Thought I’d start a new post to explain what I did. Here’s the history. Bought a float plane back in November did a pre buy on it and one of the things they found was a faulty alt light. Supposed to of fixed it. Ferry pilot commented that light was coming on. I didn’t fly it all winter and have about 30 hours on it and it didn’t act up for me until a couple weeks ago when it would stay on after starting but then go out. Last flight it stayed on the whole trip so I posted a question to this forum and got some good advice. Started by checking all wires at the alternator and fuse all good. Andy suggested the switch under the dash which is a relay so found it and disconnected the red wire going to it from the alternator. Using my test light I started the engine and had power coming on the wire. Hooked it back up and the light stayed on. So I took the test light and tapped the switch and the light went off. So I guess my problem is the switch “relay”. Sometimes you just have to hit with a hammer. Thanks everyone for the advice.

Glad you found the problem and it wasn't expensive to fix. I've never opened up one of those alternator off warning light modules but apparently there's actually a relay in there. Relay contacts eventually corrode or tarnish and fail to make a good electrical connection. Sometimes whacking them will jolt the contacts into a better position. If you can open the module up without damaging it, you can burnish the contacts with fine sandpaper unless they are pitted. I wouldn't think they are pitted since there's not much current flow needed to operate the light. However, it's not usually possible to burnish off all the corrosion and it will eventually happen again.

I had a for sale thread on here of a bunch of random new-old-stock parts I got. Alternator warning switch was one of the random parts and to my surprise, both modules I had for sale sold quick. Must be quite likely to fail.

In my M7, I've never seen the light come on. Including when the master is on but the engine isn't running. I suppose mine doesn't work either but I have a voltmeter on the panel so I'm not concerned.

-Asa

I have the following problem on my MX 7-180. In July during a flight, I have the alternator light coming on and a voltage that drops to less than a dozen volts (battery voltage). Alternator switch to OFF then to On. Good operation for a few minutes then the indicator light comes on again.
I changed the voltage regulator which is also overvoltage (new Lamar B-00371-1) and everything is back to normal
Unfortunately after about twenty hours of flight I have this problem again ....... Anyone have an idea ???

Did you remove the original overvoltage relay if it was still installed? I was having the same symptoms and thought it was my overvoltage relay failing. I decided to replace the voltage regulator with built in over voltage protection like you. I removed the overvoltage relay and haven't had any issues since.

Have you checked the connections for the inline plastic fuse holder near the alternator? It supplies current to the alternator field winding from the alternator control unit. On my MX-7-180 the fuse holder was not supported well and vibration eventually broke the wire going into the ring lug on the alternator field post connection. That stopped the alternator from producing any output and turned on the alternator off light. The solution is to redo the ring lug and tie wrap both sides of the fuse holder to a wire bundle so it won't vibrate.

gdflys wrote: ↑

Sat Oct 16, 2021 2:21 pm

Did you remove the original overvoltage relay if it was still installed? I was having the same symptoms and thought it was my overvoltage relay failing. I decided to replace the voltage regulator with built in over voltage protection like you. I removed the overvoltage relay and haven't had any issues since.

I have a LAMAR B-00371-1 box which is over voltage and voltage regulator that I replaced in July. I have done about twenty hours since then and I have this problem again ...
I find it hard to believe that it is this regulator which is still broken, the first one has done nearly 1800 hours of operation

andy wrote: ↑

Sat Oct 16, 2021 4:12 pm

Have you checked the connections for the inline plastic fuse holder near the alternator? It supplies current to the alternator field winding from the alternator control unit. On my MX-7-180 the fuse holder was not supported well and vibration eventually broke the wire going into the ring lug on the alternator field post connection. That stopped the alternator from producing any output and turned on the alternator off light. The solution is to redo the ring lug and tie wrap both sides of the fuse holder to a wire bundle so it won't vibrate.

Not Yet I will do that......

JUBERT wrote: ↑

Sun Oct 17, 2021 12:28 am

andy wrote: ↑

Sat Oct 16, 2021 4:12 pm

Have you checked the connections for the inline plastic fuse holder near the alternator? It supplies current to the alternator field winding from the alternator control unit. On my MX-7-180 the fuse holder was not supported well and vibration eventually broke the wire going into the ring lug on the alternator field post connection. That stopped the alternator from producing any output and turned on the alternator off light. The solution is to redo the ring lug and tie wrap both sides of the fuse holder to a wire bundle so it won't vibrate.

Not Yet I will do that......

Good advice, thank you
By checking this fuse which was a little hidden in the strand of wires .... I found the fuse in a little engine oil .... cleaning, then contact bomb ...... and it is perfect now (45 minutes of flight without problem ......) Thank you all