Pertronix/Compufire/Tach

[pyrOman]

I've run both the Pertronix and the Compufire and found no real noticeable difference between them. (Didn't have a tachometer back then!) After frying both of them - - eventually I got another Pertronix. Now, I have been running points on the Bug's engine and there has always been a slight "miss" going on that bounces the tach needle just a bit. Recently I scored me a distributor that has a Compufire in it so I threw it in the Bug's engine and when I fired it up the tach's needle was bouncing hysterically all over the place!!! Barely steady at all at idle and just plain going crazy upon acceleration!

At first I thought maybe I screwed up the tach or it just lost it but before swapping it, I went back with the points distributor and the wildness stopped!?! Anyone here ever had this situation happening before???
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Some people are so busy being clever they don't have time enough to be wise.

[Dale M.]

Yeah ... Electronic module had poor ground where mounded to dist points plate... Cleaned area where module mounted and checked ground at dist body and made sure mount screw was tight... No problems after that... But that may not be you problem...

Dale
_________________
“Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.

"Kellison Sand Piper Roadster" For Street & Show.
"Joe Pody Sandrover" Buggy with 2180 for Autocross (Sold)
============================================================
All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[pyrOman]

I have them cheap Sunpro tachs from FLAPS in the Dormy and the Bug and both behaved the same way with the Compufire. I will try it in the Plump Truck which has a VDO tach. If it does the same thing then something is amiss with this Compufire when used with a tach, though it works fine as far as running the engine is concerned.
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Some people are so busy being clever they don't have time enough to be wise.

[larryducas]

Put a diode in series with the tach input. This fixed mine. 1n4003 from radio shack is fine. Almost anything works
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71 deluxe, 1968 type 1 engine rebuilt, 009 distributor, pertronix module, Empi 38 EPC / DGES carb

[pyrOman]

I had to put one in for the VDO when I got it since it was not only erratic but way off too! I did take the diode from it and tried it with the Sunpro, it didn't make a difference.
_________________
Some people are so busy being clever they don't have time enough to be wise.

[tkelley]

I had the same problem with my compufire and
the needle bouncing with the engine running. I tried
using inline diodes, checked and cleaned grounds,
all sorts of things...I ended up changing out the coil
and all the problems went away.

Tom
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When the sun doesn't rise, we'll replace it
with an H-bomb explosion or a painted jail cell...

[pyrOman]

[pyrOman]

I did replace the coil and though it helped, decided to go with points instead of the Compu-Fire diz. Ran bitchen....

Until this happened!

https://www.thesamba.com/vw/forum/viewtopic.php?t=...mp;start=0

So I bought a fresh Pertronix, just installed it and it fired right up! Sweet!


However, I didn't get to actually drive it until this morning going to work. Though it starts right up, idles fine and runs good, the tach now reads WAY high! Normally at 70 to 75 it read around 3500 rpm and felt like it too. Now with only the Pertronix being different, it reads 5000 rpm! WTF?!?!?!?

Though the difference between the points and the Pertronix might have advanced the timing some, I can't see why that would make the tach reading high!?!?
_________________
Some people are so busy being clever they don't have time enough to be wise.

[Dale M.]

Is tach set for 4 cylinder?...

Dale
_________________
“Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.

"Kellison Sand Piper Roadster" For Street & Show.
"Joe Pody Sandrover" Buggy with 2180 for Autocross (Sold)
============================================================
All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[pyrOman]

But of course! Everything is as it was and worked fine before this swap. Again, I do suspect the timing needs to be reset now but I just found this out and I'm at work. I will retard it some but can't really check it until I get home and put the light to it.

FWIW, I have the same tach on the Dormy and have ran Pertronix and points without any difference in readings.
_________________
Some people are so busy being clever they don't have time enough to be wise.

[larryducas]

put a diode in the tach sending line, worked for me
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71 deluxe, 1968 type 1 engine rebuilt, 009 distributor, pertronix module, Empi 38 EPC / DGES carb

[mikedjames]

What you are trying to do with a diode is to kill off "ringing" which is a part of the way the coil works when it fires. The tacho counts 2 pulses instead of one sometimes. When I built my own rev counter the first version claimed that idle was 60K RPM because of the ringing.

Changing coils or condensers will change the ringing .

Another route is to filter with e.g. a 1000 ohm resistor from the coil -ve to tacho input terminal and then a 0.1 microfarad capacitor to ground across the tacho input (e.g. a condensor).

Tacho--------- 1k resistor ----------- coil -ve ----> contact breaker or module
|
Capacitor (condenser)
|
Chassis


In rhe end I built this circuit into my tacho sender computer.
This filters out the ringing.
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[pyrOman]

Well, how 'bout them apples?!?

I think I did mentioned that once at work I did retard the timing a bit. Doubt that really "fixed" the problem but apparently the thing just calibrated itself!

On my way home from work it went back to normal! Idled fine and readings at speed were correct. Still wondering why this happened?!? Unlike the "jumpy" issue I had before with the Compu-Fire, the way high readings yesterday were steady as could be.
_________________
Some people are so busy being clever they don't have time enough to be wise.