Tachometer off after pertronix install.

[Rattler1]

Yes I have been searching for days on this.
I installed a new blue coil and pertronix ignition replacing the points. My tachometer now reads 100rpm at idle and only 1000rpm on the high end. The needle is steady and doesn't bounce. I've triple checked my connections. Tach wire to negative side of coil. Dip switches were never touched. What's is going on with my vdo tach????

[ashman40]

Not exactly the same brand as yours, but here are some suggestions from HotSpark for fixing tachometer problems when using their electronic points.
https://www.hot-spark.com/Tachometer-Fix.pdf

I've heard of the diode fix for tachometers, the resistor/capacitor fix is a new one.
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Rattler1]

I was given the same info from pertronix. Thank you. I'll report back when I try it.

[ashman40]

Also found this old diagram for VDO tach install. It has the DIP switch settings. I wasn't aware that the tach signal could come from the alternator instead of the ignition coil. Not sure if this applied to your tach since you didn't confirm the make/model.

I also found this technical bulletin from VDO on the use of a diode in the tach signal wire but it is unclear how the wiring changes as it uses terminal numbers on the tach. But clearly there are times when a diode may be needed.
https://www.vdo-instruments.com/media/instructions...meters.pdf
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Pez]

[Rattler1]

Update. Spoke to pertronix. They said install a diode. No help. Spoke to VDO. They said same thing. No help. CB performance said must use matching flame thrower coil. That did not help. I ran out of options and went back to points and works normal again. I quit.

[Glenn]

[Cusser]

[Jimbug57]

An Arduino could be programmed to convert the signal from the coil into what the tach is expecting to see. They cost less than 5 bucks and run on 12 volts. You'll have to program it using C+.

You could use a Raspberry Pi because it's easier to program, but costs more and not nearly as fast as an Arduino for something like this.
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[gorbur]

I had a similar situation.
Mine is a Pertronix Ignitor module with the same VDO tach that Ashman posted the instructions for. In my case I had no tach movement. To fix it it added a 680 ohm resistor from the tach signal line to +12vdc to add a strong bias. Solved the problem and tach's been working fine since. At the time i did put the scope on it and i do remember seeing a fairly nice square signal from the Pertronix. But passed that I cant really remember what it showed me to go down the biasing path. Just wanted to get it working i guess.
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[vamram]

Hmmm....i never had issues with the vdo tach and compufire ignition on my '72 Super. I have my '73 wired for it, just haven't figured out where to mount it without ruining the big dash or removing my radio for a gauge bracket.
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[gorbur]

I spoke too soon. I have a Pertonix Rev Limiter installed and using the tach output from it was where i got into problems. Look like the tach was working fine with the Ignitor module. When I added the Rev Limiter its tach output didn't drive the VDO tach correctly. The Rev Limiter tach output needed the biasing to get the VDO to work not the Ignitor. Would have made zero sense if the biasing was from the +12v to the tach lead which is essentially the coil.
Sorry about my confusion.
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73 - Ghia Cabriolet, currently under resto
76 - Type1, 1776, full restomod

[Starbucket]

From ratwell=
.12 Why isn't this tach reading correctly?
eGauge's advice: Will VDO tachs work with points ignition or Pertronix?

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. 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 cylinder.

I think this is a long winded way of saying that you should connect a diode between terminal 1 of the coil and the tach wire that goes to the gauge. The cathode end (silver band) has to be facing the tach side, not the coil side.

VDO wiring instructions (eGauges.com).

[Rattler1]

The diode did not help me. That was the first thing I did.

[ashman40]

I thought I would add a contrasting comment from some work this weekend with a buddy. Before we started working on his '68 Beetle he had a OO9 distributor with a Pertronix Ignitor I module installed. In the dash we had recently replaced his large no-name tachometer (mounted under the dash) with a small 2-1/6" tach that fit into one of the small gauge holes in his dash. The small tach was working fine connected to the Pertronix ignition in the OO9 without any diodes or resistors.

As part of the work this past weekend, we replaced his OO9 with an SVDA which had points+condenser. After adjusting the points and setting the timing with a strobe light, we noticed that his tach would occasionally jump to a high rpm for a moment. The idle rpms and rev rpms matched a diagnostic tach connected to the #1 terminal of the coil so we know the tach was producing accurate readings, but the dash mounted tach would jump around 400rpm every once in a while (just for a moment) both at idle and while holding a steady fast rpm. There was not change in the engine sounds or the old analog tach connected to the ignition coil so it was only a dash gauge reading fluctuation.

I am considering adding a diode to the tach wire to see if it helps. But this is exactly the opposite of the OP's problem. Here, a distributor with electronic points works fine but swapping to point created a condition where the tach readings would spike.
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AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Xevin]

[ashman40]