Alternator shunt value
 

Aircraft - C337A, #337-0452, 1966. Symptom - Electrical system under day-VFR ops manages OK but under higher loads will eventually trip an alternator circuit breaker, the load then shifts to one alternator and if load is not shed it will shortly thereafter trip its breaker.
In trying to troubleshoot this problem it became apparent that the EI volt/ammeter was falsely indicating amperage. In trying to determine why, it appears that the instrument inself is OK, the present working hypothesis is that it may not be calibrated correctly for the original equipment shunt value.
The immediate problem is to correct the ammeter display so the first question is: does anyone know the alternator and battery shunt value for this acft. or will it have to be physically measured? This value is not stamped on the shunts, does not appear in the parts or service manual, is not known by CPA or EI tech staff. Cessna part no. is #813489.
Any further thoughts on other possible causes of the ammeter problem or experience with troubleshooting the overload problem would be appreciated.

Thanks, Steve G.

I had the same problem in a 337B. Once the breakers started popping, I could not get them to stay set until the aircraft had been shut down for a while. It was maddening when they popped two different flights in instrument conditions. The fix in my case was one I wouldn't have expected.

The main alternator breakers (35 amp) were original equipment. I replaced both of them and have never had the problem again. Breakers tend to break down over the years, I guess.

Hope the fix is as simple for you.

See you in OK city?

Steve:

I just installed an EI VA-1A volt ammeter in my 73. I put in the "external shunt" model. I have 38 amp alternators. The VA-1A required 3 shunts, 1 for the battery, and 1 for each alternator. Each of the shunts was an EI 50 amp shunt.

Hope this helps.

Mark

Ahh. Found this one out the hard way. Most shunts are designed for 50ma. The one used in the 66-67 Skymasters are 40ma. From what I understand this is an odd ball shunt. I think EI could calibrate this for you. Perferably you send in your shunt (pain to get out) and the amp meter at the same time. That way they get it right on.

I had the same problem too. Couldn't get my bus to reset, leaving me with a flight without power. Replaced my breakers and life has been good.

Richard

Jim, Mark & Richard: Thanks for taking the time to reply, I appreciate the information. I'm wondering, Mark, if you also replaced the rotary selector switch and ran all new wiring from the shunts to the switch to the ammeter and how long it took to replace those original equipment shunts. They look like a 20+ hour job to disassemble enough of the panel and wiring harness to gain access. And Richard, I'm curious to know the rest of the story concerning "the hard way". Hasta la vista, Steve G.

Steve:

My aircraft never had a volt meter or ammeter. I put everything in new. I used a rotary switch from EI, along with their shunts and meter.

I mounted the rotary switch just under the landing gear "down" light. I mounted the meter in an unused 2" hole on the right panel. I mounted the 3 shunts on a 4" by 6" plate located on the firewall between the starter contactors and the alternator circuit board. It took me about 8 hours all together, and about half of that time I spent trying to find crimpers and terminals for some of the large gage wire I was using at the shunts.

Mark

Hard way = try = then fix

It's alot of work taking the panel apart, putting in a new gauge, and then tearing it out, then in, then out, one more time........

I used the original shunts, = saves money. Then had the gauge calibrated to the shunts.

Replacing the original (never worked right to begin with) volt, amp meter was one of the best improvments to the Skymaster. I like simple panels, but I also want to know what is happening. Now in one gauge, no switches, I can see the battery current, front, rear currents and volts. Bliss.