Tachometer / RPM indicator

[mikemiller]

I have an old 2000 lt which has been converted to 2500 now used in a motor glider. The Tacho is coupled to the alternator and is proving unreliable. I have been to the local Midas and Auto Spares shops and found nothing. This is bound to be a common instrument in the more modern VW engines. Can anyone throw some light on how they are coupled. Pointers would be much appreciated.

[Dawie]

The engine in your Grob (powered glider), probably uses a magneto instead of points. That may be why they use alternator speed to trigger a special revcounter. Some diesel engines also trigger their revcounter by the alternator. Obviously belt slip will affect accuracy.

Please post some pictures of your Grob and it's engine. Does it have twin plugs per cylinder?

[mikemiller]

You are correct. It uses a magneto and is the version with a single plug per cylinder. The previous owner did tell me that when the original Tacho packed up they replaced it with one from a truck - this ties up with your answer re diesel. The motor is in pieces at present, so can take pics of inside as well, if there is reason. There is some form of sensor on the block with its wires cut, perhaps this picked up from a magnet that might be on the bendix? But I have not noticeds it. I read somewhere that a tacho is on the market that uses one of the plug leads to obtain a trigger, one removes a bit of the shielding and coil a sensor wire aroubng the thing. Maybe I should explore this if one generally uses the distributor. Thanks for replying.

[buggyfan]

@DAWIE

WIll a SP type tach work here??

[Dawie]

The revcounter from a SP beetle would not work, as it is connected in series with the power feed to the coil. It senses the change in coil current. A magneto generates it's own energy, and does not need external power and does not use an external coil.

Some 70's VW engines had a sensor in the case next to the flywheel, near the top mounting nut/bolt. The flywheel had some small holes, and the sensor picked up the change in magnetic field when the holes passed as the flywheel turned. This tiny signal went to the diagnostic plug. At the dealership, the car was plugged into a special computer. Signal from crank position sensor was amplified, and a printout including some other data was made.

Think there was 6 holes in flywheel, (6 events per revolution), and you need 2 events per revolution to drive a "4 cylinder 4 stroke" tachometer. You probably have a lightened or special flywheel, where the starter sits at a 90 deg angle like some industrial engines. Unless your alternator is a special type that doubles up as the starter...

The spark energy can be sensed near each ignition lead, amplified, and used to drive a rev counter. Another way is to have a sensor near the flywheel teeth. Divide the 130 teeth by 65 using electronics,and you have 2 events per revolution. This can be used to drive a standard automotive revcounter that senses a change in voltage. (Not the SP beetle type or early Smiths type that senses current change which connects in series with power feed to coil).

The aviation instrumentation companies may have converters available over the counter.
Engine management experts like Gotech may have a solution as well. Or a good electronics expert can make up something.

By the way (off topic), one even nowadays get adaptors that clamps over the metal injector pipe of any old diesel engine. This senses injection timing, and drives a standard timing strobe light. To check injector pump timing and advance. This works by detecting the tiny expansion that occurs in the metal pipe when injecting at a few thousand Bar pressure...

[eben]

You can buy an laser/optical handheld rpm sensor from china for EUR5 from china these days

[mikemiller]

Many thanks, Dawie and Eben.
I in fact do own a hand held device, which is ideal for confirming readings, but not for panel mounting which I need. Dawie, I am in awe of your knowledge. All things considered it seems to me that I should explore the Tacho that picks up from an HT lead. I think I saw it on the Aircraft Spruce site. Time to start trawling the web. Your inputs are much appreciated.

[retrovan]

What I have done in the past, is to wind a coil around the spark plug lead, if you have a 2 pulse rev counter, use two coils on two plug leads. 1 & 3 or 4 & 2.

You can increase the number of coils to fit the rev. counter.

This will give you positive readings, and have no direct coupling to the motor. so safe for aviation.

OMO

Herman