“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.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Transferring parts from old alternator to new alternator: First remove the nut and lock washer holding the pulley, keyed shim and alternator cooling fan to the alternator shaft. I used the alternator mounting bolt wedged between the fins of the fan to immobilize the shaft and a 22 mm box end wrench to loosen the bolt. It was a little hard to remove the pulley. I did not have my puller readily available so I just slowly drove off the pulley by wedging a bolt end between the housing and the fan and tapping the bolt a couple of times, repositioning the bolt and tapping again until I drove it off the shaft. I transferred the pulley, keyed shim and fan to the new alternator. I used the new wave washer and nut that was supplied. The new alternator shaft has a hex head recess so it was very easy to tighten the nut held by the box end wrench and the shaft held by a hex head socket. The next part to remove is the condenser, which reduces radio interference from the alternator (it is the black plastic box with the spade connector at 9 o’clock and grounding screw at 6 o’clock on the top picture of the back of the old alternator). I connected the condenser to the spade connector at 7 o’clock and mounted the grounding screw at the tapped hole at about 5 o’clock on the bottom picture of the back of the new alternator.

Installing new alternator: I oriented the old alternator adjusting bracket on top of the new alternator and from underneath the car, re-positioned the alternator into the bracket which is bolted to the block. I installed the lower alternator mounting bolt first and then from above installed the upper bolt (but only finger tight). I installed the used belts, adjusted the belt tension with the 13 mm adjusting nut and then tightened the three 17 mm bolts (two on the back of the adjusting bracket and the lower one on the alternator bracket).

Wiring to the new alternator: The existing harness was modified. The spade connectors of the two red wires were trimmed away and new heavy duty ring lug connectors were crimped onto the two red wires (8 gauge). The thin blue exciter wire was not modified except for freeing it from the old black plastic connector plug. (The frayed sensor wire for the thermostat temperature sender was repaired with a butt splice connector). The two red wires connect to the large unmarked post (usually called B+) on the new alternator and the thin blue wire connects the spade connector on the small post of the D+ terminal. People on the forum have expressed concern over current carrying ability of the stock wiring, so I added additional wiring. I purchased 2 feet of red and black 6 gauge marine grade tinned copper battery cable wire and 6 gauge lug connectors. I installed one end of the extra black cable to the alternator housing (under the condenser mounting screw) and the other end to the negative battery cable where it attaches at the body. I installed the extra red power cable to the large post (B+) and the other end to the positive terminal in the junction box beneath the battery. All the connection ends were treated with electric contact cleaner and then electric contact conditioner.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Testing new alternator: With all the wires connected to the alternator, I temporarily connected the battery and took voltage readings with the car off and then while running. I got -12.63 volts with the car off and with the car running I got -14.40 volts. So I shut of the car, disconnected the battery re-installed the battery tray and battery and checked the battery voltage closely over the next few days.

I panicked when the glowplug light failed to illuminate on startup, I thought I created an improper circuit. I disconncected the new 6 gauge power connection (red wire) and retested the glowplug light. It still would not light up, I figured the bulb decided to burn out at that exact moment when I replaced the alternator. (It ending up being worn contacts in the ignition switch, as yesterday I could get the glowpug light to illuminate by jiggling the key in the number two position.)

At idle, with heavy load, the voltage doesn’t dip below -14.00 volts and the battery voltage stays a stable -12.77 volts when the car sits overnight. No troubles with charging or voltage for the past 9 days. Yay!

‘Desert Rose” is back in service as a dependable daily driver and she is ready for new lighting options and a new ignition switch.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

hi mate

help me with simplified scheme from pic bellow
which 1,2,3 to A,B,C has to be hacked?

1 and 2 to A or just 1 to A....etc...

thanks

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w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered

Hi cho,


The wires from 1 & 2 connect to A with new ring wire connectors.
The wire from 3 connects to B with the spade connector (or new ring connector if the B on your new alternator doesn't have a spade connection).

The condenser spade connector on 4 goes to C.
The condenser mounting screw on 5 goes to D.

The extra ground connection cable goes from E to the body where the main ground cable from the battery attaches. Alternatively, the extra ground connection cable can go directly to the negative battery cable.

The extra positive connection cable goes from A to the terminal block (picture 1 on post 3). Alternatively, the extra positive connection cable can go directly to the positive battery cable.

I hope this is helpful.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

.

great "wire by wire" explanation

thanks mate !!

.

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w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered

Hi Alex,
Thanks for the great write-up, I will be converting "Bertha" my 80 300sd.
Ross