Electrical problem Rotax462

Hi,

Don't know if anyone can help.

A new aircraft to me has problems with water temp and tacho guages. The tacho sometimes works accurately but mostly won't go above 4000 revs even with foot to floor (Engine absolutely fine). Usually on take-off the water temp guage flickers wildly and settles at cruise speed.

Maybe after a landing, the tacho would read the true revs and on go around read past 4000 revs.

The previous owner said an auto-electrician wired a battery to a 12v supply cable coming out of the rectifier but the prev. owner disconnected the lot as it kept burning out the battery. He also said that he had also experienced problems with flickering water guage etc but if he switched on carb heat, it all corrected and both guages read true. He also flew all the time with carb heat switched on.

My findings today with a voltmeter are as follows with engine running:

With carb heat off: water & tacho guages read a little low and tacho wouldn't read above 4000 revs. Also the 12v supply lead read about 12.9volts.

With carb heat on: water and tacho guages both increase slightly and interestingly that 12v lead jumps from 12.x to way past 20 volts! This explains the burned out battery scenario!

So I think it could be a duff magneto, regulator/rectifier unit or both.

Could this be so?

Azhar

Hi

I'm not an electrician, but an ex mechanic. I would suspect the flickering guage problem is earth related, aka fixes itself up with carby heat on, presumably giving an earth.

From a vague memory of electric theory from a quarter century ago, an alternator needs a return earth as well trhough the regulator if external. Without such I recall the voltage can get high. Maybe the regulator is missing it's earth as well??

Just an idea. Micgrace

Electrical probs

Thanks MicGrace,

The water temp gauge flickering was solely due to a very poor magneto to engine earthing point.

I had a look at the electrical carb heat and the wiring is a *** mess under all the plastic heat shielding. I'm going to rip it out and restore it back to standard wiring without battery.

Just one question, with the 2 phase (4 wires) coming out from the magneto, are both coils joined together in parallel which then feed the rectifier/regulator?

And one very last question, how on earth does the water temp gauge get a reading from the AC side of the magneto? The tacho I can understand but I thought water temp gauges need probes like the EGT and CGT does.

Regards,

Azhar

Your magneto should be 3 phase with the centre of three coils wired together. Giving a common plus three wires. You should be able to measure equal ohms from the common to all three outputs and double the ohms from output to output.

Your carb heat elements should consume a lot of power, giving you a voltage drop, which would help with your tacho problem. Tacho's being a voltage comparator have problems when their supply voltages vary. One thing to try is supplying power (not signal) from a small battery and see if your problem still exists. If it does the tacho may be mechanicaly damaged like a bent needle or something but if it solves the problem then you "just" need to sort out your mag, rect, reg!

I don't know where the water temp sensor is on a 462, I'd have assumed it would be on the head.

hi

Not being an expert on things electrical, but having some practical problem solving experience, everthing seems to work best if power is taken from the battery direct to some sort of central distribution block and the alternator/generator is wired with it's own circuit to the battery. Naturally, all items should be fully earthed or earth return.

As for temp guage ie the sender, they earth through the switch with direct power to the guage(although this sounds like this isn't ). Although some have a voltage reduction device to supply instruments the case

Might help. Micgrace