Burned up my diodes in 300D alternator. R&R difficult - PeachParts Mercedes-Benz Forum

HughO · #1 08-20-2024, 03:16 PM

Dumb dumb careless stupid. Yup I reversed my cables jumping my 85 300D with my Cummins. This can fry a lot of things like alternators. Thank goodness no ECU or motherboards! I got the engine running and I checked the charging. and had none. A diode check showed diode death. I have 3 W123 cars which I have had for 2 decades and have never replaced an alternator. Brushes a few times but that is a 10 min job.R&R on a W123 alternator is a terrible job as some of you know. It's an access issue. There are 2 17 mm bolts, a bottom pivot and an upper securing bolt after you adjust the belt(s) tension. I did the job with a Bendpak commercial lift and it still took 2 hours. To do the job you remove the air cleaner and turbo boost tube. Loosening the lower 17mm pivot bolt was easy. The upper 17mm is inaccessible with normal wrenches and sockets as it is out of sight behind the high pressure AC line bracket. I had to make a 17mm wrench with a 90 degree sideways bend to loosen it. Very slow process loosening it a few degrees at a time enough to spin the 13 mm belt adjustment nut with an electrical ratchet. Belts off. Then I had to remove the plate protecting the charge plug and access to the 6 mm nuts was a bear. Terrible access. Last job after taking off the charging plug was driving the bolts back out of the alternator but the The AC bracket was tight to the bolt making that impossible not to mention no easy way to tap the bolts back. The AC line/bracket is attached to the alternator engine mount and that has to be backed out enough to get the alternator bolts to clear. You need to clean and polish the bolts and nuts so they spin super freely because you have to feed them back in and tighten by hand. A little silicone grease around the bolts helps as well. Reassembly and remounting my reman Bosch from ******** went relatively easy. I am an old man and doing this under the car on a creeper would have been an all day affair if I even could have done it. I would advise anybody who doesn't have my special bent 17 mm wrench and a lift to skip the job and have it done by folks with the proper tools. I have two older Toyotas. The job on them takes 15 minutes as the alternators are on top of the engine where they belong out of the the mud and water. I speak german and I was using all my long forgotten swear words. Access on my 240D looks much easier without AC and a Turbo.

Diesel911 · #2 08-20-2024, 11:45 PM

Quote:

Originally Posted by HughO

Dumb dumb careless stupid. Yup I reversed my cables jumping my 85 300D with my Cummins. This can fry a lot of things like alternators. Thank goodness no ECU or motherboards! I got the engine running and I checked the charging. and had none. A diode check showed diode death. I have 3 W123 cars which I have had for 2 decades and have never replaced an alternator. Brushes a few times but that is a 10 min job.R&R on a W123 alternator is a terrible job as some of you know. It's an access issue. There are 2 17 mm bolts, a bottom pivot and an upper securing bolt after you adjust the belt(s) tension. I did the job with a Bendpak commercial lift and it still took 2 hours. To do the job you remove the air cleaner and turbo boost tube. Loosening the lower 17mm pivot bolt was easy. The upper 17mm is inaccessible with normal wrenches and sockets as it is out of sight behind the high pressure AC line bracket. I had to make a 17mm wrench with a 90 degree sideways bend to loosen it. Very slow process loosening it a few degrees at a time enough to spin the 13 mm belt adjustment nut with an electrical ratchet. Belts off. Then I had to remove the plate protecting the charge plug and access to the 6 mm nuts was a bear. Terrible access. Last job after taking off the charging plug was driving the bolts back out of the alternator but the The AC bracket was tight to the bolt making that impossible not to mention no easy way to tap the bolts back. The AC line/bracket is attached to the alternator engine mount and that has to be backed out enough to get the alternator bolts to clear. You need to clean and polish the bolts and nuts so they spin super freely because you have to feed them back in and tighten by hand. A little silicone grease around the bolts helps as well. Reassembly and remounting my reman Bosch from ******** went relatively easy. I am an old man and doing this under the car on a creeper would have been an all day affair if I even could have done it. I would advise anybody who doesn't have my special bent 17 mm wrench and a lift to skip the job and have it done by folks with the proper tools. I have two older Toyotas. The job on them takes 15 minutes as the alternators are on top of the engine where they belong out of the the mud and water. I speak german and I was using all my long forgotten swear words. Access on my 240D looks much easier without AC and a Turbo.

At one time or another we have all spent time in that zone.

From pictures I have seen on old 300Ds the AC hoses and so on are not all run the same. I don't recall havening an issue except with the very bottom pivot bolt. And even that was not terrible.

However, I was a diesel mechanic for 18 years and obstruction of what you need to get at is just normal.

BillGrissom · #3 08-23-2024, 10:01 PM

I have much easier access to the alternator mounting bolts after I removed the EGR valve (used Rollguy's block-off plates) and ran the H.P. hose straight over to the condenser (secured under radiator support). Perhaps they made that convoluted routing to make factory assembly faster. But my 1985 CA 300D has a heat shield on the aft end of the alternator which still makes wrench selection fussy. I wonder how I changed the fan belts after I first got the car, with all that clutter in the way. I remember now, ~2 hrs of cussing like you.

vwnate1 · #4 09-02-2024, 08:09 PM

I too discovered how not fun this job was in West (by God) TEXAS on my '78 300CD in a parking lot under the sun using the thin factory open end wrenches .

After you've done it a couple times it becomes a 15 minute job . this is very typical of all the German vehicles I've worked on over the decades ~ there's a specific way to do things, once you discover it things are easier if not easy .

Might you possibly be willing to post a picture of the wrench you made ? .

I'm always keep to learn new things .

Of, if you no longer need/want it, send me a PM and I'll buy it .

Lastly, everyone who drives a W123 Diesel should know that most F.L.A.P.S. will have the 1985 300D alternator in stock and it has a slightly higher output and bolts right in .