| Author |
Message |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 17, 2005 - 9:03 pm: | |
I had my bus running the other day Checking the alt. volts. It was holding
at 28 volts stedy at the batts. checking the 12 tap off the alt
(factory used for the A/C relay and daytime running lights) I get
volts from 10 to 12 and all over. I noticed the daytime running lights would flicker before I changed them when I went all 12 volts lights with a battery equlizer. I dont
use any power off the 12 volt tap now and the 24 volt side seems
fine.
Any one know how the 12 volts get out of the Alt I was thinking it was just 1/2 of the 24 volts side but the 24 volts side stays at a stedy 28 volts no load or with heavy load and the 12 volt never changes stayed at 12 volt but now its acting up.
If its a seperate part of the alt and will not give me problems down the road with the 24 volt side That will be ok I dont use the 12 volt side any more but if its tied into the 24 volts side and the 24 volt side will fail I need to find out what the problem is.
All the wires are good, ground is good. I have not realy got into
looking very hard yet just took some meter readings. The regulator volts are steady and the 28 volts is steady.
I hope some one here has some time repairing or rebuilding the big 24 volt oil cooled 300 amp alt or know where I can look to find the problem.
If any one has a extra 24 volt alt to sell I like to have a spair.
Thanks
Brian 4905 Klamath Falls Oregon |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 17, 2005 - 9:30 pm: | |
Brian, I'd venture that one of the diodes in the 12V arrangement, more accurately, the group of diodes grouped to comprise 12V, is in its death throes. They "tap off" 1/2 way to get the 12V.
Onward and Upward |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 17, 2005 - 9:47 pm: | |
But are the diodes also part of the 24 volts? Or just for the 12 volt ? If it only for the 12 volt and dont affect the 24 volt Then I will not worry to much about it.
Brian 4905 Klamath Falls Oregon |
Jerry Liebler
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 17, 2005 - 10:02 pm: | |
Brian,
I've replaced the stator on mine so I had it all apart. Though mine is a 12 volt, they are really the same in how they work and how they are put together, they even use the same stator and diodes, only the field is different. What you are calling the 12 volt tap is called the "R" terminal on mine.
The stator is a 3 phase "y" connected source. The "R" terminal connects to the common connection between the 3 windings while 2 diodes connect to the other end of each winding. The symptom you are seeing means there is an unbalance between phases. The unbalance could be due to a blown diode(easy) or a short or open in the stator (expensive). Take a long look at your manual and a few deep breaths, open the cover over the diodes, disconnect and test all 6 with an ohmeter. If it's a diode, Luke'll probably have them, If it's the stator as mine was, or diodes for that matter, Faulkner Automotive Electric co. (503 227 3567) in Portland has them. My stator cost $176+$40 for the core and about another $7 for the 'O' rings. Hope it all goes well, if you want you could give me a call, 606 423 9854 (I'm now moved from Portland to Kentucy)
Regards
Jerry 4107 1120 |
Tom Caffrey (Pvcces)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 17, 2005 - 10:23 pm: | |
Brian, I'm betting that there is nothing wrong with your alternator. That terminal's voltage will go up and down, depending on the signal that the regulator sends to the alternator.
Jerry describes the constuction correctly, but the power available at the 12 volt terminal is dependent entirely upon the output at 24 volts required of the alternator.
The relay terminal runs about 12 volts because one of the connections is located midpoint from the 24 volt output. The other end of that circuit is fed through the negative diodes in the 24 volt circuit.
It was never intended to power much of anything. Its only purpose is to signal that the alternator is putting out power. I don't believe the daytime running lights should be connected directly to that terminal.
The fact that your 24 volt output is steady tells me that the regulator and alternator are doing their job within the loads presented to the charging system.
If you get one of the smart regulators for your bus, you should find that there is a provision in the regulator to require a small charge from the alternator at all times so that the voltage doesn't drop off at the 12 volt terminal when the batteries are fully charged.
If the voltage should rise more than can be tolerated, there is a connection to cut inside to disable this feature.
I hope this helps you figure out what you need to do.
