Rev counter tachometer tach problems

[leifanten]

Hi all,
I have a problem with the rev counter on my 1969 Elan +2. It seems to read only 50% of the engine speed. The car is fitted with Allison electronic ignition, and seems to be rewired, as I have read some of the other threads on similar topics and could not navigate the colour schemes as described.

I have the five binder workshop manual and to my surprise (or lack of ability to find it) I could not find any wiring diagrams in that whole yard of binders..... If I have missed it, please point me in the right direction.

Now, back to the rev counter, does anyone have experience with this type of issue? How is the schematics supposed to be for it to pick up the right signal? If it is an issue due to the electronic ignition, is there a way to bypass the circuit to give the rev counter the signal it is looking for?

best
Leif Andre
Houston TX

[leifanten]

So... it looks like this is a real popular topic in previous posts. Sorry for asking again about something that has been dealt with so thoroughly in the past.

Basically the answer is: If you have an electronic ignition and an RVI instrument, you need to convert it to a RVC (or similar modern setup) instrument

Here is some useful links I found on this in case others have same issues on their cars:

http://members.shaw.ca/tsmit/tachmod/tachmod.html

http://members.shaw.ca/tsmit/tachmod/tachmod0.html

Smiths%20Tachometer%20Conversion-R1.pdf

(828.93 KiB) Downloaded 1410 times

( I hope this attachment went well, because it is a real gem for those who wants to do the conversion at home)

Leif Andr?

[collins_dan]

The car had a Lumenition electronic ignition and RVI tach that didn't work when I bought the car. I replaced the ignition and coil with Pertronix and everything has worked fine since.

If you do decide to have the tach rebuilt, I had Nisonger rebuild my speedometer. They did a good job and do tach conversions as well. http://www.nisonger.com/electric-tachom ... rsions.htm

Dan '70 S4

[leifanten]

Dan, thanks for the reply. Which of the Pertronix systems did you go with? Ignitor I, II or II or the Flamethrower? Do you know the difference between the Allison (optical) system and the Pertronix that makes the latter work and not the former? Is it a predictable solution or is it random (i.e. some get their RVI tach to work with PErtronix and some dont)

cheers
Leif Andr?

[collins_dan]

Mine is igniter I and a flamethrower coil. I chose Pertronix, because I had heard from the group here that it worked and that has been my experience. I don't know of others for whom Pertronix has not worked with their RVI tach. You might contact Pertronix and ask them. I called them up the other day as I was trying to install their digital rev limiter. Very nice people. Called me right back. As I recall the Lumenition was optical as well. I'm not sure how the Pertronix works, it just does. I bought mine from Ray http://www.rdent.com/ , he's knows which of the Pertronix works with the various Lucas distributors.

Dan

[rgh0]

The Lotus curent pulse sensing standard tach will work with most electronic ignitions provided 2 condtions exist

1. The tach itself is in good condition - in particular the capacitors in the circuits are prone to drying out and failing leading to faulty readings

2. The elecronic ignition is wired so that it does not inject extra current pulses into the feed to the coil that the the tach is sensing. In particular the power supply for the electronic signition should not come from the coil supply which is how many of the wiring diagrams that come with after market electronic ignitions show it coming from unfortunately

Both these problems are cured by installing a new voltage sensing tach mechanism that senses the voltage pulses on the earth side of the coil but both can be solved without it either.

cheers
Rohan

[collins_dan]

"Both these problems are cured by installing a new voltage sensing tach mechanism that senses the voltage pulses on the earth side of the coil but both can be solved without it either."

Rohan,

Can you explain how to do this? I have a Pertronix Igniter electronic ignition and have just added a Pertronix Digital Rev Limiter, both of which are connected to the positive and negative sides of coil. The tach always bounced slightly, but now that I have 3 leads coming off the positive side of the coil (Igniter, Rev Limiter & Tach) and 2 off the negative (Igniter and Rev Limiter), the bouncing is more pronounced. This may seem crazy, but having the fan on seems to also add to the bouncing.

Thanks, Dan

[paddy]

2. The elecronic ignition is wired so that it does not inject extra current pulses into the feed to the coil that the the tach is sensing. In particular the power supply for the electronic signition should not come from the coil supply which is how many of the wiring diagrams that come with after market electronic ignitions show it coming from unfortunately

I think this is the key part. In the RVI tach setup the supply from the tach to the coil +ve is intended to supply the coil only and cannot be used as a general purpose +ve supply. This means I think both your electronic ign module and rev limiter need to be powered by a separate, direct +ve supply (on whenever the ignition is on) instead of using the coil +ve terminal.

I suppose there is also a chance that someone has wired your fan somehow to the coil +ve supply - although unlikely it is worth checking.

Paddy

[collins_dan]

So you think the negative is OK, but I need to run another line to supplier the positive to the dist and rev limiter? Dan

[rgh0]

Yes connections on the negative side of the coil ( in a negative earth car) that sense when the voltage drops to zero as the points ( or electronic igntiion) draw current through the coil are generally OK. Anything taking current on the connection between the tach and the coil postive can inject extra current pulses into the system and cause the tach to do strange things and bounce. The pertronix power supply and the rev limiter power supply needs to come from a separate switched 12v supply not from the feed to the coil from the tach. If the electric fan is wired into this circuit somewhere it also needs to be changed to a separate switched supply.

cheers
Rohan

[collins_dan]

Thanks. Being a complete electrical novice (but willing to learn), can this be accomplished by running a new line from the fuse box to a relay to power both the rev limiter and the electronic ignition? If fuse box is an OK source, does it matter which side or which spot the line comes off? The fan is wired in this fashion. Thanks, Dan

[rgh0]

Yes a wire from an ignition switched source in the fuse box should be ok. Due to the low current draw you should not need a relay for the electronic ignition or the rev limiter. If the fan is wired to a relay from a switched source then you could tap into that wire at a suitable point before the relay. The power to relay coil and then to the temperature sensor to switch on the the relay needs to also come from the fuse box source and not from the igntion coil positive.

cheers
Rohan

[collins_dan]

How do I know what are the ignition switched sources to the fuse box? I have an S4. Thanks Dan

[collins_dan]

I took a look at the fuse box and I have a couple of options. It would be a lot easier to connect to the input side than the output side, would this be OK? If not, there are 2 open on the output side, one on each, but it will be a tight fit. Can I connect both the electronic ignition as well as the rev limiter using this method, or just the rev limiter? What gauge wire should I use? Thanks, Dan

[rgh0]

I would connect to the input side if this is an easier location and put an in line fuse in the line. I would run a signle wire to the nose where the rev limiter and electronic ignion are located and feed both from this wire. I would run a wire of similar size to that used by the electronic units themselves.

Cant recall which fuses are switched in the standard Elan fuse box as mine has a non standard fuse arrangment but you should be bale to tell by measuring with test light or multi meter or from the wiring diagram in the manual

cheers
Rohan