64 Cherokee 235 Ammeter Bounce?

kenjr

kenjr

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KenJr
Hey Folks,

We have a 64 Cherokee 235 that we just did some electrical work on and now we've got a crazy ammeter bounce.

We had a starter issue so we replaced the stock starter (a rebuilt from the 90's) with a Sky-Tec. While we were in there, we replaced the starter solenoid, the master solenoid (attached to the battery box) and a new battery.

Now, when the master is on and the engine is running on the ground the ammeter looks like a metronome bouncing from end to end at about 200 beats per minute. In the air and apparently with some manifold pressure it settles down where it's supposed to be (around 20 amps).

I haven't done a ton of testing yet - going to go out tonight with my CFI to get night current and he's an A&P so maybe he'll have some thoughts but while it was bouncing all over I was shutting everything down - strobe was off, nav lights off, radio stack off...nothing stopped the ammeter bounce.

I've read a few things - some folks say replace the master switch, others say get under there and clean up the connection points. Curious if anyone else has seen something similar and has any ideas?
 
nope definitely not the strobe. That was the first thing (and the nav lights) that I cycled. The speed of the sweep on the ammeter is way faster than the strobe lights so it's not even the same frequency.

We cleaned up all the grounds before putting everything back together - we're grounding the master solenoid to where it was before (in our plane it's in the back behind the cargo area) and we didn't have this problem prior to all the other changes.
 
nope, didn't change the VR. Idle RPM is ~800. But even during the runup it was banging all over the place. Only evened out once we were up in the air.

I'm going to pay more attention to it tonight if I go up and will have my CFI/A&P in there with me to monitor what's going on a little better at the various phases.
 
kenjr said:
nope, didn't change the VR. Idle RPM is ~800. But even during the runup it was banging all over the place. Only evened out once we were up in the air.

I'm going to pay more attention to it tonight if I go up and will have my CFI/A&P in there with me to monitor what's going on a little better at the various phases.
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Sounds like a voltage regulator issue, or a problem with the alternator overloading and kicking out. It stabilizes as the battery gets charged back to normal after the start sequence drain. I'd start by bench testing the alternator, it may have a bad winding.
 
The alternator field switch is bad about developing contact resistance and acting something like a turn signal flasher. That varies the field current which cycles the alternator output. It doesn't look like anything you did should have initiated this symptom. You can jumper the switch and see if that affects the ammeter indication.
 
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Were the alternator brushes changed?
 
nope - we haven't done anything with or around the alternator in I don't know how long. The alternator itself looks brand new - but it's not of course...probably been on there 10 years or so I'd guess.
 
fwiw - I saw a bouncing ammeter on my cherokee because either the master was bad or the wires were not screwed tightly on the master. I replaced the master and the problem went away.

If you hadn't touched the master when you did that electrical work, just consider it bad coindince if the master suddenly is bad.

Anyway, hope you can find the problem without going crazy.
 
We had the same issue on my 66 Cherokee 180. After reading quite a bit on the problem re decided to try the simplest solution-clean everything. Apparently as resistance builds up in the system it can cause abnormalities. Solved the issue for $0 (I did buy lunch for my A&P who looked over my shoulder).

Of course, dirty contacts isn't the only potential issue here, but it is worth trying before something complicated/expensive.
 
kenjr said:
nope - we haven't done anything with or around the alternator in I don't know how long. The alternator itself looks brand new - but it's not of course...probably been on there 10 years or so I'd guess.
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Shunted or unshunted ammeter ?:dunno:


My gut feeling is it might be the new master contactor though..
 
we replaced the master solenoid at the battery - not the master switch. The connections at the battery are as good as they are going to get - so, it's gotta be somewhere else.

Admittedly, not sure what the difference is between a shunted and unshunted alternator - have to get back to you on that one. :)
 
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kenjr said:
we replaced the master solenoid at the battery - not the master switch. The connections at the battery are as good as they are going to get - so, it's gotta be somewhere else.
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The solenoid itself may be faulty. Can you hear it 'rattling'? If not, personally first place I would go is a bench test on the alternator and I'd do a load test on the battery at the same time (not likely to show anything if it starts well, but sometimes batteries have funny issues).
 
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Thanks Henning. We could do that but not the first place we'd look as both of those are BRAND NEW - the battery and the $300 master solenoid hanging off the battery box.

I'm guessing its a wire integrity connection or we've exposed an alternator issue (VR or brushes, etc...)
 
kenjr said:
Thanks Henning. We could do that but not the first place we'd look as both of those are BRAND NEW - the battery and the $300 master solenoid hanging off the battery box.

I'm guessing its a wire integrity connection or we've exposed an alternator issue (VR or brushes, etc...)
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As I said at first, most likely place to look is in the alternator. Take it and the voltage regulator to your local auto electric shop and have him throw it on a bench and test it. However, never in this day believe because you have a new part that it's a good part.
 
kenjr said:
Thanks Henning. We could do that but not the first place we'd look as both of those are BRAND NEW - the battery and the $300 master solenoid hanging off the battery box.

I'm guessing its a wire integrity connection or we've exposed an alternator issue (VR or brushes, etc...)
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300 dollar master contactor .. WOW...:hairraise::yikes:

Here is a great quality one for 26.00..... Prob only good for experimentals though......

http://www.bandc.biz/continuousdutycontactor.aspx

Ps... Bad diodes can cause really funky side effects..:yes:
 
N801BH said:
Ps... Bad diodes can cause really funky side effects..:yes:
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Now there's a possibility. If they replaced the diode that's found across the coil of some contactors, they might have gotten a Zener diode by mistake. That one has a breakdown voltage that could cause that wildly swinging loadmeter.

Dan
 
Last time I saw this it was a 28-140 and I traced it to a bad diode in the rectifier. The bus was basically seeing a small AC component.
 
N801BH said:
300 dollar master contactor .. WOW...:hairraise::yikes:

Here is a great quality one for 26.00..... Prob only good for experimentals though......

http://www.bandc.biz/continuousdutycontactor.aspx

Ps... Bad diodes can cause really funky side effects..:yes:
Click to expand...

nope that's not the one. We replaced that one as well. Our cherokee is a push button start so that's the one that gets power to the starter. It's $100 for a 'certified' unit. Ridiculous...probably not a damn thing different between that unit and the one we bought.

There's another unit - much more robust that the master switch connects to that powers the buss that's attached to the battery box. That's the expensive one.
 
Skylane81E said:
Last time I saw this it was a 28-140 and I traced it to a bad diode in the rectifier. The bus was basically seeing a small AC component.
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If by rectifier, you mean the diodes in the alternator, the frequency of that AC component should be much higher than a mechanical needle could follow.
 
Graueradler said:
If by rectifier, you mean the diodes in the alternator, the frequency of that AC component should be much higher than a mechanical needle could follow.
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at low RPM it sure did follow, and that was the give away, as you throttled up the swing on the meter went away.
 
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