Tach runs backward

[Dan Owens]

Guys,

I have a weird situation and I need some suggestions or opinions. Yesterday morning when I cranked my car up the tach shot over to the red line and was "operating backward". Just the tach itself not the actual engine operation, that was normal. Red line at idle and as I increased engine speed it would go down in RPMs, essentially running backward. Engine ran and runs fine. it did this for about 2 or 3 minutes then just reversed itself and functioned fine the remaining time while it was in use. The next time I started the car the same thing, then reverses itself after a minute or so and is fine until shutdown, but every time I start it fresh it does this now. Nothing has changed, no work done to the car prior, this just came outta nowhere. What the hell is this all about? I'm guess the tach itself is going bad or a sender somewhere???? Guys give me some knowledge here cause I got nothin!

[topcarbon]

sounds kind of cool.

[DRACO A5OG]

You may want pull the Tach , check the connectors for looseness , maybe crack her open and check the solders for a good contact, use a Big Magnifying glass.

If cracks are found, re-flow them.

Clean all contacts while you are in there. GO to Home Depot and get a wire label pack to keep all wires in order.

I suspect over time our babies solder do weaken and crack causing non-continuity, just had to re-flow my fuel guage.

just my 2 cents.

[Innov8]

Sounds like you have a jumpy tach which seems to be related to the VR or Alt. I am currently working out the same issue and while I am currently sorting an iginition problem... I did some research on this here... try searching "Jumping tach" to see the other threads. You'll read about checking the voltage across the battery while the tach is jumping and you'll probably find that your volts fluctuate with the jumping. Good luck!

[Dan Owens]

Thanks for the input but the tach isn'r jumping it just reverses itself. No other gauges are affected and after a small period of time reverses again to run properly. If it were the alternator it would continue the symptoms sporatically while the alternator was fluctuating so I think I may have a different issue.

[86 911 Targa]

Originally Posted by Dan Owens

Thanks for the input but the tach isn'r jumping it just reverses itself. No other gauges are affected and after a small period of time reverses again to run properly. If it were the alternator it would continue the symptoms sporatically while the alternator was fluctuating so I think I may have a different issue.

Dan,

If all of the above do not resolve your tach anomaly,
this might be the issue.

Your VR may have an intermittent issue (diodes).

Good luck,

Gerry

[Joe Bob]

A multimeter will confirm overcharging. Anything over 14v at 2500 rpms is a bad thing.