Tachometer filter

[Bonnewagon]

it was put there for a reason

What is the reason anyway? Not trying to be a dick but I have swapped a lot of factory tachs into my cars and I have never used a filter. I've never even seen one on factory tach equipped cars. Does the lack of one affect the tach reading? Radio? Inquiring minds want to know.

 

[foxtrot]

What is the reason anyway? Not trying to be a dick but I have swapped a lot of factory tachs into my cars and I have never used a filter. I've never even seen one on factory tach equipped cars. Does the lack of one affect the tach reading? Radio? Inquiring minds want to know.

A ‘tach filer’ is an RC filter utilized to ‘smooth’ out the tach signal going to the teach. Will remove any voltage spikes.

 

[Bonnewagon]

Will remove any voltage spikes.

Voltage spikes that hurt the unit or affect the reading? I thought tachs were dampened to slow down the needle so it didn't bounce around. Or am I confusing it with the other gauges?

 

[69hurstolds]

I think they were used more frequently on older cars Bonnewagon

Newer vehicles with the circuit boards on the back of them, etc., don't normally require it.

 

[foxtrot]

Voltage spikes that hurt the unit or affect the reading? I thought tachs were dampened to slow down the needle so it didn't bounce around. Or am I confusing it with the other gauges?

The RC circuit would be utilized for the dampening that you mention. The newer tachs could easily have a dampening circuit built into them..

 

[michmalibuman]

Good morning Cwc, I got a nice original tach wire and filter from Kevin for mine. He might be able to help you if you're not in a big hurry. He's on this site and might ring in. He's great guy and a lots of help.

 

[Bonnewagon]

utilized for the dampening

Ok that makes sense. I guess I never noticed any bouncy movement before. I tend to use only factory in-dash tachs whenever possible and I always thought they were pretty stable.

 

[airboatgreg]

What is the reason anyway? Not trying to be a dick but I have swapped a lot of factory tachs into my cars and I have never used a filter. I've never even seen one on factory tach equipped cars. Does the lack of one affect the tach reading? Radio? Inquiring minds want to know.

If I had to make a guess it would be a Radio Frequency Interference damper and a clamping condenser to reduce voltage spike after the coil fired which may save the ignition module and other electronics. Remember this was at the relative beginning of the auto electronics age. As to the tach...I don't know. Spike??

 

[foxtrot]

As far as ‘spikes’ goes, I was thinking that any overshoot or undershoot on the tach signal could possibly cause the needle to ‘bounce’. An RC filter would reduce the edge rates due to the RC constant and hence reduce the undershoot/overshoot. I could be wrong but thats what I was thinking..