Bouncey Tachometer?

what makes my Tach bounce around with the RPM? I have never seen a tack bounce around like this one. The gauges i have are the vdo's from VS. Also when you blip the throttle it will shot strait up to 6K and i know this cat be right.

Mike

Mike jusy wondering what type of ignition are you running? Points & condenser or an after market electronic type. I bought a pertronix module for inside of my distributor and the VDO website said to put a resistor in the line to the tachometer. I havent installed mine yet so I cant tell
how it will act yet.

I'm running a Petronix in a Bosch 009 and my VDO replica tach (Brazilian) works fine without a resistor or anything other than stock wiring . I had hooked up a Compufire and CD ignition and that worked fine too, but the CD ignition had a separate/discrete wire for an electronic tach signal.

Check all of your ignition/tach wiring connections.

Mike I had a similar problem with my VDO tach. There is a diode or a resistor (very tiny) that go's on the coil lead to the tach. Mine was defective (I over heated it soldering it to the wire. I found that Radio Shack has them, something like two for 70 cents. Once this was replaced,(no solder) the tach functioned perfectly.

I'm using a Comufire (or something like that) ignition and have the erratic tach symptom too. What size resistor do you put in the line?
BTW was running a petronix unit but the magnets weren't 90 deg. from each other. #3 was firing 23 deg. early, Petronix sed it wasn't their fault (??).
Bill

Hi, I copied this from e-gauges.com vdo section

Will VDO tachs work with points ignition or Pertronix?
Answer: Yes, with diode in line on the signal lead. Instructions are as follows:
Purchase a diode #1N4005 from a local electronics store such as Radio Shack or Circuit City. Cut both ends of the diode so each is approximately 3/4? long. Crimp 1/4? female spade connector on the end of the diode with the silver band. Crimp butt-splice connector on the other end of the diode. Crimp the opposite end of butt-splice connector to the wire connected to the ignition signal source. Connect 1/4? female spade connector used in #3 above to terminal #4 on the back of the tachometer. Connect a ground (-) wire to terminal #3. Connect a switched 12-volt power wire to terminal #2. Set switches for appropriate amount of cylinders.

Bill D.

I think it might be a good idea if we were clear on which VDO tachometers we are all talking about (there are different ones). My Brazilian VDO Porsche 356 replica tach (as used in Intermeccanicas) works fine with points/condenser, or a Petronix, or with a Petronix and CD ignition. No resistors or diodes required.

I did not know you need a diode inline so thats whats going to be wrong with my tach. I am running the pertroncix ignition and i have the VDO gauges from Vintage speedsters. I will try the diode and let you know if it works.

Thanks
Mike

I'm curious; basically in this instance a diode would be used as a "one way" shunt, e.g. to eliminate a back cirucit? Also, since most VDO electronic tachs use the same basic circuitry, I wonder why mine (and I assume, some others) works fine and others don't?

Me too George. I have the same setup as described, Pextronix igniter and the VDO replica guages which come from VS, and I don't have a "bouncy tach." Any fluctuations in my tach reading seem to represent slight changes in RPM I notice at idle.
John H.

I do remember a bouncy tach situation on my car from last year. It happened when a Bosch silicone core coil secondary wire (Bosch "blue" coil to distributor cap) went bad; this setup was using a Compufire and a CD inginition. The secondary wire went which in turn fried the Compufire. During the short period of "meltdown" the tach was doing crazy things; luckily the moment of truth occurred in the driveway. The crazy tach fluctuations may have come from a dying transistor in the Compufire sensor.

I replaced all secondary wires with stainless core items, replaced the Compufire with a Petronix, and left the CD ignition disconnected and tach operation went back to normal. Haven't reconnected the CD as I think it was damaged at the same time, and I haven't got round to installing a new one yet.

(Message Edited 10/18/2002 1:58:24 PM)