85 300D overcharging - again
 

About 2 weeks ago my battery went dead. The battery is fairly new so I suspected my alternator. I charged the battery and it was OK for a day or two and went dead again. I checked the alternator and it was not charging. I charged the battery again and drove the car. I noticed that the windows were working well (for no charging) so I got home and checked the voltage again and now it was at 16.7. I changed the regulator from a known good alternator on my shelf and viola!, 13.8 volts. All was fine for a week now. I put an aftermarket radio in today and it kept shutting off. Finally I realized that it shut off everytime I accelerated. I turned on the lights and blower fan and that ended that. So... it's over charging again. I noticed in a picture of a new alternator that it had a ground tab beside the main plug. I do not have that. Could that be making it over charge? I hate to buy a new regulator, when I'm sure the one I put on was fine. Can anything go bad in the alternator itself that causes this? Thanks, Andy

Andy,
You need to clean all the connections, not just on the outside, both 1/2s need to show bright clean metal. Your earth strap likewise.

Is the idiot light coming on when you turn the key & then going out when the car starts?

Please report back after cleaning & tell us what the red light on the dash is doing.

85 300D over charging again
 

Yes, I've done all the cleaning. I'm fanatical about this kind of stuff. The dash light works properly. When you rev the car up, it makes too much voltage. I will check the engine strap tomorrow. I read about guys running a jumper cable from the alternator to the negative terminal but they don't say what it cures, under charge I'm thinking. I noticed that when I was at speed, the blower fan increased in speed as well. It's odd to me that another regulator would exhibit the same issues after a week or so. Thanks for the reply and I'll keep you posted.

Jumper lead is acting as an earth.
If you are killing regs, it may be a bad earth allowing it to happen. It is making too much voltage because the reg is telling it to (providing too high primary voltage).

"Jumper lead is acting as an earth." In American type English "Earth" translates to Ground.

You ought to consider having a Free Alternator test done.

Better, would be to have an Alternator Test done at someplace with a Scope. From the wave form display they can tell you if any of the Diodes are burned.

If there is a poor ground any were from the Alternator to the Battery the increased Resistance can increase the Voltage. That could even mean that where the Alternator Bolts up to the Engine has a poor ground due to Corrosion or the Engine to Chassis Ground Strap contact area is not good.
The Idea of running a Jumper Cable to the Alternator Housing and to the (-) Battery Terminal is to bypass all of the other Ground connections. And, it will do that with the exception of the Ground area between the Voltage Regulator and the Alternator Frame/Housing.

Remove the Voltage Regulator and scrape down to bare metal the area where the Regulator Grounds against the Alternator Frame at one of the Screw Holes.

85 300D overcharging again
 

Well...tonight I ran another ground to the engine / alternator and tested the voltage. 16.7 If I turn on the lights and fan, it drops to 13.8 - until I rev it up and then it's back to 16. I'm wondering about the battery junction (on fender well by battery) though. The PO had messed this all up and I got a new one and installed it. Could a wire be in the wrong spot and let everything run fine except the voltage? I've only had the car for 5 weeks so maybe it's been high all this time. I can't find a wiring diagram that shows the three poles individualy and what wires go to them. Neither my books or my CD Rom show it in any detail.
Any thoughts on this? I'm going to get the alt. tested later tonight and the battery as well. I'm still wondering about a ground to the inside of the alternator though. I've seen pics of a spade terminal next to the main connector on new alternators. I do not have a wire for this (that I can see). Thanks for all the help guys. Andy

Depending on the brand of the regulator some where flakey and not reliable. Many aftermarket parts for starters and alternators are cheap but they also can be junk. Go grab one off a wreck and try it is one alternative. You cannot totally rule out that the regulator you put on has not failed.

85 300D overcharging again
 

No doubt Barry. It is a Bosch and when I installed it a week or so ago the voltage was perfect across the board. I'm wondering if the car is killing them?

Classic sign of a failed reg.
Now you need to work out what caused it.
My guess is that you have a bad connection that some times disconnects the alt main wire. It then has no battery on the end & if left running fries the reg.
On my second 300D I had to replace the main wire from the alt to the + terminal of the battery. It could also be the earth/ground strap.

If grounding is the problem, there is a possibilty that the alternator is not grounding properly to the engine even though the engine is well grounded to the chasis. The w123 cars actually come with a ground strap that connects from the alternator housing to the engine mount bracket on the same side.

85 300D overcharging again
 

EZRider - this ground strap for the alternator is what I have been asking about. Mine does not have one. I did ground the alt body to the battery but this made no difference. Maybe the damage is done and it won't? I'm still trying to find a reliable wiring diagram for the junction box that all these things go to. Thanks for your reply.