Digital Rev counters

Pete · Post by **Pete** » Mon Feb 11, 2008 12:00 am

I have had suspicions about the accuracy of my Westach rev counter for some time, so today I fitted an MGL rev counter Rev counter.

After starting my engine it showed 9999 revs, so obviously it is even more suspectable to the noise that was causing my westach to over read.

I have a Bendix dual mag on an Acro VW engine, there are no other electrical devices on the plane that can be interfering, anybody got a suggestion on how to get rid of the noise on the rev counter input ?

tnowak · Post by **tnowak** » Mon Feb 11, 2008 8:16 am

1. Make sure you route the tacho cable as far away as possible from your mag switch leads.
2. Keep the sensor cable away from all HT leads.
3. If no improvement consider using screened mag switch cable (if not already using screened).
4. Check you have good grounds at your tacho instrument and at the other end of the sensor cable.
Tony

cardiffrob · Post by **cardiffrob** » Mon Feb 11, 2008 10:27 am

Same prob with a VW. Reads double rpm. Seems that pulses get picked up through the ign leads plus the effect of having a wasted-spark system.

Can you shield just the one lead you are picking up from? Braiding from braided fuel lines might do it? Just slide it off the fuel hose. Cheap from Partco. I assume it is one of those inductance types that loops round a single ign lead?

Rod1 · Post by **Rod1** » Mon Feb 11, 2008 11:05 am

I have an MGL E2 and also had “noise”. The solution was to turn down the sensitivity and fit a resistor. All works fine now. Parts for Aircraft provide very good support.

Rod1

Mike Mold · Post by **Mike Mold** » Mon Feb 11, 2008 1:27 pm

Rob, if your Westach is reading double I recall the "fix" is to snip the loop of wire exposed on the back of the instrument. Worth checking with Barry Smith.

Pete · Post by **Pete** » Mon Feb 11, 2008 7:37 pm

Folks thanks for the advise

It wasn't reading double, more like x10. tick over revs reading > 10k

My westach tacho seemed to be non-linear and appeared to read high over the whole range.

The DC supply to the tacho is as short as I can make it, I had taken the feed fot the tacho from the mag switches, I guess these are unlikely to be very clean, perhaps I really need to take the supply direct from the mags, then the problem is I will only get revs for one mag.

I have today ordered a Honeywell gear tooth hall effect sensor, will see if that is any better,

Pete · Post by **Pete** » Sun Feb 24, 2008 7:02 pm

Hi, I have now fitted a gear tooth sensor, it sense 2 small pieces of steel attached to the prop hub. The sensor is 15mm diam and 30mm ling with a mounting lug. I am lucky, I found a handy luck on the engine case that allowed me to mount it in an almost perfect location.

I connected the sensor to the rev counter using screened cable, and yippee I get a steady reading which confirmed my suspicions of the Westach counter.

The only thing that I did find was that the screened cable I used was really too thin, and it's capacitance was such that the rev counter stopped sensing at 2350 revs, I fixed that by using a 12v supply to the tooth sensor, rather than using the 5v out from the rev counter.