1985 300CD

I just installed a bosch 120 amp alternator from this guy Mercedes Alternator W116 W123 W115 W114 W111 W107 High Amp 120 Amp Generator | eBay

I made a new wiring harness since it has posts on the back instead of spades. I used #2 welding cable to the terminal on the fender and I spliced the D+ in to the wire that comes from the dash.

I fired her up and the battery light stayed on so i checked and my voltage was 12.5 which was just battery voltage. I revved it and the voltage jumped but went back to battery voltage at idle. I pulled it and took it to an alternator shop, they loaded tested it and said it worked just fine. I asked how fast they were spinning it and they said their machine was spinning about as low as it will go, 600 rpm. I ended up buying a new h8 interstate battery from him and just reinstalled the alt.

The pulley is 67mm O.D. which is slightly smaller than the stock 65 amp bosch. At idle there is no charging. My idle is about 750/800 rpm. The alt starts charging at 900/950 rpm. He has a slightly smaller pulley he will put on. It's hard to find anything smaller than 67mm when it comes to double groove pulleys.

If I can find a smaller pulley, do you think it will make a difference? Battery and voltage regulator are new, and dummy light works fine.

Provided your custom wiring is correct, I'd suggest check/ clean/ replace if bad the battery cables and ground strap. Also wire brush the alternator mount points since that is the ground path for it. You had the alternator bench tested so it's not the alt. Belt properly tensioned/ not slipping?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The wiring is correct. I thought about running a ground wire from the alt to the chassis in case the connection through the mount was bad. The battery cables are in great shape and the chassis ground point has been sanded.

The belts are good and not slipping. When did Contitech move their production to China?

Its not unusual for a high amp alternator to show little or no improvement over stock at low RPM. The pulley may help, just make sure its within the adjustment range of the belt system. Did the rebuilder supply the alternator curve by any chance?

Need more information.

Are you using an external regulator? ( did you try the old one? )

Where are you checking voltage?

What voltage did you get with the engine at say 2,500 RPM?

What was the batteries state of charge when you tested voltage? A discharged battery will show low voltage at idle until it charges up.

There's an outside chance you got a bad alternator? I had a 150 amp alternator because of various electrical goodies, and it bolted right up and worked just fine. I did upgrade the cables. Plus, I went to a double pulley but the pulley size was the same. If it's a NA application, you will need to get a custom crank pulley. The only thing which comes to mind is a grounding issue.

Did you check connection at the starter? Batt + cable goes to starter then up to terminal box where alternator connects.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Another thought is that if the regulator does not provide enough field voltage, charging voltage at idle will be low. This can especially be true with a high amp alt that needs greater field voltage and has less idle charging output.

Did you try full fielding the alternator at idle and check charging voltage? If the alt puts out more voltage at idle when full fielded, you have a regulator / wiring problem.

I forgot to mention -- worst case scenario, take it to Autozone or Advance Auto and have them spin it. They can diagnose it for free.

About the pulley, the smaller the pulley faster it will run. If the old pulley was bigger than the actual, your alternator now is spinning faster than the old one. It concludes that pulley is not the problem. I have in mind only one reason for this problem: alternator brushes and or rectifier/regulator are not working properly. If it is the rectifier it is easy to replace. I replaced mine not removing the alternator from the car. A not working properly rectifier or regulator will give this kind of symptom. Dashboard red light will light up showing battery not charging and low voltage to the battery.

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Nyck - TX - USA

Reading through this again, I'm concerned that the light doesn't go out at idle. And that may be the clue: there is an extra set of diodes in the alternator called the trio, which supplies D+ power to the regulator when everything is running. If one of the trio diodes is bad, there may not be enough power going to the regulator at low speeds. You'd still see a fairly good charge at high speeds, because the six main diodes are ok. It would not be apparent on a test rig if the rig supplies 12V directly to the D+ terminal. You'd need to rig it so that something like 4 or 5V is supplied to D+ at startup. Have him put a resistor inline with D+.

In other words, bad alternator, ineffective test. Drive the car to the shop and have him verify the current output on the car.

FYI, 600 RPM on the test machine is 200-300 engine RPM, depending on the pulley ratio.

Pulley ratio is one thing that many stores where alternators are tested do not take much consideration. I really agree with you that RRPM in test machines and in the car must be the same when alternators are tested.
Good point.

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Nyck - TX - USA

On the Alternator Connector the 2 large slots should always have Battery voltage even when the Key is off.

The smaller slot should have no voltage until the Key is turnd on and then the voltage will drop a bit.

The Bulb in the charging light needs to be at least 3 watts.

Below is a drawing of a test cord I made using an alternator connector. The 3 watt bulb in the circuit takes the place of the Alternator Charging Light the supplies enough resistance to excite the alternator to charge.

Some of our members have added an extra ground wire from the alternator housing to the negative battery termnal.

Attached Thumbnails

   

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84 300D, 82 Volvo 244Gl Diesel

That's a very dangerous tool, especially with a 110A alternator. If the battery state of charge is low, the entire 110A would be carried by the two skinny wires. For about 5 seconds that is, before they go incandescent.

Resistance isn't what excites the alternator, it's voltage. An interesting test would be to compare the voltage at D+ and B+ while the engine is running (easy to do with his terminal configuration). If it isn't exactly the same as the B+ voltage, then the D+ circuit is compromised internally. But then we already know that, because the light comes on at idle...

I think the bad D+ diode is a good idea. On the bench he probably had a higher amp signal sent to D+ than my litttle dash light would supply.

No info came with the alternator.

Here's an update: With the new smaller pulley installed, i get 13.8 volts at idle. But....... The dash light is on and I dont get any charging when I first start the car. A slight tap and release of the pedal and she starts charging.

Also, when I start loading the alt, like turning on a/c with fan on high, aux fan running and headlights on, the voltage drops to battery voltage at idle. Once I get up to just over 1000rpm, voltage will jump up closer to 14.

Shouldn't the voltage regulator keep the output near 13.8v no matter what the load or engine speed?