M998 Alternator Question

[SchmidtLegal]

So, I bought my 1989 M998 (6.2l) running for $8,500 in October. Got it home, and ran fast and furious and lubed it, replaced the batteries, fixed fuel return lines, resealed power steering gear box, replaced some rubber boots, made safety equipment upgrades, replaced/repaired lights and gauges, got it re-titled and road legal, finally 4-man soft top kit on it. So right now, I'm at about $13k into it. Don't get me wrong, I love it. I like to go outside just to look at it.


To the point: I've driven back and forth to work a few times and around my small town. Turns out, the batteries aren't charging. Got stuck in the dairy queen drive through the other night after I turned it off to order my ice cream (didn't get the ice cream). Had to push it out of the drive through and tow it home. I'm not a mechanic, I'm a working mans lawyer. I'm bleeding money and I can't afford to make a costly mistake. My alternator shows to be putting out 24v max (volt gauge used to stay in the green, started to flux into yellow and then back to green, now just yellow). Pulled the cap off the 60amp prestolite and it looks rough. I'm assuming the alternator, which appears to have been installed at the factory back in big hair days, is going to meet its maker. I've gone through the manuals, looked at the diagnostic manuals, I've checked other threads and none directly help. The learning curve is almost too huge to overcome. *** If I buy a reman or new 60amp, without having the time or knowledge to further diagnose, could I be buying a part I don't need, or buying a part I need but not buying the other parts that I need to go with it? Is it as simple as plug and play? This thing is consuming me and I need to get back to work. (FYI - I don't need a lecture or to be made to feel dumber than I already do).

 

[Dock Rocker]

Have you put a volt meter on the battery and the generator to see what it’s actually putting out? If not start there.


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[Dock Rocker]

Also make sure you unhook your batteries of you are monkeying around with the electrical. If not you could cook your start box.


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[SchmidtLegal]

Have you put a volt meter on the battery and the generator to see what it’s actually putting out? If not start there.


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The generator is putting out 24.1 volts at about 1500 rpm. The batteries when running appear to go up 3 volts with no lights or other gear running. I really noticed the problem while driving at night with bright lights and heater blower going.

 

[sue]

Should be putting out 27-28 volts

 

[papakb]

View attachment TM9-2920-225-34 Alternator Repair.pdf

There are 2 styles of 60 amp alternators both of which are supplied by Southern Automotive. The Leece-Neville 60 amp alternators have a regulator that looks like a bullseye when viewed from the back side. The Prestolite 60 amp alternator regulator looks like a sunburst. Southern Automotive can supply any parts you need for them.

DISCONNECT YOU BATTERIES BEFORE DOING THIS!

The regulator is the most common failure point. It can be removed by removing 6 screws from the rear cover and pulling it straight out the back. The are either 3 or 4 wires on spade terminals that connect it to the main body. The regulator itself is not repairable without some special equipment but the 50 ohm, 50 watt resistor on it is easily replaced. The brushes and brush block are the next common failures and they will generate lots of carbon dust that can short things out. Make sure to blow it out when you replace the regulator.

The gauges on these trucks suck. If you are reading +24v it's probably the battery voltage alone. If things are running properly the alternator should be putting out around 28 volts and that's what the short white line on the gauge is supposed to indicate. A better place to measure battery voltage is on pins Y & T of the STE/ICE connector beside the shifter.

 

[SchmidtLegal]

View attachment TM9-2920-225-34 Alternator Repair.pdf

Thanks!

 

[SchmidtLegal]

Should be putting out 27-28 volts

Got it. So at 24.1, it's obviously under performing. Suggestions? Scrap it and get another?

 

[papakb]

They're so easy to fix unless it's burnt just take it apart, clean it up, blow it out and replace the brushes and regulator. I'd much rather spend $200 than $600. $400 buys a lot of beer!

 

[Milcommoguy]

Also, If you or a mechanic are to remove it or replace... BE SURE to watch for any spacer /s (due to lost or missing) between the lower alternator mounting flanges. If there is a gap between front and rear flanges when seated in place on the engine mounting posistion, there's a good chance of breaking the alternator cast aluminum mounting ears with a "SNAP" and a ah Sh&t.

Do not tighten to pull out the gap to hold in place.

CAMO

 

[osteo16]

Start here... Check voltage, then see what exactly what you're dealing with. It's a good video on 60 amp regulator adjustment and output. If not in spec, continue with other measures.. 28-28.5 volts output is where the sweet spot is at about 1500rpm.... After that, if things aren't going good I would suggest a thorough cleaning with compressor and (lectra motive) or some other 'electronic safe cleaner) available at Wally world or any auto parts store. Brushes might be crudded up reducing output. Thinking about pulling mine apart for fun to clean up. If I do in next few days I will post on my M99Ocho build post.

I hope this helps and is a better reply than most will attempt to post. It honestly takes 2 minutes to post an appropriate response to someone's question if you know what their skill and knowledge are versus just posting some short answer that is vague. I like to give the Most in my Post..

Ocho out

 

[SchmidtLegal]

They're so easy to fix unless it's burnt just take it apart, clean it up, blow it out and replace the brushes and regulator. I'd much rather spend $200 than $600. $400 buys a lot of beer!

