16.75 Volt Overcharging - Battery Boiled

[Axle]

I bought my PD-4106-1395 last August.  Took it thirty miles over Labor Day weekend '13 with no problems.

Replaced two 7 year old 8D starting batteries this spring.  Had trouble with them holding a charge and assumed there was a parasitic draw.  Drove the bus on the road (about 14 miles) for the first time since last August this past week and the single 8D that was connected boiled over (gas fog coming out the compartment door and a foul odor throughout the bus).

Turns out the battery struggles were not due to a lack of charging or a phantom draw, but likely because I was pumping 16.75 volts into them every time I started the engine.

So far, I replaced the 8D that I destroyed (only one hooked up when I drove it), cleaned up the battery acid all over the compartment and hooked a brand new battery up to start the engine for trouble shooting.  The new out of the box battery tested at 12.5 volts.  I replaced all four battery cables (they were in terrible shape) with new 2/0 cables, professionally built at the local Detroit shop and pulled the ground bracket in the battery compartment and cleaned it and the bay wall up so everything was shiny and tight.  I pulled the fresh water tank to get to the voltage regulator and started the bus again.  The meter immediately showed 16.75volts, so I tried adjusting the recessed screw, but no voltage change measured at the battery.  I checked connections at the voltage regulator and all seemed tight.

I pulled the voltage regulator and replaced with one from the spare parts box that came with the bus.  I called the PO and he was not sure if the spare was good or not, he had pulled it from another bus when he bought a replacement engine.  With the unknown spare voltage regulator attached, I restarted and got the same 16.75 volts at the battery with no changes to voltage when the recessed screw was turned.

The original voltage regulator is a Delco Remy Model 9000551 and Serial 888 with NEG / FLD / POS terminals.  The nameplate is pretty worn and the model and serial are my best attempt to read the numbers, but if they seem incorrectly stated, I may have mis-read them.

With a new battery, new cables, a good battery ground and tight connections on the VR terminals, two different voltage regulators (though not sure if either are actually good or bad), is there an obvious reason that I would be getting 16.75 volts at the battery and the voltage regulator adjustment screw would have no affect on the voltage?

I have the manual and have read through the sections for the generator and voltage regulator, but it is over my head and I feel like I'm just flailing at the problem instead of methodically troubleshooting it.  I've already burned a couple days of vacation working through this, so any guidance to help me get this resolved would be appreciated.

Axle

[luvrbus]

The voltage regulator is not grounded GM's are famous for losing the ground on the regulator   

[bevans6]

The way the regulator works is it senses the battery voltage, compares that to the alternator output and turns the field of the alternator on and off very rapidly so that the alternator puts out an average voltage which is at the level desired.  When the field is on the alternator charges, when it's off the alternator is off, and the end result when connected to a good battery is the battery sees a stable output (it acts as a stabilizing filter, kind of) of around 14 volts.  So in order to work properly the voltage regulator first needs a very good ground connection, a very good way to sense the battery voltage and then a connection to the field terminal of the battery.  Mostly the issues are around old crusty corroded wiring, so look for a bad ground and maybe install a new wire from the battery positive terminal directly to the sense terminal on the voltage regulator.  Since the alternator is putting out a voltage at all, you know the regulator is putting out a field voltage signal and it's getting to the alternator field terminal.

Brian

[luvrbus]

Could be someone installed a 12 volt charging relay in place of the 6 volt the 50D alternator uses   

[Axle]

Thank you for the replies.  Looking at the manual and voltage regulator on the counter, I noticed that the manual refers to a terminal labeled switch but the regulator has a terminal labeled neg.  I also realized that the generator tell-tale lamp the manual refers to is not lighting at all.  From the manual it seems like it should come on at start, then go out since the generator is putting out plenty of voltage.

Can I just jumper the neg terminal to ground near the voltage regulator to check if the voltage at the battery drops?  