Tom Caffrey PD4106-2576
Suncatcher |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 17, 2005 - 10:55 pm: | |
Thanks some good tips.
Also Tom The running lights are factory set up that way in the wiring digram for a 4905.
I will have to look tomorrow whenI get back home I think the R termanal and the 12 volt side was on differnt terminals but I dont remember.
I will pull the cover and check the diodes tomorrow and make some phone calls.
Im hoping its nothing. But I want every thing to work new.
Brian 4905 Klamath Falls Oregon |
Stan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 18, 2005 - 9:41 am: | |
I am completely baffled by some of these explanations. Many common tachometers run of the R terminal and the tach reading is only determined by the speed of the alterantor and has nothing to do with the battery.
My MCI drawings show the R terminal coming off the AC side of the windings with the return through half wave rectification of either of two windings to ground. That is, the R terminal is the positive half of the AC voltage from from all three windings 120 degrees apart. Since it is isolated from the battery by the diodes it remains as half a sine wave and not as DC.
Can you all expand on your explantion or point out the error in mine? |
Pete RTS/Daytona
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 18, 2005 - 7:38 pm: | |
actually it's full wave rectification on the "R" terminal - The lower half of the alternator sine wave is inverted - so the frequency is twice that of the a/c sine wave output - trust me - i've wired and calirated bus tach
Note: For Detroit Diesel Delco 270 Amp 24 volt alternators "R" terminal hookup (2.93 gears x 3 pole stator x 2 (full wave rectified) = 17.58 pulses/rev) - The "R" terminal output is 12 volt -
Pete RTS/Daytona |
Tom Caffrey (Pvcces)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 18, 2005 - 10:43 pm: | |
Stan, the tachometer is only a pulse counter; it does not care what the voltage is so long as it is within the limits of the tachometer electronics.
The reason I said that I didn't think any substantial load ought to be hooked the the R terminal was because that voltage operates some safety circuits for the starter and other systems.
I wouldn't think that you would want to influence the output by very much. However, it may be skookum enough not to make much difference.
For what it's worth.
Tom Caffrey PD4106-2576
Suncatcher |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 19, 2005 - 12:33 am: | |
I dont know how much load the R terminal can take But I do know it can run the low beam headlights on my 4905 as built from the factory with daytime running lights. So I would think you could easly run a radio or CB off of it if that was your only 12 volt avalble.
And today when trying to check why My R termanal voltage was going up and down the other day it worked fine about 13.6 or so holding steady.
So I guess tomorrow I will check the diodes got home to late today.
Brian 4905 Klamath Falls Oregon |
Stan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 19, 2005 - 9:56 am: | |
Pete: That is very interesting. I didn't know that there was any way to get full wave rectification when the output was tied to the AC line. I believe your numbers, I just don't know how you get them. I will have to assume that the MCI wiring diagram is wrong.
In theory, since there is no center tap in any of the windings, a full wave rectifier will give full alternator output. A half wave rectifier gives only half of the AC output which will be close to 12 volts on a three phase alterator when measured with a normal DC voltmeter (ie not an RMS or Peak voltage meter).
Brian: Since the R terminal comes off the same windings and the same diodes as the main output (although at half voltage) the limit to the output current will be the size of the lead wire (inside the alternator) to the connection bolt.
Now back to your original problem. Normally you will not see any voltage swings on a meter when connected to an 8D battery, which is what you are measuring at the 24 volt output. The R terminal does not have any battery or capacitance connected to it. You can connect a 12 volt battery to it and see if it still moves around but I think what you are seeing is the normal changes occuring as the voltage regulator constanly adjusts the field current to make up for any drain on the battery, however small. |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 19, 2005 - 4:09 pm: | |
Stan ,
My R terminal was also hooked to the day time running lights and they also flickered that Im shure is not right. Im pulling off the cover now to check the dioeds juat to be sure and I want to look in there any way.
