tach not working

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340dustr

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Question:
My car 1970 Duster 340. A while ago the engine would not start-no spark and after much investigation, removing the tach feed wire from the neg side of the ign coil, the engine would start & run. Fast forward to now-after getting the instrument cluster rebuilt incl tach the engine still wont start with the tach feed wire attached to the ign coil. When you remove the wire, it starts like before
Any ideas?
 
340dustr said:
....after getting the instrument cluster rebuilt incl tach the engine still wont start with the tach feed wire attached to the ign coil.
Click to expand...

1....Might be the tach unit is bad regardless of who "rebuilt" it

2....Might be a wiring error or a short in the tach wiring.

3....What do you have for ignition system?

If you have MSD CDI (or any CDI) ignition, you don't hook the tach to the coil.

I would start by pulling the tach wire loose up in the cluster. If the engine will then run (watch the wire it will be "hot) then it's wired wrong at the tach or a problem in the tach.
 
67Dart273 said:
1....Might be the tach unit is bad regardless of who "rebuilt" it

2....Might be a wiring error or a short in the tach wiring.

3....What do you have for ignition system?

If you have MSD CDI (or any CDI) ignition, you don't hook the tach to the coil.

I would start by pulling the tach wire loose up in the cluster. If the engine will then run (watch the wire it will be "hot) then it's wired wrong at the tach or a problem in the tach.
Click to expand...
Car has Mopar electronic ignition w orange box. Tach was rebuilt and supposedly tested just before installing cluster. if you disconnect tach wire at coil eng will start & run. Traced tach wire from coil to gauge and didn't see any issues(shorts). Do you know where the tach red wire at gauge gets its feed?
 
have tried just the wire without the tach itself hook to it the wire ? maybe that wire is grounding out , before it gets to the tach . at least it would point to the grounding issue .
 
If it's all factory it MAY be in the shop manual. You can download them free at MyMopar. Some of that "accesory" stuff is poorly documented.
 
Use an Ohm meter and measure the resistance of the tach wire to ground, when not hooked to coil. I would think it should measure 50k to 100k or more when connected to tach. Something low, is a short due to wire pinch or missconnect, or shorted internal component.

The red wire is likely fed from IGN1, the blue wire that goes to high side of coil ballast, and regulator.
 
Just a guess... tach wired wrong. That instruments connectors aren't marked. There is just 2 threaded studs mounted in a little piece of board. These can be poked through the housing in either position during assembly so marking the housing wouldn't be true.
The shorter of these 2 studs is signal. The longer is 12 volt. Although a male spade terminal is added for the signal wire connection, that could be placed on either stud. Stud length difference is the only true key.
If it is hooked up backward and grounding via the 12 volt line, we know a chassis ground is present.
A dedicated chassis ground wire should have been added at lower center mounting screw whether there is a tach in that center pod or not, and especially where there is a tach. All of my rally inst' panel customers knew these things before installing.
 
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