Does the tell-tale not lighting give any indication where the neg/switch circuit is open?

Is the 12 volt vs 6 volt charging relay mentioned the relay next to the regulator on the bay ceiling?

Thanks,
Axle

[gus]

I'm guessing there is a problem in the wiring that makes the VR think the batt is low and then makes the field voltage too high.

Yes, run a wire from the neg post to a good ground. My 4107 had this when I bought it although I now have it disconnected with no change.

luvbus,

What is a "charging relay"?

[Axle]

Thank you all for the quick replies, especially on a holiday.

I think I have it solved.

Based on the feedback in this thread and the manual info, I started working back along the ground leg.  What I found was quite a surprise.

Turns out that the PO had added some docking lights that were controlled with a solenoid.  The negative wire of the VR was actually connected to the hot side of the solenoid (I can't possibly guess why, other than it was a case of getting the wires mixed up). 

Last fall when I took the bus out, I never threw the switch that actuated the solenoid, so the VR never was connected to the hot side, though I still don't understand how it was getting to ground.  This spring, as part of my efforts to rip out all of the questionable wiring and sketchy add-ons, I pulled the dock lights since most were rusted out and not working anyway, but left the solenoid in place in case I had a future use for it.  There were two wires on the hot side of the solenoid that I didn't know where they went, so I left them until I could determine their use.  One of those wires had the negative side of the VR spliced into it.  During some bus cleaning, the switch for the solenoid was thrown by one of my family members and I never noticed it had been turned on, since it was not something I had ever really used.

It looks like with the solenoid engaged the voltage regulator was getting voltage to both the pos and neg terminals.  I guess this also explains why the batteries were losing their charge over a few days.  First I boiled them when I ran the engine, then I had a constant drain on them when I shut off the engine.

When I bought the bus last summer, it was the fifty year old wiring that scared me.  After a few months of ripping things out, it seems like I should be more scared of the 'new' wiring.

Either way, it was your help that focused me on the ground issue and lead to uncovering the wiring mess.

If anyone knows and cares to share, I'd be curious to learn how the VR was getting to ground when the solenoid was not engaged.  Other than that, thanks again.

Axle

[bevans6]

Charging relay may also be called the field relay, and it is just the relay that switches power on to the regulator positive terminal.  Controlled by the master switch on MCI's.  There is also a cut-in relay that is powered by a half nominal voltage coming from the center of the diode bridge that is switched to ground by the low air switch that delays power to the AC and heating blowers until air pressure is built up.  Looking at the MC-9 schematic I can't for the life of me figure out how the alternator turn on is delayed until air pressure is built up, maybe it isn't on MC-9's but it is on my MC-5C

Brian

[gus]

Axle,

The VR on my bus is grounded to the bulkhead by the mounting bolts. Sometime before I bought the 4107 someone must have thought it had a poor ground and added the wire. Since it is now disconnected it has made no difference. My current problem is getting full batt voltage to the VR pos terminal.

Sometimes mounting bolts corrode but turning them out and re-tightening usually solves that problem. I have a Dodge minivan that sometimes has this problem with the WW motor mounting bolt

Brian,

Thanks, never heard it called that but makes sense. My 4107 has appx the same engine run circuit you described except a relay from the alt output delays power to the blower until the alt is working instead of air press. Using air press for that makes no sense?? Also mine prevents blower operation, not alt operation, to keep from discharging the batts. I don't understand why yours would ever need to prevent alt operation?

That circuit is now the source of my problem since I'm not getting enough voltage to my VR pos terminal and having to jump it directly from the batt.

[bevans6]

The reason for the delay for air pressure is that, on an MC-5C, the alternator and the air conditioner are both belt driven, and the belts are tensioned by air pressure into cylinders.  So the delay lets the cylinders get pressure and the belts are tight before the loads are applied.

Brian

[gus]

OK, we GM guys have a hard time remembering that weird setup:)