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M923a1 alternator voltage gauge readings

ChuckM

ChuckM

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rickycotte

rickycotte

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Hello Chuck,

Thanks for your post. Do not despair...your input on the 21si is of great value. It is just that I like to see all the possibe alternatives and of course I would feel inclined to change the alternator alltogether. Although I understand there is some modifications to be made.

The reason I was inquiring about the regulator is because this particular truck was rebuilt in 2009 and I got it with only 800 miles on it and some 25 tach hours. I do not think the alternator would be too old. But, on the other hand, these do not seem to be very reliable.

I have seen some other alternatives also for a direct replacement which makes the possibilities endless. From $110.00 (as Delco 24si is) to thousands of dollars. I got a dimensional drawing of the AMA5104UT alt and will compare what it takes for retrofitting the Delco unit.

Again, thanks for the info!
 
rickycotte

rickycotte

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Ok, I finally got to the adjustment screw..no luck. 24.3V either way...lock to lock.

Dukeman, when you say a 2" pipe spacer, surely you mean 2" long. But, what did you use? 1/2" ID or 3/8" ID? Would you be more specific? Thanks.

I have included some pictures I found at the APS website which I feel might be useful for others seeking info on this thread. :roll:
 

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Dukeman

Dukeman

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I think it was a 3/4" pipe diameter, just what ever fit over the bolt, the diameter size wasn't really important. I took some pics, but I haven't had a chance to get them off the camera yet. I will try to do it tonight.

It wasn't a problem to drill out the existing pulley. 7/8" was the perfect measurement.

I still want to know what to do with the two small wires that I asked about above. If you figure it out let me know.
 
rickycotte

rickycotte

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Hi,

Thanks for the info. I replied a few threads ago about what I found regarding wire 568 and 566. The 566 circuit is to operate a relay in the safety box to prevent starter engagement while the engine is turning. Circuit 568 just comes from ignition switch to excite the alternator. I understand these Delco 21si alternators are self excited so it might not be needed.
2cents
 
J

jwesley74

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Englewood, FL
I just replaced the alt in my M923, used a delco 24v. It was recommended by alternator repair shop. Took it home, had to drill MV bottom bracket out to fit 1/2" all thread and top slotted bar had to grind to make slot bigger for 1/2" bolt. It only required Pos & Neg to be hooked up. It was self exciting. Started truck up and it was in the green on the voltage meter. Prior to replacing it was in middle of yellow. New alternator cost me $145 and about $8 in all thread & nuts from Home Depot to make it work. Now I have a problem I can just get a new one from most auto parts store. Sure beats $400 for replacement MV alternator. Figured I burn this one, I'll get one from Autozone or Advanced with warranty. Then replace for free if I burn 'em. I don't expect them to last through mud/water we go thru.
 
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jwesley74

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BTW, the alternator shop put the double V pulley described above, in rickycotte post, on my alternator. All included in the $145 I paid. He said it was better cause it was smaller than the MV alternator pulley and would spin faster. Lastly, I forgot to mention the new alternator is a 70 AMP vs the 60 AMP MV alt I took off.
 
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dfwernig

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Thanks for all the info. Followed the procedure, but could only get my alternator down to 30 volts, figured that was a lot closer than the 35 it was at. So, I reconnected the battery cable, I had disconnected it with the truck running. All good. Then I noticed the dash gauge, which had been above the green in the red, was now right on the line between the green and the yellow. For some reason that escapes me, I disconnected the battery again and the truck stopped running. Reconnected the battery, and it started right up. Even when I had the alternator down to ten volts the truck had kept running with the battery disconnected. Any thoughts? Thanks in advance.
 
rickf

rickf

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My thought is that you are going to be buying a lot of regulators and electronics if you insist on running it with the batteries disconnected. You are spiking the system every time you disconnect while running and with no load the alternator could very well put out 40-50 volts and blow out everything on the truck. The regulator will most likely go first though since these regulators do not like spikes at all. You can get away with this with a generator but not an alternator, at least not for long.
 
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dfwernig

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Okay, thanks for that. I only disconnected it so I would read alternator output and not read the battery charge . So, it is hooked up now, and I am just a volt or two over specs. So I will try again and leave my batteries connected. Thanks for you input.
 
rickf

rickf

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Alternator output is totally dependent on the load it sees from the battery so disconnecting them really tells you nothing other than it is working. It used to be done all the time with generators and it kind of carried over as alternators came in. The old guys will say it will do no harm, well I have replaced a ton of bulbs and electronics that says otherwise. That is why you get the good reading when hooked up because it now has the load to read. the batteries also absorb ant spikes in the system.
 
S

SLOrazorsedge

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San Luis Obispo, Ca
Well I'll revive the thread. My M936A1 wrecker will occasionally pop over into the RED on the volt meter and the LED headlights will flash on and off on low beam. It doesn't matter if the lever light switch is off or on. I can get the lights to stop flashing by putting them on high beam with the light switch on. The truck originally came with standard headlamps and I added Trucklight LED headlamps. How in the world can headlamps flash without having the lighting switch enabled? Why would an alternator spike into the red only occasionally?
 
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SLOrazorsedge

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San Luis Obispo, Ca
Solved the flashing headlight issue, one of the headlight 3 stud bolts rubber isolator was loose resulting in a bad ground. Just replaced the alternator belts today. The alternator spools up fine, no noise, etc. After the truck warms up, the volt gauge will spike hard right into the red. I shut the truck down for 10 minutes, fired it up again and voltage was normal in mid-green. Its hard to imagine the regulator is inconsistently bad? I've never seen one inconsistently spike voltage, my experience is they are dead or ok. Any suggestions?
 
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