Humorous Tach Act

[Manitop]

I searched for the issue, and found many similar, but not the specific issue I'm having with my tach. At idle, when I'm at a light for example, and the turn indicator is on, the tach needle starts to jump like its on a trampoline: goes up a few hundred at first, but with each "blink" a little higher until it is pinging up to the top of the range and falling back for the next jump. Each jump is synchronized with a blink of the light. Only does it at idle, only with turn signal on (either left or right, and turn signal works fine) and the engine runs just fine. Car is 89 930. Any thoughts much appreciated...

[ManniB]

I would look into the ground connenctions in your dash (specifically the tach ground) since this is how the turn signal can influence the tach. Maybe the additional current drawn by the turn signal raises the ground level (of the tach)enough to cause a malfunction of the tach. That only happens at idle since the power supply/battery voltage is low in that situation.

Regards, Manfred

[cdrik915]

Control the alternator , and its regulator.

[Manitop]

Manfred,
My knowledge of electronics is limited, but your comments make sense and are consistent with the facts. Thanks for the suggestions, much appreciated.

[smokintr6]

My tach started to malfunction right before my voltage regluator went out...

[Joe Bob]

Put a meter on the battery terminals when the tach is bouncing, spikes over 14.2 volts can be a wonky voltage regulator. If it dips below 12.5 at idle it can be a failing alternator.

[Bob Kontak]

Originally Posted by Joe Bob

Put a meter on the battery terminals when the tach is bouncing, spikes over 14.2 volts can be a wonky voltage regulator. If it dips below 12.5 at idle it can be a failing alternator.

From my statistical mass of one failed voltage regulator across 18 years, I found the voltage at the battery was simply wandering 3-5 tenths of a volt all the time but that was enough to cause the tach to bounce around erratically sometimes and the headlights to dim and brighten enough to notice. Not exactly your symptoms but as JB says put the voltage meter on the battery when idling. If rock steady, vary the load, lights, AC, etc. and check again. If all ok move to the ground theory.

[Manitop]

voltage at the battery is steady 13.52-13.54 at idle. Is that too low?

[Bob Kontak]

Originally Posted by Manitop

voltage at the battery is steady 13.52-13.54 at idle. Is that too low?

That looks great. Main thing is the variation is only a few hundredths of a volt. Good sign.

[Bob Kontak]

Originally Posted by Manitop

voltage at the battery is steady 13.52-13.54 at idle. Is that too low?

That looks great. Main thing is the variation is only a few hundredths of a volt. Good sign.

If your battery is fully charged it may be a little less. If it's partially drained, may be a little higher. Like tenths of volts but steady.

Check it with lights AC on, etc. Load it up and see if anything odd happens. The reading will drop, but again, steady is the key.