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-   -   Factory tachometer problem (http://www.clubcobra.com/forums/superformance/137351-factory-tachometer-problem.html)

wstaab 10-10-2016 04:31 PM
Factory tachometer problem
 
Factory gauges with a MSD Street Fire Capacitive Discharge Ignition and MSD distributor.
I have checked the wiring per MSD and the Superformance MKIII wiring diagram from Blas Costagli and everything appears correct.
Problem: tachometer works until you turn on the instrument lights.
Light in tachometer illuminates and the tachometer reads “0”.
Turn off the instrument lights and the tachometer starts working again.
Tachometer has an internal bulb. Connector to the rear of the tachometer is properly seated.
All the remaining gauges work normal with the instrument lights on or off.
Any one seen a similar problem and a solution?

patrickt 10-10-2016 06:21 PM
It almost has to be a bad ground path for the bulb in the gauge. Instead of the bulb having a nice, clean ground path back to the negative terminal of the battery, the electrons are taking a "short cut" across the workings of the tachometer, and the result is that the tachometer doesn't work. Try running a wire, with alligator clips on both ends, from the negative side of your battery and clamp it on a metal portion of the gauge (try a couple spots) and see if that temporarily fixes it. And if there's a nice loose ground wire running to the gauge, try jumpering to that as well. Sometimes the "jiggle" test is your best friend.

jhv48 10-10-2016 07:35 PM
Check your charging system. Is your ammeter running on the positive side while driving? When my tach did that, it was because my alternator had quit and I was running off the battery.
It put out just enough power to run the engine but turn on the lights and the tach stopped working. Just a thought.

That ground thing may work too.

Blas 10-11-2016 12:56 PM
Pull bulb in tach and see if problem goes away...

wstaab 10-11-2016 04:20 PM
I am going to pull the tach and check the continuity in the connector that mates to the factory wiring. May be a problem with the ground wire in the connector. Wiring diagram says the bulb is internal in the tach.

JBCOBRA 10-12-2016 09:42 AM
On mine, if any of the associated wires are too close to anything ignition related my tach gets weird

patrickt 10-13-2016 10:58 AM
Quote:
Originally Posted by wstaab (Post 1405883)
I am going to pull the tach and check the continuity in the connector that mates to the factory wiring. May be a problem with the ground wire in the connector. Wiring diagram says the bulb is internal in the tach.
And, before you dismiss the problem as being a bad gauge or bad connections in/to the gauge and/or bulb, perform the same test, meaning rev the engine and turn the lights on, but do the test with the alternator disabled so that the car is running on battery power only. Here's why: If the alternator is putting "polluted current" out, the additional load of the lights increases the pollution. That pollution can affect your tachometer readings. An oscilloscope would show that in short order, but running on battery power only (just revving in your garage or an around-the-block drive with the lights on) is an easy test. This can be a tricky bug to find when you get one -- and Blas's test may or may not point you in the right direction. Here's a YT video on it for those that are really interested:

Blas 10-28-2016 08:59 AM
Any follow-up here on this issue?

wstaab 10-29-2016 10:47 AM
Pulling the tach this weekend to check wiring.

wstaab 11-10-2016 08:05 AM
Problem is no power to the red/white wire that supplies the tachometer bulb.
The red/white wire is connected to a ground somewhere because is checks for continuity.
It appears that when the instrument lights are activated the power is back feeding through the red/white wire and using the tachometer ground to complete the circuit which causes the signal from the MSD unit to fail. Trying to find the wiring problem would mean pulling the dash which is not an option. I may try to disconnect the red/white wire and splice it to a instrument light feed on another gauge.
Appears like a wiring problem from the factory. Car number 3126 delivered in 2015.
Car has 400 miles on the odo.

patrickt 11-10-2016 09:51 AM
Quote:
Originally Posted by wstaab (Post 1408311)
Problem is no power to the red/white wire that supplies the tachometer bulb.
The red/white wire is connected to a ground somewhere because is checks for continuity.
If you can't easily find the glitch in the harness itself, I would just cut the wires, tape off the sides going to the harness, and then run a clean feed from one of the gauge bulb feeds that works properly, along with a nice clean ground feed. Remember, a hot side feed to a filament bulb will show continuity to ground (through the filament) when there is no 12v+ on it.

Blas 11-10-2016 11:46 AM
Ah, so pulling the bulb would have been a good test it seems. Could the fault be in the connector plug itself? The small contact wires in the bulb plug perhaps bent an shorting? Splicing the power for the tachometer bulb to another bulb power feed is acceptable. The power for the instrument lighting all comes from the dimmer anyway.

wstaab 11-10-2016 05:38 PM
I checked it at the connector which was disconnected from the tach.
The red/white wire at the connector show 0 volts dc on a digital meter with the instrument lights on. When you check continuity to the main ground lug in the fuse panel the red/white wire at the connector shows is is grounded. When the instrument lights are on the bulb in the tach is illuminated. If the engine is running the tach reads "0" when the instrument lights are on. Turn the instrument lights off and the tach functions normally. Next step is to disconnect the red/white wire from the connector and verify tach functions normally with the instrument lights on or off.


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