Erratic Voltage
 

Hi Everyone

Looking for some guidance, I am experiencing the following situation.......

1. Overvoltage tripped, front alternator light came on just a second before overvoltage.

2. Reset during flight following procedure, issue returned immediately.

The following checks and rectifications have been carried out.

1. Voltage front alternator, erratic at 29.2 to 30.5 through to 2100RPM, even at idle it remains erratic at 29.2 to 30.5.

2. Voltage rear alternator is stable at 27.7 through to 2100RPM and at idle.

3. Regulators checked, front and rear deemed functional.
(Decided to replace with zeftronics anyway).

4. Over voltage sensor checked and deemed functioning correctly
(Decided to replace with new from Cessna, 49 years old, time for new)

5. Front alternator checked and found to have a slipping drive as well as internal bearing issues. Only 260hrs since O/H, replaced with complete off-shelf O/H assembly and new drive.

6. Diodes checked and found to be functioning correctly.

Why I need some guidance.....

The front alternator continues to deliver erratic voltage to the buss bar at 29.2 to 30.5 (and occasionally higher), it is as if the voltage regulator is not being told to regulate the voltage, and we are getting raw voltage from the alternator to the buss bar. I have checked all wiring and did find a bad connection at the regulator connector, fixed but no change.
When I switch to the rear alternator it controls fine at 27.7v with almost no change. So we did a quick change of regulators by swapping the front for the rear as quick test.
Even after the swap the previously front regulator now on the rear, it was working fine at 27.7 and the rear now connected to the front was erratic at 29.2 to 30.5, keep in mind these are both new zeftronics

So the issue seems to be with the front system, any thoughts?

As a footnote, during the resto every switch in the panel was replaced with new from Cessna including every terminal connector, and we have re-checked the connections.

Try this link on this forum . Hopefully it works as I’m not good at this tech stuff. Electrical issues can be quite nemesis in my ownership experience. Good luck my friend.

http://337skymaster.com/messages/showthread.php?t=5238

Rereading your post, (AND I AM NO EXPERT, just a 21 year owner who watches my shop work and learns as much as I can, without turning wrenches (that is not fun for me)), I did observe your comment in what appears to me to be quite a voltage disparity between your front and rear alternators. Your front is likely out of spec high and your rear is likely out of spec low. Please confirm this with an expert but I am reasonably certain the target voltage is 28.4 volts with either or both alternators functioning properly.

I do not know what negative results might occur with the disparity you reported but that does sound abnormal to me. In my experience minor voltage fluctuations continuously occur across my system (1973 C337G model) as observed on my digital volt meter however turning one alternator or the other off and running on a single alternator does not change the displayed voltage appreciably. Additionally, the fluctuations that I do observe running on both or single alternator output is on the order of plus or minus a half a volt. In other words, my volt meter displays a range between 28 to 29 volts with the average being closer to the 28.4 volt target.

I think/hope this info is accurate. Please confirm with someone smarter than me. Good Luck!!

Check impedance across alternator switches and master
 

While the resistance across the contact may show fine with a DVM, under load they aren't.

While these only have an amp or so running through them, anything less than perfect and they act like resistors, which allows voltages to jump around all over the place!

If you pop the switches out from behind, you may find a volt or more across the CLOSED contacts. No kidding.

You can pop switches out, then just blast like crazy with cleaner (brake, contact, corrosion X), while cycling them a few bazillion times.

Re test until voltage drop os at or near zero.

From personal experience, I am making a $1 bet this will solve your issues.

Hi I'm reposting something I put in the forum a while ago, as it may be hard to find with some edits for this situation and model >>

I think it helps when trouble shooting to think of an alternator as a current controlled voltage source. The output voltage depends on the field excitation current, with a higher current value leading to a higher output voltage. The regulator looks at the output voltage of the alternator and then raises or lowers the field excitation current for the field winding in the alternator to maintain the correct output voltage from the main winding. Since the output voltage is not where it should be, but not zero, the field windings, field excitation circuits and regulators seem to be working. I have a 1969, On this plane one regulator feeds both alternators. On your aircraft, I looked up what I believe is the schematic for the 1974. It looks like this aircraft has two regulators and both of them have their own alternators with diode blocking to set which one is driving the electrical system. Since your problem appears to effect the front alternator, I would suspect a problem of higher than normal resistance in the leads going from the buss supply to the regulator for the front alternator, meaning check all the connections from the buss to the B input on the regulator. High resistance anywhere in this lead will starve the regulator for current, leading to erratic output voltage from the alternator. The regulator is exceptionally sensitive to this issue. It doesnt take much to cause a problem. I had a problem with voltage instability that was plaguing me for some time. When I put in a new avionics stack, I had the avionics shop look into it. They did a really smart thing and starting jumpering from the buss bar forward, bypassing components until they saw a stable output result, then replaced all the crimps between the buss and the good result point. I had already had my A&P replace all the switches in an attempt to fix it without a good result. Voltage is now rock solid. If you decide to try this yourself, DO NOT bypass the regulators. This could fry your airplane as the output of the alternators in an uncontrolled state can exceed 60V. Based on my experience if you have an avionics outfit that is good I would take the plane there. They are much better with electrical issues than your typical A&P.

As a follow up to this and in concert with the previous post, I recently noticed that the voltage for my system was erratic, rising as high as 28.9 volts but jumping around. It would settle down if I cycled the regulator switch back and forth between reg 1 and reg 2. It would then start rising and jumping again as I was flying. I replaced this regulator switch with a new one ($100!) and the problem is solved. Both regulators rock solid at 27.8 volts. So the basic message is any résistance in the regulator circuit will cause this sort of issue as the voltage drop along the sense lead to the regulator becomes meaningful. Cleaning the switch with a solvent may also fix this but there is a lubricant in there that will be washed out by this so you will probably see the problem come back sooner.