's reverse logic thing all the time.


I still love this car, but it does keep me busy.