Figure 1

Figure 2

We have had several member mentioned that there tach is not accurate and bounces around. To date this has been diagnosed as two possible problems. The first is using an aftermarket ignition system and the second is wiring aftermarket electrical products (fuel pumps) to the coil circuit.

Aftermarket Ignition Systems:

For those of you who have been having Tachometer problems with an aftermarket ignition system, the following is text from Denaploy, the manufacturer of Luminition. I was directed there by a Luminition person after enquiring about a Tachometer problem.

British vehicles, with negative earth, fitted with a tachometer as standard were fitted with a Smiths Instruments device. These units come in three general types depending upon their triggering method. The first is mechanical and usually driven from the distributor shaft and has no electrical interface. Common in sports cars of the 50’s and 60’s.

Before 1974 (and on Lotus Elans) the electronic tachometer were current impulse triggered. This means that the tachometer sense wire is connected in series with the ignition coil, usually between the ignition switch and coil/ballast positive as in Fig 1. Sometimes the power for the tachometer was shared internally with the ignition switch end of the sense wire giving only three terminals. Smiths unit of this type can be identified by the letters RVI on the face of the tachometer. Designed in the days of contact breakers, they give inconsistent results when used with electronic ignitions. Many installations will work but some will produce erratic or no movement of the indicating needle. Some suggestions for rewiring the ignition circuit to eliminate the problem have been tried with limited success. Later electronic tachometers are voltage pulse trigged and the single sense wire is connected directly to coil negative as in Fig 2. Smith unit have the designation RVC printed on the face. As far as we are aware there are no problems using this type of tachometer with our Optronic ignition. It is important that owners can tell the difference between the two types of tachometer as incorrect wiring of the unit can cause damage to the electronic ignition module. If used the tachometer sensing wire(s) must be connected as appropriate for that particular design of tachometer.

As mentioned above some owners have tried alternative wiring for the sense wires with RVI tachometers with limited success. Our recommended solution if you wish to retain the benefits of electronic ignition is to:

• replace the RVI tachometer with a later RVC model if available or aftermarket equivalent or
• have the existing RVI tachometer converted to the later internal electronic.

In both cases a minor wiring change must be made to the vehicles normal ignition circuit.

The company in England that we usually recommend to carry out conversion is:

SPEEDY Cables, The Mews, St Paul Street

Islington, London, N1 7BU UNITED KINGDOM

Phone 0171 226 9228

Fax 0171 704 9542

Does anybody know of companies in North America that will do the conversion???

Aftermarket Electrical Products:

Hmmmmmmm. So is this related to the problem I have with my tach which jumps around whenever the aftermarket electric fuel pump (which is powered off the coil) clicks on? I have a Twin Cam (with standard points, not the Luminition), and the problem is that the tach seems to "droop" for a split second. The frequency of the droop is proportional to the engine speed and throttle position. Therefore, at idle the tach will droop for a split second about every second or so (which corresponds to whenever the fuel pump clicks on). But at heavy throttle at around 4k and above, the tach droops so frequently that it basically never goes above about 3000 to 4000 rpm. So after reading this post about Luminition, my hypothesis is that the tach may also be confused by the current which gets diverted to the electric fuel pump. Any one else have a problem like this? I guess the obvious fix is to wire the fuel pump into some place else, so where does everyone suggest?

Yep, not much doubt since the tach is current activated, not voltage activated. The easy fix for you is to drive the fuel pump (which draws a lot of pulsed current) with a relay (which will draw a negligible and constant current) actuated by the the line that presently drives the fuel pump, and drive the fuel pump through the relay to fresh solid wires from the battery (with an inline fuse, of course). A standard 30 Amp relay and socket can be had at a NAPA or Carquest place. The same is true for anything else that is attached to the ignition line. BTW, since this is a current path problem your problem should also be cured by attaching the fuel pump to the ignition wire on the 'upstream' side of the tach since the pump current would no longer be pulled through the tach sense loop. I'd recommend the relay as it will relieve the rest of the wiring (possibly the key switch) from the fuel pump load and increase reliability due to shorter power line runs for the pump.

I put in the relay (a $12 Bosch unit) yesterday, and now the tach works great.