Hello friends,

I was in the process of rebuilding my alternator...essentially changing the regulator module. I found only one diode, and that checked out OK. Of course I mangled the crap out of it getting it to the meter, so I had to resort to attaching a "prosthesis" because I couldn't find a replacement. This has me wondering?

If this is a 3-phase alternator, why can I find only a single diode? Also, is there a rating or number for this diode in case I wanted to pick one up at an electronics store?

I have checked everything I could find in the alternator. Aside from the brushes being totally spent and not knowing the condition of the regulator transistor, the coils are not open and the bearing seems OK. Before I put the thing back together with the new regulator module and the surgically restored diode with a slight disability, am I overlooking something that may come back to bite my ass when I head for South Dakota this weekend?

Thanks for the advice...

Joseph Monk
1985 190E War Pony

Was it a big diode or a little one? The big diodes are arranged as a 3 phase full wave bridge configuration. They usually have some heat sink fins on them. They dont look like a classic diode. More of a power rectifier assembly. They are impossible to check individually without disconnecting the stator wires. If one diode in the assembly is bad, it cannot usually be replaced as a single diode.

There is also several small diodes which provide power to the voltage regulator.

So which diode (assembly) was it?

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Thank you Dave for pointing that out. Now you've given me several questions.

Yes, the diode I checked was the small (zenor?) diode and it was good. I also see the difficulty in testing the main diodes.

If I cut the stator wires, how should I reconnect them? Will heavy duty solder work?

The diodes appear to be brazed which is understandable, however, how the hell did those things get brazed together without incinerating the bakelite module or the diodes themselves?

If, in fact, one of the diodes is bad, where might I find an assembly with mandatory travel this weekend?

I don't know for a fact that any diode is bad, but I wanted to check since I have the bad boy split open...you know sort of like an incidental appendectomy.

I am thoroughly convinced that both brushes are shot, both having less than 1/4" closer to 1/8" remaining. I have the new regulator module and am ready to close it back up. But heck, I just don't like the idea of doing double work if I have a bad diode.

Nor do I feel like paying upwards of $150 for essentially (six) $1.49 diodes, another voltage regulator, and a hunk of metal and windings that have experienced the very same wear and tear, if not more.

I'm open for alternatives to buying a rebuilt alternator. It just rubs me the wrong way...only so much can fail. If time and distance were not a factor here, I'd be more willing to play Dr. Kildare with these diodes. But I'm still willing to give it the college try if the re-connection and possible diode bridge replacement problem can be resolved in a very timely manner.

Thanks again for all of your input

Joseph Monk

I concur with Dave. If you only had 1/4 to 1/8 of an inch of brushes - that most likely was your problem.

The only way to test the power diodes is to apply a rotating magnetic field to the stator coils and look at the resulting waveform at the battery terminals with an oscilloscope. Analyzing the rectified waveform will show the condition of the diodes. The bottom line is put it all back together and test it. The small diode is not a zener just a regular 1n4004 or something like that.

The power rectifiers are spot welded or plasma welded. They can be disconnected and re-soldered but, you better know what you are doing. Generally the problem is in the regulator or the brushes. Rarely but sometimes it is in the windings and power rectifier assembly.

The diodes can be had at any shop that rebuilds alternators. All the rebuilders do is: brushes, bearings, regulators & diodes.

Sounds like you dont have the right parts to the job and probably cant get them, so I would just cave in and buy the rebuilt one. Your first mission is to locate a source of parts. After that its easy.

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

OK, your wisdom prevails. Plasma welds? Wow some high tech stuff, ennit? I put the alternator it back together again last night. My nephew is going to install it back into the car today... good to have helping hands. Hopefully the problem is cured. Thanks again.

Joseph

After about 3,000 miles of travel through this land, from Ohio to South Dakota and back...total backstreets way instead of highway, I had no problems with the alternator, or car, for that matter. I am convinced that the only thing needing replacement was the regulator module. Thanks to Dave and the others for the helpful recommendations and comments, and keeping me from looking for problems where there were none (e.g., diode cluster).

My nephew was totally impressed with the "plasma welds" BTW. He said damn, they make these cars to last! Also, if you don't think you have time to tour the country on your next trip using state routes, think it over again.

My drive time was the same 19.5 hours, whether I took the freeways or the state routes. However, because the state routes were more direct, I saved about two tankfuls of gas. And boy, driving for miles right beside the Mississippi was a real treat. The countrside is absolutely fantastic! Try the "long way" somtime...its quite enjoyable.

Joseph Monk
1985 190E War Pony (still!)
156K