W900A Alternator With Ballast Resistor

[Relic]

Hi, I have been having charging issues on my 76 Kenworth. the alternator seems to charge ever so slightly with no accessories on, but once the heater, lights etc are turned on there is a discharge situation. I am suspecting the voltage regulator.

truck has been sitting a long time and I'm thinking the problem has something to do with corrosion on contacts somewhere. I started fiddling around with it this afternoon and noticed there was a ballast resistor in the circuit, can anybody explain why? truck has 4 6V batteries, I'm guessing 24V start and 12V accessories, does that mean 12V feed to the alternator?

at one point the ammeter did show it start charging in what looked like a normal manner, but it stopped again. alternator will come out tomorrow for inspection. btw, I'm guessing that is an adjustable voltage regulator attached to the back of the alternator?

 

[Truck Shop]

Look for a series parallel switch, that old girl has one probably in the battery box. One of the things of owning old trucks is changing it over to straight 12 V.

Truck Shop

 

[Relic]

Look for a series parallel switch, that old girl has one probably in the battery box. One of the things of owning old trucks is changing it over to straight 12 V.

Truck Shop

I'll have a look in the morning, IIRC there is some sort of device toward the front of the battery box, thanks.

 

[Birken Vogt]

Can you draw us a rudimentary schematic of what you have? Any unknown components just labelled as such. A picture is worth a thousand words.

 

[Delmer]

On anything that's been sitting, check the brush holders for corrosion and free movement. The brushes can be fine, and look fine, but if they don't spring back then they're not making contact and you're not charging.

Could even try starting it up with a voltmeter hooked up, and whacking the alternator with a screwdriver to see if the voltage jumps up. It might help to know the orientation of the brushes to know which way to whack it, or just go all over the back end.

If nothing else some voltage readings will give something to go on.

 

[trucker1]

I had an old Cummins once that would do that. Drive it all day and it would not show charge, then the next day it would charge fine. I took the alternator off and took it to a local fellow to have it checked out. He told me that was a "self energizing" alternator, something about if the alternator wasn't turning fast enough, the regulator would not turn it on. I guess because that Cummins was a slow turning engine, only about 1900 RPM, then it was right on the edge of starting to charge. He did something about wiring in a diode of some sort so it would charge at an earlier point. I don't know exactly how it worked, but it did. After you warm the engine up, crank it right up against the governor for a couple of seconds and see if it starts to charge normally. the ballast resistor would have nothing to do with it, I think someone has put it in there to reduce the output to 6V, which doesn't make a lot of sense to me. Trucks of that era should have 4 12V batteries and the aforementioned series parallel switch to convert it over to 24V for starting.

George

 

[Delmer]

The only place I've seen ballast resistors is to replace a voltage regulator, and it will work sort of. I doubt a semi would be that hacked up, but you never know?

The sticker on top should tell the voltage of the alternator, or it could be stamped/cast somewhere. Should be simple to figure out from the wiring also, the big wire to the alternator is connected straight to the batteries (or in this case the parallel switch). I've never had to mess with parallel switches so the others can handle that. Shouldn't make any difference if you have 4 6 volt batteries, they'll just be hooked in sets of two in series to make 2 12V batteries as far as all the wiring is concerned.

 

[mekanik]

That is an old Motorola alternator probably 60 or 70 amp. And yes they used
a ballast resistor. I don't remember the reason. As a previous poster
mentioned the truck should have a series parallel switch in the battery box.
The SP switch connects two pairs of batteries in series to send 24 volts to
the starter for cranking and connects them in parallel for charging.
If one pair of batteries is not charging it's probably the SP switch.

A Delco 21/22SI alternator should fit existing brackets. I'm not sure if the
pulley from the Motorola will fit the Delco. It would be a good upgrade.

Terry

 

[Truck Shop]

IMO. The first thing to repair and make reliable is the starting/charging system and it will cost $$$. Like said above go with a later Delco alternator, but for
the money install a 24SI-160 amp alternator, you can get pulleys to fit it. Next replace the batteries with 31HD screw top 12-V's and install a 42MT 12-V
starter. You will have to make-up the cables but they will be lug type and those are easy to make. Buy doing this you will eliminate the SP switch, a lousy
set of 6-V batteries and a very outdated alternator and starter. The lights, switches and breakers are all 12-V anyway so you won't have to mess with any
of those things. Once done you will be glad you did it. just my two cents.

Truck Shop

 

[Relic]

Spent a few more hours on the old girl today, and am slowly becoming familiar with things. have to remember, this is my first freight truck of any sort... there is a bit of a learning curve involved here.

I do appreciate the advice, all of it good from what I can figure, but the job in front of me is to get the old girl roadworthy. I would love to jump on a new electrical system, steering box, 18 speed transmission etc, but I need to get her to a 'run what you brung' level. the transmission and diffs seem to run smooth and fairly quiet in direct, but there is some serious complaining going on in overdrive. I don't know if I will have to change some gears/bearings there yet, so $ will be held tight in the meantime.

The alternator did come to life, briefly, yesterday by fiddling with wires. couldn't get it to come back today so I took the regulator off and had a peek. nothing bad that I noticed, brushes still have good spring load against commutator. I will take the alternator to Kamloops this week to have it tested and fixed the cheapest way possible.

