DIY - AL129X alternator in a 1978 W116 300SD

[Alec300SD]

“Desert Rose” started having alternator trouble a few weeks ago. Because of cold weather and rain, I started using more electrical circuits (lights, heater blower motor, wipers, radio and double glowing on cold mornings) during my commute.

At first the battery light began to flicker at idle, even going out at higher revs and then eventually stayed on all the time. I checked for loose belts and loose connections, cleaned the battery terminals. I pulled the voltage regulator, it looked worn and one of the brushes was sticking, failing to rebound when pressed.

The back of the alternator including the connector plug was oil-soaked (seepage from the crankcase oil vapor recovery line going into the U-tube on the turbo) so I installed a new Bosch voltage regulator, charged the battery overnight and solved the problem…or so it seemed. The battery light came on again during the last ten miles of my commute the very next day. The alternator was only putting out -12.2 volts at idle and -12.4 volts at higher revs.

In retrospect, cleaning the alternator connections with some brake cleaner may have solved the problem. However, I planned to eventually upgrade the alternator anyhow, because I want to update the lighting from stock sealed beams to either euro lights or Hella euro code halogen lights. So it was time for a new alternator.

To prevent oil seepage in the future, I will be installing a catch can (like I have on my Volvo wagon), where only the relatively clean mist is vented into the air cleaner housing.

From what I could gather from reading previous threads and posts, the best upgrade for the OM617 motors seems to be the 115 amp AL129 X. People have reported installing them successfully in W123s and Squiggle Dog installed a similarly sized 80 amp AL69X in his W116. So I felt confident attacking this upgrade. I sourced a rebuilt (in Germany) AL129X from my indie mechanic. The cost will be between $215 and $265 depending on whether or not my undersized core will be accepted or not.

Below are the steps I went through. Overall the process went smoothly.

Safety first…as always! I worked in my driveway as I don’t have a garage to use for work on my cars. A heated garage with a lift would be nice! Anyway my setup was: PS front wheel on a Rhino ramp (to have more room to access the alternator from below), PS rear wheel chocked (to prevent rollaway), radio off (to protect the radio microprocessor from spikes when reconnecting the battery), and then I disconnected and removed the battery (no sparks from tool contact) and battery tray (easier access to wiring). I used a large flatten corrugated box to lie on when under the car (prevent scrapes from crawling under car and stay warm) and nitrile gloves.

Removal of old alternator: Unplug harness from temperature sensor on thermostat, unplug harness from alternator. Loosen the two 17 mm bolts on the back side of the alternator adjusting bracket. Loosen the 17 mm bolt on the bottom of the alternator bracket. Loosen the 13 mm turnbuckle adjusting nut (ccw) on the alternator bracket until there is enough slack to remove both alternator belts from the alternator pulley. Remove the two alternator mounting bolts. From underneath the car, remove the alternator with adjusting bracket attached. Use two hands as it is heavy and you need to pivot the alternator twice to remove it from below. I found it came out easiest with the pulley pointing at the ground and the 13 mm adjusting bolt pointing at the engine block.

Note: you may want to remove the windshield washer pump from the frame rail first. On removing the alternator it slipped and stuck the top of the plastic housing on the pump and broke off one of the barbed hose fittings. Time for a new or used unit.