Weird tach problem

[clean96GST]

So my cars an automatic and It shifts really high when you put it on the floor so today i was testing how high it shifted when it was wide open and I was doing about 90 in third at 5000 and then the tac guage just went crazy up to the max then back down and up again but it wasn't actually happening in the engine...so what could it be?

 

[b-morr]

So the engine wasn't doing anything, just the gauge?

 

[clean96GST]

Yeah I'm pretty sure it's just the guage....i probly would of felt it if it was the engine also

 

[b-morr]

The needle went up as high as it could go? I would say check the connections the the tach and wires.

 

[clean96GST]

yeah it just rapidly went back and forth

 

[luv2rallye]

Probably connector (vibration might make it fail at certain rpms) but could also be alternator is outputing too high of AC voltage (riding on the DC voltage due to bad internal diode or regulator) or voltage spikes when it gets up to higher rpm's under electrical loads. Try with high beams and rear window heater on in neutral at various engine rpms to see if it happens. Also measure charging voltage at battery (both DC and AC) at high and low rpms with electrical loads on and at the rpm where tach is flakey. A bad alternator would have a pretty consistent failure pattern. You could disconnect the alternator to eliminate it. Another rare failure is the coil filter capacitor intermittently failing (eg. shorts at certain voltage or vibration or heat). You can just unplug it to eliminate it.

 

[STI2TALON]

Trace the wires to the sensor and plug and see if there are any breaks in them. Or dab silicone ont he outside of the connector and plug to give it a better hold