1991 m923a2 electrical charging issue

[CPDOG]

All

we finally decided to post a thread concerning our issue, keep in mind we have been at this very intensely for over 2 weeks...we now need all opinions both main stream and off the wall!


ISSUE

-the main issue is that my voltmeter on the dash is reading mid yellow to mid red and jumping around erratically. (21vts to 35+vts)
-at idle it is high yellow and at 1000rpm it is at 35+, I stopped at this point. then on another run it is all fine up to full rpms of around 1800-1900 then she spiked to the mid red zone, but regardless the jumping around never ceases.

PAST RELATED ISSUE
-So, having been thru one original alt burn up...it seized up going down the road and the pulley broke the key in half and the belt flew off. (crazy day here guys!)
-So we have just accomplished the mod to a 21si alternator. all went great with this! (with awesome assistance from fellow members "manders" and "74M35a2".... thanks again guys) if you haven't done this mod, look into it!
-so I went for a test drive and it still doing the exact same thing.

ACTIONS TAKEN/RELATED BACK GROUND ON THE TRUCK
1A) the tech man is broken out.
1) new batteries (but did drop test and gravities on old batteries , both were sat)
2) cleaned every ground to bare steel, applied dielectric grease and verified each ground with greenlee dvm (we stopped counting ground points at over 100.. just fyi) there are still more under the dash we are going after...
3) replaced the battery switch on dash.
4) verified voltages thru cables at the alternator and at the batteries (it was constant) and no identified shorts other than the erratic fluctuations.
5) removed the new alt and took it to Blanchard electrical in Tacoma Wa and they thoroughly rung it out its still fine and I had them test my 2nd prestolight alt too and its fine...which means this existing issue was here all the time
6) at one point only during the low voltage portion the abs light came on but clears.
7) the only switch I have not tested or replaced is the start switch.
it always indicated a little low in the green zone, but now we are seeing that this issue may have been here the entire time and it just finally let loose.
9) the truck starts/runs great. this truck is pampered and is our camper and parade truck. the maintenances/upkeep are up to par.

Lastly, we have poster sized print outs of the electrical system that me and the wife have combed over with our minimal electrical knowledge and all the data and possible issues literally traces back to the goes to the PCB (protective control box) we have read many post here concerning the PCB and its role in the charging process....with this said do you think we are on the right track and if so is there a way we can check this PCB here at home prior to us having to buy a new one?. any input/recommendations here is greatly appreciated.

if we have missed any details or need to see pics just ask and we will respond to it.

V/r

 

[Suprman]

If the dash meter is correct and the voltage is going up to 30+ volts then you have an alternator issue or the regulator wire is damaged somehow. Nowhere else on the truck for that high voltage to come from unless you have 3 batteries wired in series. I have seen several bad dash meters though. Always best to verify with a known good handheld meter. The pcb is 2 relays. One turns power on to the fuel solenoid and a few other things when you turn the start switch to run and the second relay controls starter crank. If the truck starts and drives properly then the pcb is probably good. Not much to these trucks electrically. Check the regulator sense wire.

 

[CPDOG]

suprman

thanks for the reply.
we did use 2 different dvm's. (at some point we questioned them too)

that regulator sensing wire you refer to, do you mean the actual hot wire to the alt from the battery? the reason I ask is on the new 21si we only hooked up the ground and the hot wire. the other 2 smaller wires are capped off now.

V/r

 

[Suprman]

The mil alt regulator uses an external sense wire. The new alt (which I have no experience with) may have all internal regulation. Then you would just need the positive and ground hooked up. Something is making the alternators regulator think its under voltage and its putting out too high voltage. You
can post a pic of your batt box and the alt so we can try and make sure everything is hooked up right. But if you have more voltage than your batteries put out the alt is the only place it can come from.

 

[CPDOG]

superman

got it .... we are taking pics and will post soon


v/r

 

[CPDOG]

superman

1) here are some pics, the battery well is kind of hard to make out. up at the alt you can see that the 2 small wires are no longer hooked up. as the unit is self exciting.
2) also, since we last emailed you we have been busy troubleshooting this issue. see if this makes any sense to you...I am doing continuity checks just for the heck of it....with the engine off, the battery negative is disconnected, the battery switch is in on position, the start switch is moved to the next position up but not the starting position and I have a dvm hooked up to the positive terminal at the alt so we put the other end of the dvm on the positive battery terminal and get nothing, we put it on the negative terminal and it beeps??? then we take the battery switch to the off position it doesn't play a part in anything. we then take the start switch to the off position and the dvm stops beeping...?? but it gets weirder here, when we hooked up the negative cable again it doesn't beep on the negative cable any longer it now beeps on the positive cable..ha-ha. so did we just find an issue with the start switch or does it behave in this manner out of a design.

lastly we ran the truck for about an hour lights and blower on/off and the voltmeter gauge only spiked to the red (31volts)once this time but it did fluctuate with rpm's from high yellow/low green to mid green. does your truck fluctuate like this or does it stay constant for you?


thanks again for the assistance


V/r

 

[74M35A2]

Could you have the + and - wires connected wrongly on the back of the alternator? With both black and the same length, they could easily be confused as soon as removed, as the red terminal cover is not stock. Or, I'm thinking you could have the B+ wire of the alternator going to the + of one battery only, so it only sees 12V and is always trying to push voltage up.