Thanks for the info! .......And, a beer sounds really good. Maybe not at 8:45 a.m. but soon......

 

[SchmidtLegal]

Also, If you or a mechanic are to remove it or replace... BE SURE to watch for any spacer /s (due to lost or missing) between the lower alternator mounting flanges. If there is a gap between front and rear flanges when seated in place on the engine mounting posistion, there's a good chance of breaking the alternator cast aluminum mounting ears with a "SNAP" and a ah Sh&t.

Do not tighten to pull out the gap to hold in place.

CAMO

Got it. Thanks.

 

[SchmidtLegal]

Start here... Check voltage, then see what exactly what you're dealing with. It's a good video on 60 amp regulator adjustment and output. If not in spec, continue with other measures.. 28-28.5 volts output is where the sweet spot is at about 1500rpm.... After that, if things aren't going good I would suggest a thorough cleaning with compressor and (lectra motive) or some other 'electronic safe cleaner) available at Wally world or any auto parts store. Brushes might be crudded up reducing output. Thinking about pulling mine apart for fun to clean up. If I do in next few days I will post on my M99Ocho build post.

I hope this helps and is a better reply than most will attempt to post. It honestly takes 2 minutes to post an appropriate response to someone's question if you know what their skill and knowledge are versus just posting some short answer that is vague. I like to give the Most in my Post..

Ocho out

Thank you! I tried to adjust the voltage as described and it did nothing. Stayed at 24.2 or so volts. I decided to go ahead and just replace the darn thing. Stung a little, but just a visual examination of the generator (even with my untrained eye) suggested that, even if I get it to work this time around, it won't be long until I'm swearing at it again. Eriks Military sold me a reman for $510 plus shipping. I've had good luck with those guys: They are reasonably priced and fast on shipping.

 

[kvoyson]

So, I bought my 1989 M998 (6.2l) running for $8,500 in October. Got it home, and ran fast and furious and lubed it, replaced the batteries, fixed fuel return lines, resealed power steering gear box, replaced some rubber boots, made safety equipment upgrades, replaced/repaired lights and gauges, got it re-titled and road legal, finally 4-man soft top kit on it. So right now, I'm at about $13k into it. Don't get me wrong, I love it. I like to go outside just to look at it.


To the point: I've driven back and forth to work a few times and around my small town. Turns out, the batteries aren't charging. Got stuck in the dairy queen drive through the other night after I turned it off to order my ice cream (didn't get the ice cream). Had to push it out of the drive through and tow it home. I'm not a mechanic, I'm a working mans lawyer. I'm bleeding money and I can't afford to make a costly mistake. My alternator shows to be putting out 24v max (volt gauge used to stay in the green, started to flux into yellow and then back to green, now just yellow). Pulled the cap off the 60amp prestolite and it looks rough. I'm assuming the alternator, which appears to have been installed at the factory back in big hair days, is going to meet its maker. I've gone through the manuals, looked at the diagnostic manuals, I've checked other threads and none directly help. The learning curve is almost too huge to overcome. *** If I buy a reman or new 60amp, without having the time or knowledge to further diagnose, could I be buying a part I don't need, or buying a part I need but not buying the other parts that I need to go with it? Is it as simple as plug and play? This thing is consuming me and I need to get back to work. (FYI - I don't need a lecture or to be made to feel dumber than I already do).View attachment 785043

I’m in the same boat, I’ve replaced virtually everything including alternator and smart box
I’ll continue reading

 

[Mogman]

I’m in the same boat, I’ve replaced virtually everything including alternator and smart box
I’ll continue reading

DETAILS! what alternator and reg? what rig? (M889, A1, A2 etc.)

 

[Glider]

I am finding myself in need of a good regulator. I have the kind that looks like a Starburst. This will be my second replacement. The brushes looked good previously when I had it open and I blew everything out. It worked flawlessly until it quit altogether. Now its 24.3 on or off, it has voltage to the exciter. Last one was a ebay NOS. Not sure if China or not but it didn't last long.

If anyone knows a good economical source please let me know the name.

 

[papakb]

Southern Automotive: https://southernautomotive.com/cata...ucts_id/101?osCsid=usefdtolona5ogtdvlddr23ft2

 

[Glider]

Southern Directed me to this ebay link: https://www.ebay.com/itm/3134204642...JnGiMa5Uxk2Obbjh6U+ZY9THs=|tkp:Bk9SR-jHrKbfYA

Apparently, there are two sizes for this alternator that look identical(one metric one standard. As i mentioned earlier this will be my second regulator. When I bought the last one the diameter was slightly larger. I thought it was a manufacturing error put in the freezer and then tapped in with a wooden block. The code that was applicable on the alternator ended in 005. the code is easy to find with the alternator removed but difficult to see when installed. I had to use an inspection mirror to see while standing in the engine bay.

 

[kvoyson]

DETAILS! what alternator and reg? what rig? (M889, A1, A2 etc.)

M998 100 amp niehoff dual voltage
I’m finding that the ign lug on regulator the exciter is not getting power to turn on the alternator.
can’t find any schematics for that are related to that circuit . I’m wondering if my keyed ignition is possibly the problem? Thanks