Brian 4905 Klamaht Fallls Oregon |
Mike Eades (Mike4905)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 19, 2005 - 8:17 pm: | |
Brian I have a 1975 4905 and all my running lights are 24 volt. Just I thought you should know what mine is and I had U.S.Coach change some lights and they put 24 v lights in and they work fine. mike4905 |
Stan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, January 19, 2005 - 8:54 pm: | |
Brian: I think your voltage regulator is looking at batteries that are fully charged and as the voltage drops by any minute amount the regulator applies field current to bring the voltage back up. A simple test of this is to put a reasonable load on the system by turning on every electrical device on the bus (this is still a small load on a 270 amp alternator). This will make the alternator work to keep the drain on the batteries replaced. If the voltage on the R terminal is still jumping around then the most likely cause is a faulty regulator. If you have never taken a DN-50 oil cooled apart before, get some professional advice. |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 20, 2005 - 3:02 am: | |
Mike my 1975 4905 running lights was also 24 volt , My head lights are 12 volt from the factory that way running though a set of risistors to bring them down 12 volt. The R terminal is run to the low beam side to give a day time running lights that are a little dimmer then with the low beams on at night all built that way from the factory and shown in the GMC wiring schmatic book.
I also changed all my running , brake light , turn signels and removed the risisters for the head lights and have every thing switched to 12 now using a Vanner equilizer.
Stan it may be a regulator problem but so far it keeps it right at 28 volts and it looks to be brand new. Im thinking it maybe a diode or 2 the book say if the alt is disconnected with the motor running it could dammage the diodes and that is what it was doing when I drove the bus home for the first time. It had bad connections losing contact I would lose power to the alt.
I fixed all of that but the damage my have been done.
I have some of the diodes out and having trouble testing them like the book shows . They all flow one way .My ohmmeter dont have the 1.5 volt cell to test em like the book show or Im not doing it right. I could just buy 6 new ones and be done with it.
Brian 4905 Klamath Falls Oregon |
Stan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 20, 2005 - 7:56 am: | |
Brian: You are a true mechanic. If you don't understand it, don't know to test it, then just replace it all with new parts. |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 20, 2005 - 3:01 pm: | |
Well stan I can spend $200. on testing them or just spend $30. and replace em all.
And if you know how to do a leakage test using a standerd ohmmeter. Then do tell. Im not going to spend $200. on a meter I will only use once.
Your right. Cant test it, dont understand it, just replace it. Works for me.
If some one knows how to test them I done the simple test with my ohmmeter it shows them as ok but I was told there is a leakage test but need spical high $$ equipment is there a way around this?
Brian 4905 Klamath Falls Oregon |
Jerry Liebler
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 20, 2005 - 7:06 pm: | |
Brian,
If they test good with an ohmeter they are good enough. This is after all a 300 amp system so what does a couple of micro amps mater? Not at all. Also, regarding loads on the 'R' terminal. resistive loads are fine but the effective voltage is less than 1/2 of the battery voltage. The wave shape is called a haversine. It is positive halfs of a sine wave, it goes to zero a multiple of 6 times per revolution. This is what a tach counts. running a CB or a capacitive load from the R terminal would interfere with a tach's operation. |
Stan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 20, 2005 - 8:52 pm: | |
Brian: If you tested the diodes with the low voltage used in an ohmeter, that is usually sufficient. If you want to do a higher voltage test just put a lightbulb in series with the diode and connect to your battery. It should light when connected in the forward direction and no light in the reverse direction.
See, if you asked how to do it you would have learned something instead of buying new diodes (which may not be as good as the ones you took out). New diodes are supposed to be good but there are faulty ones and you should check the new ones before you install them.
If you are putting in pressfit diodes use the proper tool and if they are stud mount torque is critical on the nuts (1/4" stud is 25 inch pounds). |
Brian (Bigbusguy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 20, 2005 - 9:09 pm: | |
Thanks Jerry,
Do you think that the Gen relay the comes off the R terminal that runs the heater and A/C relay would also interfere with the tach also?
My tach runs off the blower with a sending unit.
But I was thinking if others here having problems with there tach know the some buses use the R terminal for other systems on the bus.
Brian 4905 Klamath Falls Oregon |