Try this instead: Remove both wires from the alternator and cap or tape them off so they don't touch anything, including each other. Next, take a set of jumper cables, and connect the black ground clamp to the alternator mounting ear and the black clamp on the other end to the engine block somehow. Then, take the red jumper cable clamp, and hook that from the alternator B+ to the starter B+ lead (on the starter solenoid itself, as the big cable on the back cap of the starter is the B- cable). Make sure you are connecting it to the starter solenoid terminal that has the big wire coming from the battery bank to it, and not the other starter solenoid terminal that has a short but large wire going directly into the starter motor. This gets all of the truck wiring out of the equation, as long as the starter is working correctly. This should work correctly, as long as the place that tested yours confirmed there is no over-voltage issue. If they just checked to see if it made power, then it still could be the alternator. Report back your findings.

 

[KaiserM109]

...we did use 2 different dvm's ...V/r

Because a DVM takes a second or so to settle, it is often more useful to use and old-style analog meter. It will show you the bouncing signal.

 

[CPDOG]

74m35a2 and kaiserm109

thank you for the replies....

I will definitely follow up with results on Thursday to both of you!......... and with some possible pics


V/r

 

[CPDOG]

74m35a2 and kaiserm109

guys, ok here we go!

RESULTS OF YOUR RECOMMENDED TESTING

kaiserm109
- borrowed another dvm, verified that possible variable....no issues. but through out this testing we used all 3 ..ha-ha

74m35a2
-re-verified all wires iaw the tech man electrical dwg and all my wires have the little metal id tags, but we still did continuity checks and some hand over a hand checks too. ....good to go here.
-conducted your recommended test and all went great... no more spiking was observed. but we did observe the overall lower voltage output from the alt over the previous unit and I cant adjust this unit. we are planning on running it again Saturday for longer period and try to get it to expose itself one last time.



TESTING CONDITIONS SET

- I did my testing at cold idle/1500/warm idle rpms. with the conditions set per 74m35a2. plus I had all lights, high beams and blower to high as well.
-at cold idle at I was getting 26.41 vts at the alt and 26.3 at bat. I was in the high yellow on gauge.
-at 1500 rpm I was getting 27.4 at the alt and 27.3 at the batt. and at the yellow/green line
-at warm idle I was getting 24.7 at the alt and 24.3 at batt and was at mid yellow at gauge, I hit the gas and went to 1500rpm and all I got was yellow/green line on gauge.


MY FOLLOW UP QUESTIONS FOR YOU BOTH

now with this same testing above with out lights and blower off I was in yellow/green line to mid green on gauge, 26.4 vts at the alt and it fluctuated with the rpms from idle to 1500-2000rpms with no more spiking observed and highest output was 27.4 vts. where did that spike go? do you think we have an unidentified wire issue? or are we back to a possible weak alt? we forgot to put the amp meter on the hot wire, so we will get that data later.

so, we have been in the tech man and conducted testing on the switches, pcb (6 tests here all good), starter checks(good), continuity check with all associated wiring, removed all terminals and cleaned one at a time(good) all told we have conducted about 40 tech man checks so far, etc... we think we are back at that alternator, it definitely doesn't put out enough power to in our opinion or it could just be how this model works over the military alt and we just have to get used to it fluctuating like it does?.

one small item of note when we went to fast on the blower even at 1500rpm the needle at the gauge dropped from mid green to high yellow and respectively at lower rpms too, we are going to check that guy too possible high draw on the system we never noticed it doing it that big of a drop with old alt in place.. as for the lights/high beams it had very little effect on gauge needle deflection at the above rpms. do you recall how your needle reflects on your units?

ok we are done, sorry for the long winded right up but this result of intensive troubleshooting efforts over here. we look fwd to your responses.
if we have missed any details please let us know and we will get it.
V/r

 

[74M35A2]

Anything over 25.6V is charging. Not sure of the idle output of the Chinese Delco clone alternator, but it seems it can't keep up with your loads at an idle. This is also dependent upon how discharged your batteries are, as they wilk suck power also to recharge. A 60A alternator should make 60A at speed, but much less at idle. Idle reduces output, as does increased temperature.

Not sure of the wiring issue with your truck, but we got your system behaving normally. I personally wouldn't put hours into figuring out why it was what it was, now that you have a path to operate it correctly, just wire it that way and move on. Up to you, but I don't chase vehicle wiring issues, I just run new. It saves so much time.

I would not judge much by the dash gauge. Mine was on the yellow/green border for 2 years and everything was fine. Batteries were always fully charged, and recharged quickly after my elongated cranking from running out of fuel, several times.

Digital volt meter is good. Clamp on DC ammeter is good, but a little hard to come by. Fully charged 12V battery is 12.8V, so use that as your reference point (takes > 25.6V to charge).

