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09-23-2016, 05:21 PM | #1 |
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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
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09-23-2016, 06:19 PM | #2 |
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Re: Tach needle bouncing
Problem encountered, problem solved and solution shared. Thanks Bat!
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09-23-2016, 06:24 PM | #3 |
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Re: Tach needle bouncing
Good tip i get that call from time to time...
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09-24-2016, 04:41 PM | #4 |
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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.
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09-24-2016, 08:17 PM | #5 |
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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?
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12-04-2016, 02:45 PM | #6 | |
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Re: Tach needle bouncing
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Quote:
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!! |
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12-04-2016, 03:56 PM | #7 |
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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?
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12-04-2016, 05:37 PM | #8 |
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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.
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12-04-2016, 05:39 PM | #9 |
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Re: Tach needle bouncing
Also, diodes are directional make sure you install it with the correct orientation.
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01-04-2017, 03:49 PM | #10 |
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Re: Tach needle bouncing
Can you tell me which way (polarity) the diode goes?
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01-05-2017, 11:29 AM | #11 |
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Re: Tach needle bouncing
Not certain but the tech guy at VDO can tell you.
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01-05-2017, 11:40 AM | #12 |
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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. |
01-05-2017, 11:51 PM | #13 |
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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. Last edited by Drbrown; 01-08-2017 at 09:34 PM. |
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