912UL electrical problem

Here's one that's causing me grief. Has anyone else encountered this before?:
Shortly after starting the engine the voltmeter indicates at 16v, (higher with higher RPM - meter pegged at limit, showing at least 18v @ 3000 RPM). The ampmeter similtaneously draws -8 to -10 amps (altho the amp meter may be wired in reverse, because is always reads negative 1-2 amps when all else is normal).The overvoltage reading lasts 3-4 minutes then returns to normal readings (14 v, -1A) & doesn't re-occur (or rarely re-occurs) for the rest of the flight.
There is no smoke or electrical burning smell. This happens even if the engine was recently started (still warm, no moisture, battery fully charged). Sometimes it happens after T/O. Turning off the Master switch sometimes helps bring it back to normal faster, but not
always. Placing a greater voltage demand (ie., selecting the gear retraction switch to up momentarily (amphibious floats), turning on the lights, etc..) brings down the voltage somewhat, but not back
down to normal (14v). When the voltmeter is pegged at 16v+, sometimes the Master switch does not turn off the current (it should, but it's as if there's a temporary short).
I realize that the battery gets drained a bit to crank the engine, but the engine always starts quickly (1st try), so the draw is small. The battery seems to charge well when the engine runs (-1A), and seems to keep its charge.

Some have suggested that a bad ground to the rectifier-regulator could be at fault. I cleaned & tightened that ground and any others I could find, to no avail. I checked for loose wire connections and burnt (brownish/discoloration) wire insulation but cant't find any. I read the Rotax installation manual (Sep 1989) which addresses the possible reasons
for the failure of the rectifier-regulator (pg 22 in my manual) but being a "Buy & Fly" kinda guy, the "non-obvious" stuff is Chinese to me. Others have suggested that the rectifier-regulator could be at fault, which I don't mind replacing if it'll fix the problem, but at $170, I'd like to check everything I can first. Any ideas?
Thanks for any help.

No particular experience with Rotax but...

Looks like it is either the regulator or a wire that samples the generator output is broken/missing. The DC output of a generator should NEVER be so high as it will destroy the battery.

Ariel

Carl,

Properly regulated voltage should run no higher than 14.1 - 14.5 volts at any time including after startup. That should be your standard.

Make sure your charging/regulating wiring is secure and correct - no loose screws or connections. Make sure that master switch is not at fault. I know you have checked some already but make sure nothing was overlooked.

If everything is secure and no cracked wires (sometimes a break can occur under the insulation) - I'd be concerned about the health of the regulator.

Running an over voltage can be hard on radios, etc. - and it doesn't have to take long to do the damage.

As always - be sure your gauge is reading correctly - plugging a second one into the system to check might not be a bad idea.

If you are a buy and fly guy - might be worth having a Rotax repairman or a qualified A & P take a look at it - I hate replacing parts just to find out what it isn't either - can get kinda pricey.

Sincerely,

Dave S
KF7 Trigear

if the voltage is really going that high there must not be any overvoltage protection or crowbar installed . if anything is wrong with the stator the voltage wont be high . the regulator is pretty much the only thing that can fail and cause the overvoltage . if the alternator switch wont turn off the alternator its either a switch or a relay thats no good . first things first , you have to figure out how your ship is wired since your not the builder . good luck

Thank you all for the suggestions re: my elec problem. I'm based out in the boonies so I
unfortunately can't turn to an AME for help & don't have spare gauges for testing. This
engine has 360 hrs on it & I've had it for the last 100 - electrically trouble free until
now.

Thnx for the diagram Chuck. The one in my manual is different - I'll try to figure out the
diffs. I checked the ground common to both the Amp meter & voltage gauges in case it was
causing a similtaneous false reading on both dials, ie, just an indication fault - not the
case. I had done some work on the engine last winter & had disconnected the yellow leads to
the rectifier- regulator (RR) - (the "G" connections on the RR) & may have crossed wired
them when re-connecting (not critical, I understand). In any case, I've confirmed the
continuity of that circuit. I didn't disconnect the other RR leads.

More info: the voltage is normal at 2000 RPM (14V), but increases linearly as I increase the
RPM. At 3000 RPM it's at 16V. Above 3000 RPM it increases to the gauge limit (above 16V -
18V extrapolated). As mentioned before, after a few minutes it returns to normal readings (14V).

The over-voltage may re-occur during flight, but rarely.
I gather that the capacitor (22000uF, 25V, according to my diagram) is the RR's protection.
There isn't one, so I've got one on order. All other wires look fine & the ones I've tested
for continuity are good too.

I've flown with this problem for approx 20 hrs, but am loath to fly the machine now that I
understand the potential consequences (more work to replace a burnt out dynamo than just
changing the RR). Also, it seems to be getting worse. Looks like I may have to bite the bullet...

Before you buy new parts you might want to take a garden variety voltmeter and corroborate your readings (hate to buy new parts and find out it was the gauge).

Hi, I have attached a pdf of an article that might be of use. I did allot of looking for info when I was having a charging issue.

Problem fixed.
I replaced the rectifier-regulator, which fixed the problem of the occasional over-voltages on my 912UL.
BTW, I learned from a Rotec tech that turning off the Master swit (engine running) is NOT a good idea. I did this a few times to try to "cool" my voltage problem, but he said that this creates an overload on the rectif-regul, which may damage it.
Many thnx to all who took the time to help me out - I learned a lot.