Rev counter madness....

[KML1976]

This morning, with it being such a glorious day, I decided to take my Stag to work (a nice exercise for her with it being a 24 mile round trip).

However, for the first 5 miles the rev counter didn't work at all, then it suddenly sparked into life flitting from zero to 4,000rpm before dying again.

Bizarrely, when within 2 miles of my workplace the bloody thing started working normally again, as it did on the return journey as well as on my subsequent 10 mile round trip to the Chinese to purchase a House Special Foo Yung!

Anyone else experienced this kind of misbehaviour?

[V Mad]

Do you have a Mk1 rev counter? If so, do you have electronic ignition?

[Robmsharp]

I did on a Mk1 I owned. It was fitted with an aftermarket electronic ignition which was causing the rev counter needle to bounce all over the dial. The rev counter fitted was from a Mk2 and the problem was cured by fitting a Lumenition electronic ignition.
Assuming that your rev counter has been working fine up to now and nothings changed, it sounds like the intermittent fault could be caused by a poor electrical contact either from the distributors 'signal' wire or internally within the rev counter itself. Old electronics (as the rev counter is) are renowned for electrical connections becoming intermittent. A careful inspection by someone who knows what they're looking for (e.g. dry joints on soldered connections) may reveal the culprit.

[Valleyforge]

Mine takes exactly 25 seconds to come to life after engine start-up. Its an endearing feature.

[V Mad]

In the past I found that the Mk1 rev counter did not work with (Lumenition) electronic ignitions, but oddly my current one (Powerspark generic type) is fine. That may be due to different characteristics of the ignition circuits or the way some systems are not connected up properly.

[milothedog]

In the past I found that the Mk1 rev counter did not work with (Lumenition) electronic ignitions, but oddly my current one (Powerspark generic type) is fine. That may be due to different characteristics of the ignition circuits or the way some systems are not connected up properly.

I would have thought you had the expertise to understand / explain why Chris, given your background. Could it be the 2 different ways that the systems are triggered ? one being Hall Effect and the other being Photo cell I believe?

Ian.

[Mike Stevens]

The original systems were designed to trigger off the big pulse signals seen on the coil -ve (voltage sensed) or the coil current (current sensed). I suspect that with an electronic ignition these signals are different enough to make the tacho operation marginal on some units.

I have re-calibrated the tacho on my Mk2 Stag (and PI saloon!) by feeding it with a square wave from a signal generator, set to a suitable signal level. No, I can't now remember the signal level required but I can check again. This was with voltage sensed units. I've not tried with current sensed ones. Yet!

I would agree that if it stops suddenly when you've not changed anything else, it's more likely to be due to poor connections somewhere.

Cheers,
Mike.

[pagan]

It still just registers the collapse of the coil...
How would, whether the trigger was optical or whatever, the electrical signal on the coil is the same. ?

PS
My tacho has a delay before operating, thought it was a Triumph thing.

Just changed from optical "Piranha" to Electrical "Accuspark"
The lead to the coil were old and had been spliced into before.
Renewed the wires and connectors for the coil, still has a delay but very short.

Allan

[V Mad]

Ian, always happy to explain more if it helps and if anyone is interested.

The type of trigger circuit should not matter providing that it is working properly and gives a clean trigger pulse (points often dont, they can bounce and arc!). But the characteristics of the coil switching circuit, and the associated wiring to the coil and electronic module can make a difference.

The Mk1 tach is current sensing. The shape of the current pulse is different between points and electronic switching, elctronic usually having a much faster/sharper pulse with associated high frequency oscillations. It is these oscillations that seem to upset the Mk1 tach in some installations. Two other factors that, IMO could cause tach problems are lack of proper earthing, and lack of a proper 12 volt supply to the electronic circuit (the ballasted coil+ point is not a proper supply).

[milothedog]

Ian, always happy to explain more if it helps and if anyone is interested.

The type of trigger circuit should not matter providing that it is working properly and gives a clean trigger pulse (points often dont, they can bounce and arc!). But the characteristics of the coil switching circuit, and the associated wiring to the coil and electronic module can make a difference.

The Mk1 tach is current sensing. The shape of the current pulse is different between points and electronic switching, elctronic usually having a much faster/sharper pulse with associated high frequency oscillations. It is these oscillations that seem to upset the Mk1 tach in some installations. Two other factors that, IMO could cause tach problems are lack of proper earthing, and lack of a proper 12 volt supply to the electronic circuit (the ballasted coil+ point is not a proper supply).

So what I think your saying is the optical types that have an amplifier unit could have some effect on some Tacho's. Whereas with the hall effect type, because the switching is much simpler the problem doesn't seem to occur.

[V Mad]

Sorry, didnt make it clear enough. The type of sensor, ie whether hall effect or or optical should not matter. I think that what does matter is
a) proper earthing
b) proper power supply.

My experience using the integral elec ignition (eg Powerspark type) is that this works OK on my Mk1 tach.

[jasper]

I had a similar thing
Terminal loose on back of rev counter
Hope yours is an easy fix !!
John

[flying farmer]

The MK1 rev counter on my Stag engined estate has worked fine for years with the lumenition (that's the kiss of death), but the one on my project Stag triggered by the megasquirt wasn't having any of it so swapped to a MK2 to cure it, biggest PITA was having to change the speedo to match.

Neil

[V Mad]

Neil, could you not have fitted the Mk2 guts into the Mk1 tach instead or fitted an aftermarket voltage sensing circuit?

With the Megasquirt, did you use an EDIS system?

[DJT]

The MK1 rev counter on my Stag engined estate has worked fine for years with the lumenition (that's the kiss of death), but the one on my project Stag triggered by the megasquirt wasn't having any of it so swapped to a MK2 to cure it, biggest PITA was having to change the speedo to match.

Neil

Neil,

The tacho on my old Mk1 worked fine with Lumenition too. And no, it hadn't been altered. I fitted Lumenition to that car back in the early '90s; at the time totally oblivious to any potential incompatibility problems, and it was still working the same when I sold the car 7 years ago. It was only from information posted on this forum that I learnt that the Mk1 tacho doesn't work with Lumenition