While the alt was out I took the opportunity to strip the belts off the engine and have new ones ordered. water pump and alt easy peasy, fan belts have to come from Rockford.

Exhaust comes off turbo/straight pipe/flex pipe/ and into an elbow. the elbow is rotten, looks like a fun job to replace, but it's on my list now.

As well, there are some mysterious tabs bolted to the side of the block, 6 of them in all, 5 are leaking. what are these? and is it a backyard job to seal them back up or do I need to order some parts from cummins?

The tach and the speedo are not working either, although the tach does occasionally move up to around 300-500 rpm mark it just hovers there. it does not move with engine rpm.

I have included pictures with arrows to those tabs on the block, and what I think must be the tach sensor on the bell housing. any ideas?

 

[Greatwestcam]

those are piston cooling nozzles, have an o-ring on them that goes hard with age. be careful on how you pull them out as they are plastic and brittle (at lest the N-14's are) if they crack replace them.

post some more pics of the truck if you can, love those old A models.

 

[trucker1]

On older engines the piston cooler nozzles were metal, I have no idea when they changed them to plastic. I think they were changed to plastic when they went from the small cam to big cam engines. the sensor on the bell housing is indeed for the tach. When you install it you turn it in with your fingers until you feel it touch the teeth on the flywheel then turn it back out I think one turn then lock it in place. If I remember right they only come with a short pigtail, 2 wires, and you splice them into the original wires. If it was replaced before check those connections before buying a new sensor.

George

 

[Birken Vogt]

I would not necessarily junk it or do the work to replace with a Delco, yet.

I have a Motorola alternator that is probably the same out in the yard, but it has been raining nonstop since you asked this question, so motivation has been low.

I would still like to see a hand scrawled wiring diagram even if you draw it on a piece of paper and take a picture.

 

[Relic]

I would not necessarily junk it or do the work to replace with a Delco, yet.

I have a Motorola alternator that is probably the same out in the yard, but it has been raining nonstop since you asked this question, so motivation has been low.

I would still like to see a hand scrawled wiring diagram even if you draw it on a piece of paper and take a picture.

looks fairly simple, yellow arrow points to battery power supply to alt, red arrow goes to 75 ohm ballast resistor and then on up through firewall, guessing it goes to ammeter. I have found what looks to be the right voltage regulator on ebay for $35.00. http://www.ebay.com/itm/Motorola-Voltage-Regulator-M5-198A-/182300006632

btw, I believe the piston cooler nozzles are steel, but having snugged them up, they just leak worse. I have an o-ring kit, will see if I have anything close to the right size on hand, thanks.

 

[Birken Vogt]

OK, now it is clear to me.

The other side of the resistor should be hooked to the ignition switch.

When the switch is on, current flows through the resistor and "jump starts" or "lights off" the alternator and then it starts to generate out the yellow arrow wire as you know.

The red arrow terminal takes power from the ignition circuit to start, but once the alternator is working, it uses internal generated voltage to bring that terminal up to output voltage and not much current flows either way on the resistor.

To test, remove the red arrow wire from the stud and turn on the key without the engine running. There should be 12v there. If you brush it against the stud you might see a small spark.

If this is functional and the alternator is still dead or weak, the problem is internal.

(On some vehicles it is a light bulb instead and lights if there is a problem with the alternator. If it is working correctly this balanced voltage means the light is off)

 

[Delmer]

Does anybody know how an ammeter works in this case? Assuming this is real ammeter in the dash. I've only seen the ones that have whole big wire going through them. A volt meter is a better idea IMHO. maybe voltmeter's were harder to make back in the day?

 

[crane operator]

Relic - I didn't see what transmission the kw has (you mention a 18 speed, but I'm assuming that's what you eventually want), but I noticed you mentioned noise in overdrive. 13 speeds are notorious for a overdrive whine, if you haven't driven one before, it sounds bad, but there isn't actually anything wrong.

 

[Birken Vogt]

Does anybody know how an ammeter works in this case? Assuming this is real ammeter in the dash. I've only seen the ones that have whole big wire going through them. A volt meter is a better idea IMHO. maybe voltmeter's were harder to make back in the day?

There may be a shunt somewhere so the ammeter in the dash only registers some small fraction of the current. Or this may be a real ammeter. In either case the battery and starter are the only loads on the one side and all other loads connect to the other side of the ammeter away from the battery.

If everything is working right, the ammeter will swing over pretty hard to the right when the truck is first started, then after some minutes or even hours, as the battery fills up it will back down lower and lower until it settles just barely in the positive for as long as you want to keep it running.

 

[repowerguy]

Truck Shop has the best solution to the problem, just chop out the old and in with the new. I would however suggest a Denso or a 39 MT Delco gear reduction starter. These will eliminate a huge amount of ugly weight and make access to the 10 o'clock bolt easy. The other thing is you can use 3 batteries which is a weight and money savings also. JMHO.

 

[fast_st]

Oh yeah, the resistor!! ususally they were in the harness, when you turn on the key, power goes through a bulb and a resistor, when the alternator starts charging the field current drops and the bulb goes out, if the alternator stops working, the current to field goes up and the bulb lights up.

One brush on that alternator looks filthy, polish that with emery cloth and wipe it with an alcohol soaked rag or spray contact cleaner. Make sure the brushes move freely, the thing on the back is the regulator, I'd guess any alternator repair shop could test it. Keep track if its + or - ground too.