HELP! Tachometer's gone psycho!

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packin3deuces

packin3deuces

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My tachometer worked fine until a few weeks ago. Now the needle bounces all over the place with no correlation to the engine speed. :wack: I have a '71 340 with a Mopar Performance Electronic ignition and a chrome ECU. The tachometer is a Stewart-Warner Deluxe Series 0-8,000rpm. Does anyone have any ideas how to troubleshoot this?
:banghead:
 
did you install stereo equip??

where is your hot lead hooked up and whats it running near?
 
Mine did this also it turned out to be a bad connection at the coil.
 
It's radio delete and therefore no stereo equipment. It was wired before I took over the build, so I am not sure on the hot lead. Wires run through the steering column, so I was hoping I wouldn't have to drop the column. Unfortunately, I can't pick out the wires coming through the column. Looks like I'll be dropping the column tomorrow. If wiring checks out OK, then what?
 
The tach in my truck does this when it gets to be time for new plug wires. Just a thought.
 
C130 Chief said:
The tach in my truck does this when it gets to be time for new plug wires. Just a thought.
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This is a very good point. ANY open in the secondary circuit, a bad rotor, cap, bad coil or plug wires, or even cracked/ bad plug could cause this.

Otherwise I'd check the power and ground wiring to the tach

Another thing you might do is take your meter on and check the running battery voltage

THEN put the meter on AC volts --yes that's right AC volts--and put from ground to the alternator output stud.

This will show you if you have a bad diode which adds a-symetric ripple to the DC supply line. IF you have a radio, you can also hear extreme wire problems, or this AC ripple in your radio.
 
67Dart273 said:
This is a very good point. ANY open in the secondary circuit, a bad rotor, cap, bad coil or plug wires, or even cracked/ bad plug could cause this.

Otherwise I'd check the power and ground wiring to the tach

Another thing you might do is take your meter on and check the running battery voltage

THEN put the meter on AC volts --yes that's right AC volts--and put from ground to the alternator output stud.

This will show you if you have a bad diode which adds a-symetric ripple to the DC supply line. IF you have a radio, you can also hear extreme wire problems, or this AC ripple in your radio.
Click to expand...


I put new plugs and wires in on Wednesday. Today I replaced the coil with a new one. Tach still the same. I'll check the cap and rotor next. If they are ok, then I'll do the voltage tests. Thanks for the help. :banghead:
 
I was doing some tuning yesterday. Had my hand held underhood tach hooked up as follows: positive lead to coil -ve and -ve lead to ground. Not only did the underhood tach work perfectly, but to my amazement, the psycho tach on my steering column works flawlessly as well! :scratch: When done tuning, I disconnected the -ve lead on the underhood tach 1st and the in car psycho tach still worked perfectly. :shock: But when I disconnected the underhood tachs positive lead, the in car tach went psycho again and started bouncing all over the place! :cussing: My electrical diagnostic abiility is very limited. Does anyone have an idea on this one? Any help is greatly appreciated. :prayer:
 
The only lead going to the coil should be the neg side trigger wire.

The red wire/power for the tach should go to a switched wire not coil + stud. Like a radio lead, heater power line.
 
crackedback said:
The only lead going to the coil should be the neg side trigger wire.

The red wire/power for the tach should go to a switched wire not coil + stud. Like a radio lead, heater power line.
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Cracked, my in car (psycho) tach is wired as you suggest. My hand held tach that I use when I tune was attached as described in my last post.
 
I suspect that you are getting some interference from plug wires, that is, induction in tachometer signal wire from high voltage in plug wires.

Perhaps a shielded signal wire will cure the problem.

Re-routing the signal wire so it does not run parallel in close proximity to any of high voltage conductors may also solve the problem.
 
wjajr said:
I suspect that you are getting some interference from plug wires, that is, induction in tachometer signal wire from high voltage in plug wires.

Perhaps a shielded signal wire will cure the problem.

Re-routing the signal wire so it does not run parallel in close proximity to any of high voltage conductors may also solve the problem.
Click to expand...

wjajr, thanks for your reply. Do you have any recommendations for what I can use to shield the signal wire? :salute:
 
Do you have any recommendations for what I can use to shield the signal wire
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Not off hand, Google will be your friend on that question. What I would do is relocate signal wire, making sure to cross any high voltage plug wires at 90 degrees. This will lessen any induced voltages. Also double check signal wire’s connections, and gage’s ground connection.
 
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