Low alternator output <1200 RPM on a 252

[N177MC]

I have a 252 with single alternator that does not put out much until after 1200 - 1400 RPM.  The alternator is just a 100H since new.

Any ideas on what to look for ?

Thanks!

[neilpilot]

Loose belt (assuming you drive it via belt) or, if clutch, slipping clutch.

[N177MC]

This is a Conti TSIO-360 with gear drive alternator.

[milotron]

Check for poor connections on the field or battery wiring at the alternator. It is an awkward spot to access and something may not be quite right in there.

I have dual alternators on my K and the gear drive alternator doesn't produce much at lower RPMs compared to the belt drive unit.

 

Maybe what are seeing is normal? Are you seeing low voltage or current?

[carusoam]

Has it been doing this for a while?

1) There is a history of alternator clutches going bad.  They usually leave a few signs of rubber bits in the engine oil when it happens...

2) Voltage Regulators are the magic device for controlling output from the alternator.  Which one do you have, how many hours are on it?

3) The control wire (field voltage) from the VR to the alternator is next in line to see if there is a problem.  Any break or fraying could cause a problem.  Usually an on/off type of problem...

4) Does your VR have any output adjustment?  

Alternators have variable output with rpm.  But, they should produce enough voltage to keep the red light from coming on somewhere above idle.  1200 rpm is a bit high...

PP thoughts, not a mechanic...

Best regards,

-a-

Edited February 19, 2017 by carusoam

[N177MC]

OK, that's what I was looking for - Troubleshooting Questions  .

I will investigate and report !

[jlunseth]

Well, I have a 231 and it was my understanding that the drive system was improved in the 252's, but I can tell you that the output at low (under 1200) rpms is a known issue in the TSIO360 KB/LB engines.  Some years ago it was thought the problem could be cured with a higher output alternator, so everyone started installing a 100 amp, but it did nothing.  The issue is the coupler drive ratio is just too low at low RPMs.  

I flew once with a 231 instructor at a Mooney PPP.  His first question when we started down the taxiway was, "Do you use the brakes or just put up with the flashing light."  I knew immediately what he meant, to keep the buss voltage out of the red, the choicse are to taxi at about 1200-1300 and use the brakes periodically to slow down, or just ignor the Low Volts light.

Mine works ok at 1100, good at 1200, no good under 1100, the low volt warnings will all flash.  

Now, that's the 231, and I had thought that was corrected in the 252, but I thought the 252's all had two alternators also.  So maybe you have the same alternator set up as the 231, and low volts at low idle is just a given.

The coupler is a problem, but it usually a "go-no go" type problem.  There is a rubber clutch, and either it grabs and the alternator spins, or it doesn't.  Sometimes, if the clutch is getting old, it will not grab well until the engine has warmed for awhile, but that is the only time I have seen clutch failure cause low volts rather than no volts.  If it fails, you usually have no volts.

[LANCECASPER]

1 hour ago, jlunseth said:

 . . . I thought the 252's all had two alternators also . . .

Unfortunately some early 252's did not have two alternators since it was an option. The single alternator 252's have a reputation for eating up batteries pretty often.

Scroll down to the highlighted portion of this pdf for K model alternator comments:

M20K PRE-BUY highlighted.pdf

[Guest]

With the cowls off, carefully put your finger into the alternator housing and hold the cooling fan with your finger tip.  With your other hand carefully turn the propeller, if you can hold the fan and turn the engine the drive coupling is slipping and requires replacement.

Clarence

[jlunseth]

Thanks Lance.  That article is exactly what I was talking about.  It appears to be the same issue at least in the early 252's, as in the 231's. The point is that a Low Volts annunciation is normal in this engine, at what would be considered good low idle speeds in other engines.  My "coming in" speed is a little above 1100 RPMs, but I generally idle and taxi at 1200 just for good measure.  1300 is not bad either, but will result in a fairly brisk taxi.  If you don't mind the Low Volts light, you can idle lower.  I might do it on takeoff, because the alternator will shortly be operating at full speed and normally charging the battery, but I won't do it after landing, when I am about to leave the plane (and the battery) sit.  Our OP's clutch may be going bad, yes, but it sounds to me more like normal for the particular engine, different from other engines.

PS to the OP, if you find you need a new clutch, get an MSC to replace it, don't just leave it to any mechanic.  I did once, a bushing was left out that seems counterintuitive unless you really understand the Mooney coupler design.  The lack of a bushing allowed the coupler to beat the tiny cotter pin that holds it on the spindle, and so the coupler and some parts fell into the operating engine (into the sump).  Fortunately, all pieces were found so nothing got picked up and run through the block.  But the moral is, make sure someone who understands that device, do this seemingly simple task.