Intermittent electrical issue

[flyby201]

I am having an intermittent electrical issue in my 77 M20 J.  This problem first appeared about a year ago.  After take off I will get a low battery alert on my EDM-700.  Last October I was on my way home in the middle of nowhere so I decided to turn off my master and fly home, saving my battery juice for talking to the tower and lowering the gear at arrival.  Then, 3/4 of the way home I was skirting a class C area and turned on the master to talk to ATC and everything was working fine, battery showing 14+ volts.  My A&P and I investigated the issue, checked the alternator and associated wiring and all check as ok.  Subsequent flights went off without any problems so we thought we might have inadvertently and unknowingly fixed the problem with our alternator checks, I.e.; tightened a loose connection.  Then, yesterday, the problem raised its ugly head.  Same scenario, low voltage alert on take off, then 50 miles from home turn on the master and all working as advertised.  I hate to start throwing money at the problem, but I don’t know what else to do.  We have a service manual but it is lacking a wiring diagram.  I was thinking faulty master switch but everything else connected to the master works, it just doesn’t charge the battery.  Could it be a bad master relay at the battery?  I’m looking for any help/ knowledge about how to approach this problem.  Thanks in advance.

Steve Beckerdite 

N201HL

[ArtVandelay]

If a bad battery relay, then pulling the field CB, the voltage should drop to zero. If it drops or stays below 12.8 then It’s likely a voltage regulator problem. Of course it could also be a bad connection somewhere like in the field wire, or the EDM 700. Can you confirm the low voltage?


Tom

[Andy95W]

The master switch does two things, it energizes the master relay to connect the battery to the system, and it also energizes the voltage regulator and alternator.

If your master relay was bad, I would expect different symptoms such as weak starter cranking or insufficient battery charging.

It sounds more like something is knocking your alternator offline.  Usually this ends up being the overvoltage relay.  Turning off your master switch for a bit allows the overvoltage relay to reset, which is why it then works when you turn your master back on.

Another possible cause is described by Don Maxwell:

http://donmaxwell.com/fluctuating-ammeters/

[anthonydesmet]

Just a thought because I have a 700 also....I.e. there old, old wiring, old probes, old display and electronics, etc. and have had mine go off once for overtemp.....actual temps on CHTs Andy EGTs never crossed threshold then found my default for warnings was set for CHT at 390. after never touching the setup page.  Just hadn’t flown in 5 weeks (which never happens). Anyway, you may have the sensor sending to your 700 some spurious inputs.  Do you have an actual voltage gauge on your panel you can cross check? Next time I’d leave everything on and watch for a trend?   I might be way off but sounds like we both need to upgrade.

[larryb]

The alternator field circuit does flow through the master switch. So that switch could certainly cause the problem and cycling it could fix the problem.

[PT20J]

Verify the EDM 700 first before tearing into stuff. Mine reads half a volt high. Maybe yours reads low - near the alarm limit, or is intermittent.

[carusoam]

The cool thing about the jpi700 system... it records voltage... as it does EGTs and CHTs...

Download the data to see what was going on in the voltage department...

Send it to Savvy... and share the link here...

We can then see what was going on over time... and possibly see where and when the alternator was no longer performing...

PP thoughts only, not a mechanic.

Best regards,

-a-

[flyby201]

Thanks to all for the input.  I’m hoping Andy95W’s link to Don Maxwell’s article hits the nail on the head.  He mentions in the article that this is a common problem on early J models. Mine is serial number 24-0014 so I think that qualifies as an early J model.  I do remember yesterday during my flight reaching under the master switch and wiggling the wires connected to the master switch.  Maybe I wiggled enough corrosion off of them to make a better connection and that’s why it worked later when I turned it back on.  I’ll get in there tomorrow and give all the connections a good cleaning.  As intermittent as this has been it may be a year or more before I know if it fixed it.

[Yetti]

Check all the grounds.  Should be shiny bright connections.  When I took my alternator off the B+ wire fell out of the lug.

 

Here are the wiring diagrams

http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 2 of 2.pdf

[larryb]

The master switch in my J failed a few years ago. The rivet holding down the terminal lug to the frame got loose. The switch needed replacement. The symptom was the wildly swinging ammeter needle. But if it were just a little worse it could cause the symptoms discussed here.