407 Rev-counter. Stopped working

[Nick Challacombe]

My rev counter has ceased to function everything else within the housing works.
Does it pick up from the auto box somewhere with an actuator or the propshaft?
I have tried the highly technical method of giving it a thump but nothing.
Could someone point me in the right direction please.
I was not able to find an answer within the forum.

[dwomby]

I am away from home and can't check but I doubt it's mechanically driven. Take a look at the wiring diagram but IIRC, most tachometer are electrically triggered from the coil.

David

[Roger Morrall]

Nick

According to the 410 wiring diagram, a copy of which may be found on this forum if you search for "410 Alternator charging fault" (I put it there under my then nom de plume PEU186F) there is a white cable feeding power from the ignition switch to the rev counter. There is then another feed from this connection to the petrol reserve switch. There is another white cable which is looped through a link on the back of the rev counter which feeds power from the ignition switch to the ballast resistor and thence to the coil and the distributor.

I imagine your car will be very similar, Bristols weren’t renowned for changing things for the sake of it

The rev counter works by interpolating the rate that the distributor points are opening and deriving the revs from that. Rev counters accordingly came in 4, 6 and 8 cylinder variants.

Your Rev counter could have failed because of an earthing fault, but we can rule that one out as the warning lights share the same earth via the distributor housing

It could also have failed because no power is getting to it. But I note that the feed to the petrol reserve switch and its warning light comes off the same connection point on the back of the Rev counter, so if those work that rules that one out.

It could also just possibly have failed because the feed to the distributor that is looped through the link has fallen off - this link can be undone and opened - but I think that really rather unlikely.

The most likely explanation is that old age has caught up with it and it’s given up.

In all events you’ll need to get at the back of it, which involves disconnecting the battery, removing the steering wheel, removing the instrument cowl and very probably loosening the instrument panel to get at the back of that in order to get it out.

If you do need to get it repaired may I suggest JDO instruments who may be found via the following link Home page
I found them (actually I think a one man band) excellent.

Good luck - by the way it is a mistake to assume that it retained any accuracy over the years, even if it is a piece of electronic kit. By now it could easily be out by +/- 40% - I speak from experience, but JDO will recalibrate it for you.

[David C]

There are two options for attachment of the loop fed from ignition switch to the ballast resistor onto the tacho. Either the white wire loops around a plastic block which can be unscrewed from the back of the instrument or there are two bullet connectors and the loop is inside the tacho.
If the feed had failed I don't think you would have any sparks, so once you have checked Roger's suggestion of a bad earth it looks as though the fault is internal to the instrument.

Are you using an electronic ignition? I fitted a fancy uprated Mopar ignition which doesn't work with the original tacho. This requires conversion from current to voltage sensing. A DIY kit is available from Spiyda Ltd. Dependent on what is wrong with your unit this may be a solution for you.

[Nick Challacombe]

Thank you for your help Roger, David and dwomby.

With the information I gave you seem to have answered my question. The white wire must be the problem as everything else is OK. I have a tame auto electrician (old school) who has sorted out electrical problem on the beast for the last 20 years. I am almost 80 with limited mobility with only one leg so crawling around the limited space under the steering wheel is a bind. I got stuck once.
the car is std in most ways except for an electric fuel pump, Facet,
power steering fitted a few years ago. Brakes now sorted with a new servo. At least I can drive it with no rev counter.
This Forum is fantastic, thank you on and all, especially Kevin for setting it up.

[Roger Morrall]

We’ve all been stuck down there, but far worse is getting stuck in the same position in a Lotus Cosworth Super Seven. And that was in the days of my youth!

But neither you nor your auto electrician need to get under the dashboard to get at the Rev counter, it’s all above board as it were. But it will be necessary to remove the steering wheel etc as I mentioned previously.

My unreliable memory says that the "plastic block" mounts a removable copper link, held in place by two tiny screws. This link is then connected into the instrument itself to form another loop and transmits, by induction from the loop around the block, the "spark" current into the Rev counter. I have no idea what happens then. On second thoughts it occurs to me that if either of those tiny screws have become even slightly loose or corroded then the intended very slight induced voltage/ current might not ever reach the internals of the instrument. Another option perhaps?

I found a Facet fuel pump tiresomely noisy and reverted to the double ended SU. And never had any trouble with it.

By the way my unreliable memory also seems to recall hosting Hilary and Geoffrey Herdman for a night when they came down this way to pick up some better seats from Geoff Kingston for your car. Can that be true?

[Geoff Kingston]

Almost true Roger, the seats were for Marc Atkinson if I remember correctly and were ex 603 needed for the high backs/headrests to go into a 411.
Geoff.

[Nick Challacombe]

I would like to thank everybody for their kind replies and of course the advice therein. I have had it up on a ramp today and everything underneath is OK. After the next lot of hols I will equip my tame electrician with all the advice and get him to sort it.
Roger, Geoff has answered your recall and I am a bit envious I think perhaps those seats would have suited me beautifully.
Next job is to get them restuffed, Connelly fine but the last time they were done someone used foam and it is disintegrating, especially on the drivers side!
And no, i do not think a diet would help.
Thank you all and have a good weekend.