Hey everyone,

On my 420SEL. I replaced the stock alt with a AL129X rebuilt from AutoZone...first replacement unit had a bad bearing.. after a week I replaced it with a replacement unit..

Now a week later..the car is dead. Battery read 7V across it.

I throw on the charger..a few hours later I get the car started.

I measured battery voltage at 11.9V..clearly no alt.

I measure from the alt's power cable at the distribution block to ground...26V!

I performed continuity tests between the battery's positive terminal and the upper half of the two post distribution point.. got continuity there but nothing between the battery and the lower point... WEIRD.

So I remove the alt's cable and check voltage..50V!

I think the regulator is..broken.

Anyone know the replacement regulator part number for the AL129X?

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Resolved!

I replaced the regulator with an older one I had on the shelf here...Brushes are at 60% but I will be ordering a new one shortly.

Once I re-installed everything I found that I still had 30V at the alternator wire on the electrical distribution block.

I probed around a bit and found that the alternator terminal was not making very good connection. I fixed that and saw the normal expected voltage at the battery.

I also found that I have ~ 0.5 V drop between the alternator line at the dist block and the battery.

With everything fully on/loaded I measured 12.8V across the battery but 13.4V at the alternator line.

I'll have to re-do my distribution setup so that the alternator is directly connected to the battery, like I have in my 190E and 300D.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k