Tach needle bouncing
 

A while ago I installed a 4,000 RPM VDO tach in my '39 p/u with an 8ba converted to 12 volts and running a Bubba Chevy dizzie. When first installed the needle had a bit of a bounce to it but the readings seemed pretty accurate so I was just living with it. Then I installed a new set of solid core wires and the tach needle was all over the place and was reading over twice what it should at higher revs. A check with Mr. Google said that the tach trigger wire was picking up interference from the plug wires and to re-route the trigger wire away from the plugs and if that didn't work (which it didn't) to install a 10K ohm 1/2 watt resistor in the trigger wire which only made things worse. The company that I got the tach from suggested a tach noise filter for about $30.00 shipped. I then contacted VDO tech support which I should have done in the first place.They suggested a 1N4005 Diode in the trigger wire as close to the tach as possible. After a quick trip to Radio Shack for their part number 2761104 and less than 2 bucks for 2 diodes no more tach needle bouncing and the tach was reading correctly. If anyone is having issues with a tach needle bouncing or reading incorrectly you might try the 2 dollar Radio Shack fix before shelling out 30 bucks for a tach noise filter. JMO

Re: Tach needle bouncing
 

Problem encountered, problem solved and solution shared. Thanks Bat!

Re: Tach needle bouncing
 

Good tip i get that call from time to time...

Re: Tach needle bouncing
 

The only trouble I have had with a tach is, I trusted a guy to send payment for one and never heard from him again. Oh well he knows who he is and for me, lesson learned, lesson gained.

Re: Tach needle bouncing
 

I have an original SW tach out of a 61 or so Studebaker. I ordered the little black box from Studebaker INTL and it works , but bounces. I wonder if the diode will work for it?

Re: Tach needle bouncing
 

I installed the diode today and it works perfectly!! Thank you for sharing!!
One would think that the tech people at Pertronix would know about this, but they don't. They told me to use resistors and none of them worked!
Thanks again!!

Re: Tach needle bouncing
 

Had the same problem with a new S W tac. Where do you install the diode? 51501bat mentioned purchasing 2 diodes. Does the fix require both diodes or is the second one a spare?

Re: Tach needle bouncing
 

Second one is a spare. IIRC they came in a 2 pack. Install in the trigger wire as close to the tach according to what the tech at VDO told me.

Re: Tach needle bouncing
 

Also, diodes are directional make sure you install it with the correct orientation.

Re: Tach needle bouncing
 

Can you tell me which way (polarity) the diode goes?

Re: Tach needle bouncing
 

Not certain but the tech guy at VDO can tell you.

Re: Tach needle bouncing
 

Depends on polarity negative ground it should have the ring on the casing towards tach.
That way it cuts out anything thats not a positive pulse going to the tach.

Re: Tach needle bouncing
 

Yes, Tech at Pentronix say use resistors (as many as req'd) and that DOES NOT WORK; neither does re-routing or re-wiring (I have copper ignition wires). Got my 276-1104 diode at Radio Shack. Hope I got this right .... silver painted end is positive/cathode end. If you install backwards just won't work.

Irony: For me, nothing worked. My older Pentronix module started mis-behaving and failed. Several module replacements resulted in crazy tach readings (bouncing, double readings) and failed anyway. Converted to a points distributor and didn't need a Tach fix any more. My tach indicator needle went back to steady accurate showing. Some ignition systems come with gremlins; some are pretty frisky.