The Problem

It is often described as "bouncing" tach. After installing a Marelli electronic ignition from a 1979 Fiat Brava on my 1974 Fiat 124 Coupe, the tachometer needle would read accurately at idle, but upon reving the engine it would jump around then peg to zero. Jack Ivemeyer’s description is also very typical, "my tach acts up (bounces or goes to zero) until the car is up to running temp.,then the tach works properly or if I turn on the lights (even when the engine is cold) the tach works right..."

A Little History

Over 25 years, I’ve installed Plex units on every Fiat I’ve owned (850, X1/9, 128) and have found it to be one of the best improvements for reliability and performance. In the late ‘70’s and early ‘80’s, Marelli offered for after-market their Plex upgrade kit for Fiat models which were originally equipped with points distributor. This included kits for the pushrod 850 based motors, the SOHC X1/9 and 128, and the DOHC 124 motors. The Marelli Plex and the later stock Marelli electronic ignition share the same high output coil and GM control module set in a sporty finned alloy housing, along with identical magnetic pick-up. The only operational difference is internal to the distributors, where the Plex unit has a mechanical advance and stock electronic distributor uses a vacuum advance.

Since I'd always had good luck with Marelli’s Plex units, it was a surprise to me when the Coupe’s tach behaved as I’ve described. Its the only Fiat I’ve ever had do this when fitted with electronic ignition. In making a post on Mirafiori.com’s Forum about my tach problem with the EI, I remembered that on Marelli’s instruction sheet for their Plex they had a note for fixing potential tach problems, which involved putting a capacitor in-line on the tach wire. Of course, my set of instructions are long gone, because there never seemed any reason to keep them - little did I know. However, Pete Angel (a Mira regular) had a Plex instruction sheet and found the value of the capacitor needed. Its my supposition that other Fiat model’s tachometers and later 124 tachs have the correct capacitor retro'd internally.

The Marelli Plex instruction sheet specifies a 0.68uF 400V capacitor be added in series between the negative side of the coil and the electronic tach - which is to say, the tach wire is interrupted by the capacitor. The specific cap is tough to find as they seem to be more for audio or photographic use. I found a good supply from Digi-Key Electronics on-line - CLICK THE PHOTO LEFT FOR A LARGER IMAGE. At the time I purchased mine, they listed two different part numbers for the value needed:
PF4684-ND or E4684-ND

If you have trouble finding the correct value cap locally or on the web, e-mail me. At the time of this writing I still have a few left from the supply I bought. 

Remove the tach wire from the negative side of the coil and cut away the connector. In my case there were two wires feeding into the same connector, so I only cut the tach wire and reconnected the connector to the coil. Then install one of the spade connectors on the loose tach wire end. Build the rest of the "Tach Fix Rig" as shown in the photo - CLICK ON THE PHOTO FOR A LARGER IMAGE. The capacitor has no polarity, so you can’t hook it up backwards. Once built, the rig installs very easily. The eyelet connector fits over the negative terminal on the coil and the spade connector plugs into to its opposite sex spade connector on the tach wire. I secured the rig to the coil/contol module housing with a zip tie.  Done.

PS: Have You Checked Your Magnetic Pick-Up Recently?

Contrary to common Fiat mythology, a bad magnetic pick-up wire is not the cause of every glitch with the Marelli electronic ignition. Some well intended folks who have never heard of Marelli’s capacitor fix might tell you that your tach is flaking out because you have a bad mag pick-up or a bad ground at the tach. Sure, both can happen and do ...definitely neither are the problem here. However, its not unusual for pick-up wires get brittle with age or pinch between moving parts in the distributor. It's a good idea to check the pick-up wires often or at least keep a fresh pick-up installed every season.