Alternator on its way out?

robbyb20

Well-known member
Oct 2, 2019
146
16
Seattle
Over the past 2 weeks my alt has been displaying lower voltages when the engine has warmed up.

Following readings without any accessories on
Cold start - 14.2v
30 min running - 13.8v
1-2hrs driving 12.6-13v

Battery is holding charge/still being charged after every run at 12.5v.

Belts/pulley new within the year.

I had to replace the alt this summer after it burned out on a road trip. The original replacement was a Bosch AL9348X which at its hugest output 13.8v. I brought that back to my local LR mechanic and swapped it out for a different (same model) one which then just started to not work at all. Went to oriellys and bought an Ultima alt and that’s what’s currently in there.

While getting a new alt isn’t an issue (I have one on order), I’m interested what else could cause this. Doing a search on here returns some results but those all seemed be consistent low voltages the whole time. Mine is only after it heats up.

EDIT: rebuilt alt fixed issue.
 
Last edited:

robertf

Well-known member
Jan 22, 2006
4,411
145
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PM had a rebuild writeup somewhere on here or nas-row. After years of accepting 13.8ish I finally rebuilt one and its 14.2 all the time. Basically just replace the brushes and bearings. I forgot how much better the headlights were when these things were new.
 
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robbyb20

Well-known member
Oct 2, 2019
146
16
Seattle
PM had a rebuild writeup somewhere on here or nas-row. After years of accepting 13.8ish I finally rebuilt one and its 14.2 all the time. Basically just replace the brushes and bearings. I forgot how much better the headlights were when these things were new.

Something like this work?
 

robertf

Well-known member
Jan 22, 2006
4,411
145
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Thats it. Pick up an extra slip ring. You’re almost guaranteed to destroy the first one you attempt
 

robbyb20

Well-known member
Oct 2, 2019
146
16
Seattle
This seems to be the tutorial for anyone else that might be looking for it at some point.

 

robbyb20

Well-known member
Oct 2, 2019
146
16
Seattle
Turns out its probably grounds. Reassembled a "Bad" Bosch alternator im waiting on parts for and after putting a multimeter on alt positive and alt frame i get 14.2, any other combination that includes the battery yeilds the lower voltage.

I will taking a look at the battery ground to chassis/frame, strap from starter to frame and engine block strap (I think is there) on lh side. Read theres also one to t-case? Any other locations I should check for?
 
Last edited:

JUKE179r

Well-known member
Sep 14, 2016
621
61
Suffolk, UK
From what I've seen from past experiences is that webbed grounding strap from the LHD rear side of the engine to the fire wall is either disconnected or badly corroded. I removed it and made a 10AWG cable as it's replacement.
I also made a 0AWG cable from the alternator housing to the grounding bolt (at my battery disconnect switch) for better grounding.
InkedAlternator ground cable_LI.jpg
 

robbyb20

Well-known member
Oct 2, 2019
146
16
Seattle
From what I've seen from past experiences is that webbed grounding strap from the LHD rear side of the engine to the fire wall is either disconnected or badly corroded. I removed it and made a 10AWG cable as it's replacement.
I also made a 0AWG cable from the alternator housing to the grounding bolt (at my battery disconnect switch) for better grounding.
View attachment 60457
Thanks for the suggestion. I may end up going this route if the new ground straps i ordered (none at local auto shops) doesnt work out. I checked the grounding cable from battery to chassis and frame and cleaned them as well as the one from the starter to the frame. The starter was a bit loose (ill have to address that too) and the strap looks like its seen better days. Once i tightened up the starter its working a bit better but having gone on any long drives yet.

On the D1 there doesnt seem to be a grounding strap from the LH side of engine block to body but there is a bolt/retainer that doesnt seem to be used that i can repurpose and add an extra one there.

Im going to take the opportunity and do as much as i can for the electrical system now and hopefully not have to worry about it for a bit.
 

robertf

Well-known member
Jan 22, 2006
4,411
145
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On the D1 there doesnt seem to be a grounding strap from the LH side of engine block to body but there is a bolt/retainer that doesnt seem to be used that i can repurpose and add an extra one there.

I could be wrong, but I believe that ground strap is only on 3.9 trucks and was an afterthought for the remote mounted ignition module