 

[CPDOG]

74m35a2

hi bro, thank you again for the help!
we will follow up with anything we may find,

V/r

 

[CPDOG]

74m35a2

hi bro, me again.

what do you think of the idea of us just running from the alt straight to the batteries? and would it be as simple as (2) wires hot and ground.
do you foresee any issue with this idea?


V/r

 

[74M35A2]

74m35a2

hi bro, me again.

what do you think of the idea of us just running from the alt straight to the batteries? and would it be as simple as (2) wires hot and ground.
do you foresee any issue with this idea?


V/r

You can do that. Or you can go directly to the back of the starter and be done with it. That is how they wire over-the-road trucks now, alternator is wired directly to starter since there is large cabling to starter anyway. Gravy job. You can buy car battery cables with the correct ends at any car parts store and be done. Run 2 wires to the starter, confirm with a digital volt gauge, close the hood, and be done with it, go for a ride.

Cap the old unused wires well, as they will still be live with power and you don't want them to ground out.

 

[CPDOG]

74m35a2

that idea is way simpler......thank you. I will post pics when we are complete.


V/r

 

[CPDOG]

74m35a2

hello again, I wanted to give you some feedback on our issue with the spiking that was occurring. we replaced 2 more components the PCB and the actual start switch and wired it up normally. the spiking is gone as well. we test drove and no spiking, but at normal driving speed of 55-60 mph and about 1900 rpms it was only on the yellow/green line and when came to a stop light it went to mid yellow. I know I should have replaced only one at a time to isolate it... I completely blew this one.. ha-ha.

we did every test on the pcb and switch iaw the tech man and the only item I can come up with is something wasn't right after warm-up with a contact or something internal to the switch. but the undercharging we conferred about is still present. so we attached my strobetach and looked harder at the rpm route. sure enough it is only doing 660rpm while the engine is at 550rpm. maybe this units kick in curve is for a higher rpm? it is registering only 24.2 volts at alt and batteries. another thing that we observed at the volt gauge on dash was that at idle and when we shut down, the needled stayed right where it would be normally reading the battery voltage prior to start up, which told us we are sucking off the batteries at idle for the most part.

so I am going to put another pulley on the alt (a smaller one) and see if I can bring up the rpms on it and get it into the green zone. I am also looking at an actual delco-remy and a Leece-Neville 24volt /75amp with an adjustable regulator.

so this is where I am at I hope we provided some good feed back that will others with kind of issue will be able to use.


thanks again for the assistance and if you have any further ideas/input just drop us a line.
V/r

 

[post 5466]

74m35a2

hello again, I wanted to give you some feedback on our issue with the spiking that was occurring. we replaced 2 more components the PCB and the actual start switch and wired it up normally. the spiking is gone as well. we test drove and no spiking, but at normal driving speed of 55-60 mph and about 1900 rpms it was only on the yellow/green line and when came to a stop light it went to mid yellow. I know I should have replaced only one at a time to isolate it... I completely blew this one.. ha-ha.

we did every test on the pcb and switch iaw the tech man and the only item I can come up with is something wasn't right after warm-up with a contact or something internal to the switch. but the undercharging we conferred about is still present. so we attached my strobetach and looked harder at the rpm route. sure enough it is only doing 660rpm while the engine is at 550rpm. maybe this units kick in curve is for a higher rpm? it is registering only 24.2 volts at alt and batteries. another thing that we observed at the volt gauge on dash was that at idle and when we shut down, the needled stayed right where it would be normally reading the battery voltage prior to start up, which told us we are sucking off the batteries at idle for the most part.

so I am going to put another pulley on the alt (a smaller one) and see if I can bring up the rpms on it and get it into the green zone. I am also looking at an actual delco-remy and a Leece-Neville 24volt /75amp with an adjustable regulator.

so this is where I am at I hope we provided some good feed back that will others with kind of issue will be able to use.


thanks again for the assistance and if you have any further ideas/input just drop us a line.
V/r

Hey brother I’m having kind of the same problem with mine 9258 to put in new engine was fine box out on firewall was replaced because truck could see it when you switch truck on the crank the fuel shut off wouldn’t work and the truck with a crank with the starting switch without having to jump it at starter put on new box everything was fine and then all of a sudden battery started over charging how did you test your box

 

[post 5466]

Hey brother I’m having kind of the same problem with mine 925 a2 all new batteries when put in new engine was fine box out on firewall was replaced because truck had sat for a while when you switch truck on to crank the fuel shut off wouldn’t work and the truck wouldn’t start crank at the starting switch without having to jump it at starter put on new box everything was fine and then all of a sudden batteries started over charging how did you test your box Thank you so much in advance

 

[CPDOG]

Hey there sorry for the late reply hey when I tested my box there was a procedure in the tech manual and it spells it out over like to two sections and it goes over step-by-step and including using a voltmeter on testing it that’s a procedure i used I don’t have a tech manual in front of me right now I am sorry however there is a thorough testing section for that box also ensure the grounds are all clear take the box off the firewall and scrub Mounting points down to bare metal and clean the star washers , but I replaced the military alternator and put civilian alternator or and it has been 6 years no issues!!! good luck bye