Electrical Gremlin !

[jp9094]

Due to a recent windfall, I decided to have the original Tachometer on my 1983 r 65 LS rebuilt. When the rebuilt Tacho arrived I removed the old style (with the red over rev light) tacho I had been running and installed the original only to find the main instrument illumination not working. The neutral, generator, oil pressure and high beam lights all worked. As per Snowbums schematic I checked the voltage from the pins to the main light and it was live. I also checked the bulb and it was good I have returned the tacho to the rebuilder for checking.

When I replaced the previous tacho, the same thing occurred (No main instrument light (which had previously worked) ! In addition the rev counter only works when the ignition switch is in the run position and not in any other position. I have the Euro style ignition with : Off, Park light, Start/run, run/park light and run/headlight! Neither  the ignition switch or the tacho were messed with nor was the white plug in the back. Any ideas about what is going on??? Any and all help/ideas appreciated.

TIA

[mrclubike]

You need to check for Battery Positive on the  Grey/White wire that feeds all of the instrument illumination
The   GR/WS wires are highlighted in red
 
Does the park light and speedo light  work
If not the GR/WS may have come unplugged from the back of the fuse block

Since the High beam indicator works the ground on the tack connector should  be good
But check it any way

[jp9094]

Thanks for the reply! Yes, both the park light AND the speedo illumination work perfectly. Any other ideas?

TIA

[mrclubike]

Just make sure you have battery positive voltage on the GR/WHITE wire and battery Negative  on the Brown wire  going into the back of the tack
If you do then there is something wrong inside the tack housing

[Bob_Roller]

Thought of a possibility causing this, the bulb holder has two tabs if I remember correctly that contact copper strips on the pc board inside the tachometer .
Possible one of these got bent in the repair process and not making good electrical contact .
If this ever happens again, swap a bulb holder from a functioning position and see if it helps .

[Snitzler]

My tach doesn't work.  Can I ask where you had your tach overhauled?
TIA

[Bob_Roller]

Check the ignition coil input wires, one solderless connector should have two wires .
I don't have a wiring diagram in front of me, but I think they are both black, one comes from the ignition control module, the other goes to the tachometer .

[jp9094]

Thanks for the replies. In answer to the question I had it overhauled by "Joe's Speedometer " in Reseda Califprnia(joespeedometer@gmail.com).  I think, that what has been missed is that the replacement Tachometer does work, but ONLY with the ignition in the run position. When I turn it to the run/parking light or run /headlight position it stops! This is separate to the instrument illumination problem. Could it be a problem with the ignition switch? What is mystifying is that the replacement worked properly before I tried putting in the original, and nothing was done to the ignition switch, tacho plug.

Thanks again to all who replied.

[mrclubike]

The same wires are in play regardless of the run position
The TACH signal comes from the Coil power so we know its good
It sounds like you have a bad GROUND to the TACH assembly and you are getting feed back through lighting circuit
Make sure the brown Ground wire going to the TACH is doing its job
Just checking but Do you know how to check the ground using a volt meter
Back Prob the brown wire  with the NEG lead and the GR/BL with the Positive lead while the ignition is on with the TACH inop and if it is good you should show battery voltage
I really dought the TACH unit is faulty 

Here is the EURO spec schematic