Fixing the 50DN alternator voltage regulator on our new bus

On a 50DN alternator, poor voltage regulation is a common problem. We purchased this bus knowing that it was doing this, and the prior owner had already spent a lot of time and money working on the issue - including replacing the alternator and going through a few voltage regulators. Yes there is a modern voltage regulator available for the big 50DN alternator we use it in this video. I went through a series of hypothesis about what was causing the issue and tested them, including removing the house batteries, removing the group 31 batteries and trying 8D’s, looking for a bad ground between the engine and the bus body, trying to understand modifications done to the stock wiring over it’s lifetime, and completely bypassing the bus wiring and using new wires between the regulator and the alternator.
The new, temporary wires I was using for testing didn’t do me any favors. Short wires of appropriate gauge, directly and firmly connected are needed. Extra distance or crusty wires will interfere with the signal and add latency - the scope shows this - and interfere with proper regulation.
This is the scope I bought super happy with it, there are many similar. amzn.to/3PSbTZz (affiliate link, but purchased and used for a couple years)

[@jeffbyrd7976]

I would like to see the a/c system. Thanks for the video, informative as always on your squawk repairs.

[@ThisOldBus]

Thanks, noted. I did shoot a tour with the original owner 2-3 years ago on this channel, which is where I first learned of this bus and he did talk about it some. But I will cover it more.

[@strayhound61]

Hey Scott, you have more knowledge than I do on diagnosing this for sure. My bus has had that fluctuation in charging for 35 years. It also has the added relay (which I think was added to extend the life of the regulator) that is either on or off and cycles about 100 times a minute. When the bus has been sitting for a while and the batteries are down to 12.5 volts or so, the output voltage on the 50DN is in the 14.7 range. However, once they have had an opportunity to be refreshed for a few minutes, the output voltage drops to something under 14 and will stay at 13.7 after an extended drive. All the buses I used to drive did the same thing from what I recall.

[@ThisOldBus]

None of the other buses I’ve had did this and in this case I could hear the engine cycling under load while it was doing it, I’ll bet yours does the same thing, and it’s hard on the gears. The relay is stock, in this case a modification, but not needed with the new regulator. I would seek to resolve the issue, most likely extra resistance between regulator and alternator.

[@detroitdragon7072]

Where did you get the new regulator? Google search brings up sketchy suppliers. I gutted every inch of wire, terminals, relays out of my bus and replaced only what I needed. Tons of OE stuff that is N/A. I did the magnetic switch update for the regulator, replaced all chassis grounds with ground wires and terminal blocks-- probably the single best upgrade for any old bus. I'm running the old regulator with a single 8D, no issues at all. But.. maybe I will get a new style regulator to put in the spare parts bin..... I also moved my regulator to the rear wire compartment so it is easy to access. Plenty of room back there once you clear out all the old crapola.

[@ThisOldBus]

It’s a D698, I didn’t have to source it because it came with the bus. I was seeing them at transpo electronics (wai) before but can’t find them now. I’d buy them anywhere that seems reputable and in stock.

[@TimTurner115]

Chasing wires that someone else did the work is a pain in the back side. On a bus is not fun. Im trying to be nice about it. 😅.

[@ThisOldBus]

Yup especially when they deviated from known documented upgrades and the manual.