Tacho not working

Dave R · Post by **Dave R** » Thu May 07, 2009 5:58 pm

I reluctantly ask for help again, this time to get the tacho working. One day you guys will get fed up of this endless problem solving!! (but not yet hopefully)
I took out the RV1 tacho from my Sprite and purchased a rebuilt RVC Smiths unit (RVC 2414/01F 4cyl. 12v neg earth). I understand that the RV1 type cannot be driven off Megajolt of EDIS units. This has 3 connections on the back. There's an obvious spade connection to earth and another spade that I've fed with 12 volts via the ignition switch. The third terminal is a male bullet and I've been trying to feed the 'rev trigger' to that. I've tried the following, all connecting to the bullet terminal:-
- Megajolt tacho feed (running V4 software)
- EDIS terminal 2 (diagnostics signal)
- EDIS terminal 11 (clean tacho out)
- tee'd into coil drive/feed No 1(looked for half or double revs)
- tee'd into both coil drive /feeds and fitted diode pair with zenor (I hope I got them the right way round - the bar/stripe on the diode end represents the point of the arrow and bar on the diagram??)
- the above diode arrangement without the zenor
...........all of the above showed the rev needle remaining totally still. If I hold the tacho in my hand and twist it, the needle does move off its stop so it's not stuck.

First of all. Is there anything in the above that I've missed or got wrong?
Secondly, is there an easy way to check if the tacho is working? If I fed a wire from a car with a normal coil that's driven by a Luminition system; would that prove anything? (I have a Rover P5B with that system)

As always, all ideas welcome.
Regards,
Dave

Post by **brentp** » Thu May 07, 2009 6:49 pm

Hi Dave,

Are you able to test with both the RV1 and RVC units? If so, are you getting the same results?

Your testing was pretty comprehensive. Tapping right into the coil drive of one of the coil packs is the closes way to replicate the original distributor setup, aside from the 1/2 Revs reading. It is surprising this doesn't work- just focusing on why at least that arrangement fails to work will get you further in solving the problem.

Are you confident the 12V power supply to the tach is active while the engine is running?

sierra2dr · Post by **sierra2dr** » Fri May 08, 2009 5:48 am

I've got the same problem,I did a forum search and I'm sure I had seen a post about connecting it to PIP. I've not yet had the chance to further the problem

Dave R · Post by **Dave R** » Sun May 10, 2009 5:32 pm

Hi Brent..............progress
I've only just been able to get back in the garage, so I'm a bit delayed in my reply.

Yes, power is at the spade terminal with the engine running.

I took the tacho out of the car and wired it to the coil and ignition of my Rover P5b (V8 engine with 'standard' igntion coil apart from a Lumination optical trigger). At tick-over I got no reading and then with a few extra revs it started to read. I think it was twice the actual revs due to the V8. Anyway, the tacho works but seems to be 'sticky' at low revs.
I put the tacho back into the Sprite and wired it in with wires from the No 1 & 3 feeds to the coil going into the diode and zenor set-up you advocate in the guide. Again at tick over nothing, then it starts to read. I took the car for a run and the tacho read for,say, 10% of the time and 90% it was at zero. THe reading period did not appear to be linked to revs or engine load.
I would conclude that the EDIS coil is doing its stuff and the diode wiring works but the tacho itself has a problem. Since it it a new 'rebuilt' unit it may have a stiff region due to new components....I'm guessing here. I intend running it like that for a while and see if it settles in. If it doesn't get better I would propose to change the tacho since the wiring and diode set-up seem to be fundamentally sound.

Yet again, many thanks for your help.
regards,
Dave