sud rev counter problem

Anyone got any thoughts on this problem.
On tickover rev counter works fine.
If you rev engine needle works fine up to about 2500 - 3000rpm.
Once you get past this, rev counter needle goes back down hard onto the stop.
Once the revs drop again then needle starts registering again.

If you have had it did you manage to solve it?

rsfruitbat

Check first the ground cables... its like it doesnt stand enough current. Propably the connectors have rust and dont contact very well . Just open the panel and try to clean all connectors firts...

Funny that , I suffer from the opposite at tick over the needle fluctuates , i put in a new counter and everything seemed fine for a week or so , then the problem came back, not overly bothered about it though as the dials are only a rough guide to what the engines doing (i.e. never accurate), hope you find the problem and let me know.

I will check out the earth ground cables and report back.
I do like to have an indication of revs especially as it is a new engine. Also to pass noise test we have to have some way of indicating engine is at 4500rpm. Thanks for guidance so far.
Oggie - glad your engine is on song again.

rsfruitbat

Cheers

Originally Posted by Iain.Oglethorpe

Funny that , I suffer from the opposite at tick over the needle fluctuates.

I have that problem on one of my cars. Needle fluctuates between the 1000 and 2000 rpm markings, averaging maybe 1300rpm, and when I apply a little accelerator it drops back down to what it should be reading!

More annoying is the other car where the speedo works some trips and not others.

Hey, dont knock the Alfa rev counters. my 33 rev counter is within 100 rpm of the rolling road reading all they way through the rev range.

Mine was / is 500rpm out (when it worked)
rsfruitbat

Mine reads about 150rpm too high according to my kitset digital tacho. Must be that curious '80s Italian habit of setting all the rev counters to point to 500rpm with the motor stopped, and actually including 0rpm on the rev counter below the reach of the needle.

Hi Fruitbat - this seems like an odd set of symptoms, but there may be an explanation. First, its definately a 'Sud rev counter? Certainly the early ones, like my '78 Ti used discrete components (not IC's), which may differ from later Sprints and 33's.

The principle of operation is much the same; the pulse train from the points is integrated to produce a voltage proportional to the engine speed (frequency-to-voltage conversion) and then this level is used in turn to drive a sensitive milliammeter (the rev counter needle).

On early rev counters (maybe late models too), the integration is performed by a capacitor/resistor network with some transistors for amplification/meter drive. If the pulse train from the points is noisy, what should be a well-defined train of pulses may mush into a constant voltage at high engine speeds, and this would cause the rev counter to drop to zero.

On the face of it, this may explain your problem. I would check your points (if you have them), condenser (ditto) and HT leads first. Make sure your coil is not breaking down internally, and is connected the right way round (+Ve to ignition supply, -Ve to points/electronic ignition module and rev counter sense wire on this terminal or dedicated module output) and whether the sense wire is routed near any HT leads.

Also check the distributor cap for tracking and maybe the ignition timing if there is a possibility that "spark scatter" may be occurring.

If all that checks out, there may be a problem with the rev counter itself, but would be more likely to be the meter movement rather than the electronics, which would probably fail completely (as mine did ) rather than act intermittently.

Good luck!

Lauren

Thanks for that Spacenut. Unfortunately I an rule most of the options you have suggested out asI went dwn the aftermarket ECUend electronic ignition route. The signal is sent direct from the ECU. I have both early and lat clocks / rev counters so will let you know which resolves problem. At the moment it is on my "nice to have " list which is some way behind my "must have" list like an MOT!

Thanks though rsfruitbat

Hmmm. Perhaps the ECU rev-counter drive is marginal and drops too low to drive the rev-counter at high revs. I guess that's possible as the (Sud/33) rev counter was originally designed to pick a pulse train from the points/ignition box (i.e. 12V pulse train with a lot of large transients), so there is a lot of lossy filtering in the interface circuit, so it may lack sensitivity. Your aftermarket ECU may be assuming that your rev counter is also designed for an ECU application (quite likely these days) and may only be producing a 5V pulse train (for example).

Good luck with the MoT!

Lauren