electric system diagrams

I had a little episode on a local flight Sunday where my Alt trouble light flickered several times, then stayed on full-brightness followed by an OV warning on my EIS, voltage climbed rapidly to 36V. I popped the Alternator side of the ol' Cessna split master immediately, and continued the flight on battery with a bus voltage of 12.1-12.2 All avionics thankfully worked normally. No smoke released. Especially glad the dual P-mags continued to function normally.

I tried turning on the landing lights to load the charging system and bringing the alternator back online, got 18 volts and climbing, immediately turned the alternator off again. Tried it one more time at lower engine rpm and got a normal 13-14V range that seemed stable, no alternator light. Decided that aloft was no place to troubleshoot a runaway alternator that hadn't (yet) smoked anything expensive, and landed.

Clearly an intermittent problem that went away again before I landed, and I obviously have one of the Denso IR alternators that is not (further) harmed by turning it on and off under load, something I had never done before. I strongly suspect a wiring connector or split master problem brought on by airframe age (20 years) and poor building practices in my younger years. However, it's possible the internal regulator is toast. I'm willing to troubleshoot and to try the remove-and-replace thing. A house fire has left me with no documentation of my electric system as-is. I think I built it per Vans electric plans and when I "upgraded" to the IR Denso unit I just removed the old Vans ER alternator, the Ford regulator, the AeroElectric home made OVP module and bolted the new IR alternator in. But without wire tracing (my next step) I'm not sure of the architecture I'm dealing with.

I have current copies of Z-11, which is likely the progenitor of my current rat's nest of wires, and also Interim Z-24 - which is definitely being added soon. Anyone have a reference to the schematic Van was recommending for basic electrics in the 1990's? I sure would like to take a peek at it, as it's probably what I put in to begin with. Maybe all I need to get flying again is a new master switch and some squeezed and potted connections to the alternator, but I know there will be lots of opinions and welcome advice forthcoming

I've only been in the Vans world about 3 years now. The first thing that came to my mind was to call Vans and see if they have copies from back then. Probably in a box somewhere - pre-digital world.

Before doing anything, I would check all of the electrical connections associated with the charging system. The vast majority of electrical problems are due to bad connections. High resistance in the voltage regulator sense circuit can make the regulator think the voltage is low, even when it is not. The regulator then tries to increase the voltage even though it is already too high.

Mich48041 said:

Before doing anything, I would check all of the electrical connections associated with the charging system. The vast majority of electrical problems are due to bad connections. High resistance in the voltage regulator sense circuit can make the regulator think the voltage is low, even when it is not. The regulator then tries to increase the voltage even though it is already too high.

Click to expand...

I thought about that. Question being, does the IR alternator sense voltage from the field control lead, which is external, or from the regulator's "field power" connection to the B+ lead, which is internal to the case. If the latter, the only cure would be replacement, it seems. If the former, there are obvious wiring harness connections and switch contacts in play.

Most IR alts have a 'sense' terminal that can remote-sense voltage, but will default to sensing the B-lead if there's no voltage on the 'sense' terminal.

Most won't come on line until there's voltage on the 'I' terminal. But many will continue to operate even if the voltage on the 'I' terminal is removed. (Which means you can't use the I terminal to reliably control the alt.)

Mine was quite controllable by switching the Control lead on and off via the split master - several cycles and it obeyed every time. Output voltage level was whatever it wanted to rise to at the moment, but the whole thing came offline when "turned off" at the master. I'm not sure if there might be a voltage sensing issue in the control terminal wiring that is causing this behavior, though. Seems entirely possible a high resistance connection would incite this.

check the split cessna master switch early in the search

The split Cessna switch is a very common weak point. Just replaced mine after 890 hrs. as I was having same Alt light issue.

Thanks, Craig. I plan to. Might go with the progressive transfer DPDT switch that Bob Nuckolls favors for the OFF/BATT/ALT master. Not sure its wiring is a direct replacement for the split master, but I'll have to ponder the diagrams to see. (Which is one reason I want to see a split master schematic from the old-school spam can era).

I am inclined to approach this problem as an opportunity to rip out all my aging and now undocumented DC power distribution wiring and replace with new. Heck, I'll send the P-mags off for upgrade while I'm grounded doing this other work. A hybrid of Bob's Z-11 and Z-24 will be replacing whatever's in there now.