911- Rev Counter / Tachometer with a mind of its own

58A - 71E · Post by **58A - 71E** » Tue May 27, 2008 1:48 pm

All

Done the usual hours of searching trying to come up with a solution to my latest problem.
This week the rev counter has started to do all sorts of random things, no real pattern to it, just doing whatever it likes

It usually sorts itself out at constant engine speed e.g. on motorway or if I take it above 4,500

it soon catches up, but anything from 900- 4500 it's just pot luck really as to what it reads.

From what i've read it could be points or charging issues or the dreaded CDI

but

I had the alternator rebuilt about a year ago, so i'm not suspecting charging issues, but will check to see if I'm getting 14+v at the battery.

Also just gave the distributor a bit of a strip down and good clean/service as part of the recent engine rebuild and reset the points gap then (the car is still running points and 3 pin CDI for the MFI)

Any tips on what else I should be checking/looking for

Cheers

58A - 71E · Post by **58A - 71E** » Thu May 29, 2008 9:52 pm

Well it pretty much died yesterday, but if anyone else should have similar problems I thought I'd answer my own question:

Checked the alternator - its working fine and charging as it should

No misfiring or running problems so didnt think it would be the CDI really

Took the distributor cap off and checked the point gap - just as it should be and checked the black and purple wire attached as it should be - it was

Then noticed that the new rotor arm had some suspect looking wear marks around the contact area - i.e. the plastic was fouling and worn down, leaving a very dirty metal contact - so cleaned that up

Then looked inside the cap to see where it had been catching

My pics are always crap (must upgrade the garage cam) but basically two of the contacts for the HT leads in the cap have really got scored (this cap and rotor arm have less than 3,000miles on them)
So I'm guessing the cap had been put on slightly off from where it should have been.
Anyway, cleaned up and reseated I have a working rev counter again

Will replace the distributor cap and rotor arm this weekend but have noticed the cap has a very very slight wobble when seated so maybe the clips arent quite as they should be, but will investigate further when I replace the cap....

At least its nice to see that central dial heading north of 6krpm again

impmad2000 · Post by **impmad2000** » Thu May 29, 2008 10:33 pm

Pleased you've sorted it. I can test tachos on my test bench but it usually only proves that they work (or not !!) they aren't usually half way good !!

Cheers
Tim

bluemonk · Post by **bluemonk** » Thu Jun 12, 2008 9:21 am

Mine did a similar thing and it turned out to be the high energy coil installed which buggered up the rev counter.

Nothing coming up on the secondhand market so I've decided to strip it down and have a look

I'm wondering if it's possible to solder in new electronic components?

Probably a long shot though...

impmad2000 · Post by **impmad2000** » Wed Jun 25, 2008 1:43 pm

Just seen this.
The parts are old technology, and in some models they use a transistor that I've not yet identified. The older ones are clearly marked and whilst old, the parts are available. A circuit schematic would be useful, but I've not founds one yet nor managed to reverse engineer the circuit board to my satisfaction.
Tim

bluemonk · Post by **bluemonk** » Thu Jun 26, 2008 11:09 pm

Cheers for the reply Tim!

I've given up on the whole idea of trying to repair the old unit now and have found a secondhand replacement.

Would be great if someone could tell me where the 3 wires go before I install it? I've got redwhite left, Black Purple centre and Brown right. They might not have been the right way round to begin with..

Alan

impmad2000 · Post by **impmad2000** » Fri Jun 27, 2008 7:07 am

bluemonk wrote: I've got redwhite left, Black Purple centre and Brown right.

Redwhite = +12V
BlackPurple = Tach signal
Brown = GND

Tim