Generator Upgrade

[Nicolas Aristodemou]

I have noted on the dashboard of my App for 123 Bluetooth ignition that the voltage output of my generator under load drops to between 12.6-13V depending on load. When under no load or under moderate load my voltage is around 13.8V as it should. I had the readings verified at my car electrician and they are correct. Furthermore, I had the alternator removed and checks out ok outputting 35-40A. Voltage regulator was also replaced with new one just to be sure. Clearly a case of some form of upgrade being required. With lights on and AC running I am afraid the battery will drain. What is the easiest way to have my unit upgraded? Do larger generators fit? If so where can such units  be sourced from?

[Tyler S]

You need to check the wiring that goes from the back of the alternator to the starter B+ post. It is often melted or damaged. Usually repairing this will solve the issues you have.
Do not attempt to install a larger alternator without changing this wire to a larger gauge or things could get hot. See post below. Your numbers aren’t far off though. Is this at idle? What happens to the voltage when you add some throttle?

https://www.sl113.org/forums/index.php?topic=24660.0

[lpeterssen]

Dear friends

Agree with Tyler, you have a T30 degraded cable which is common on this 50 year old aprox MB.

A damaged insulation at the beginning, before it gets worse and causes a short circuit, allows the humidity to get in contact with the copper conductor causing corrosion which affects its conductivity and leads to unexplained voltage drops.  Also in some cases depending on how close are these conductors to chassis parts and the humidity in the environment it causes higher that normal battery self discharge rates.

I see , as engineer, that if you plan to use A/C system with this same alternator of 35A that loads are not balancing and you will have depleted batteries often.

You have to upgrade the alternator for something like BOSCH ALX66 Which is a direct fit with internal regulation, and improve original wiring harness capacity for the charge sub circuits.

In my cars I do a full job upgrading many things, but the minimum you should modify  is the cable from alternator to starter T30 (B+) post from original size 4 mm2 to at least 10 mm2.  I made some time a post explaining the calculation in all detail.

Use only MARINE GRADE FULL COPPER CABLE, which can be sourced in Amazon or any other electrical stuff provider.  A boat shop will have it.  Why this cable? Because it’s specs are much better than original cable used on your MB.  Is much more flexible, water resistance, humidity, heat and oil resistance.

Buy grade AWG 8.   You need only about 1.5 meters. Buy quality terminals in copper and solder them.

Any question per PM are welcome.

Best regards
Eng. Leonardo Peterssen

[Nicolas Aristodemou]

Thanks Tyler and Leonardo for the guidance. The wire you are talking about (generator to starter is new  and adequate. It was replaced during my alternator overhaul last year. It is clear that my problem is the output of the existing generator: 35A. Voltage is below. 13V with load even during motorway cruise. I will have to go for a higher amperage unit, I will also upgrade the cabling in that case, I just hate the hussle  of fitting the new unit unless it is an exact fit. I think the AL66X is not an exact fit...

[Tyler S]

You may want to put an amp clamp style multimeter on the alternator feed wire while applying your loads as see if in fact the alternator is actually maxed out and matches your bench test.  If it is not then there may be an issue or bottleneck somewhere else. A weak battery or one with a partially shorted cell can cause overdraw. Also all of the B+ feed to the rest of the car is fed from the back of the headlight switch. This has been known to be an issue as well.
I did have an issue on my car with low charging voltage that was solved by installing an electronic voltage regulator in place of the mechanical one. Its a direct swap with no modification needed.
Just food for thought.

[Nicolas Aristodemou]

Thanks for the tips Tyler. Very valid points.

 I have been through some calculations though: 17A for my AC unit (10A condenser fan, 4A evaporator fan, 3A for AC clutch), 3.5A for my ignition coil, 8A for my fuel pump, 3A for radio plus 5A for other smaller and intermittent loads (123 ignition, various relays and solenoids, brake lights, turn indicators, instruments) that comes to 36.5A which is the generator capacity. My battery is less than one year old. If you add to the above a load of at least 15A for night time driving then you are over the 50A demand so a lot of strain on my system. It’s either a bigger generator or no AC and no radio in the evenings.

[lpeterssen]

Excellent and accurate calculation