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Voltage issues...

Posted: Mon Dec 07, 2015 7:38 am
by Volky
Good day,

I have a 1970 Beetle with MS-II in it. Also, it is crank triggered, EBC, etc. The thing is the car runs well and I boost up to 10psi.

The alternator charges and the gauge shows almost 14v in the daytime. At night when I turn on the lights, it's a different story, it shows 12v.

The EBC gets affected I guess since I could only boost maybe 6 or 7 psi. The idle is off because of the AIC valve, too.

I checked the wiring and it looks okay, at least the outside.

Any remedy for this issue?


Re: Voltage issues...

Posted: Mon Dec 07, 2015 12:17 pm
by Dale M.
A early alternator puts out about 50 amps... With lights and radio playing and what ever else car has in stock form the alternator should produce enough power to be run everything ok.... Do not think adding EFI draws the much extra power to pull system down... Believe you main problem is 45 year old wiring and where you may be measuring voltage....

What is main tell is if you are measuring out put directly at alternator (with everything "on") and it reads 13.5 to 14 volts just above idle and also reads same at battery (directly across terminals) then you have a voltage drop problem in wiring.... IF you are only reading say 12 volts at fuse panel, (again everything on) the the main supply feeder (from front to back) is probably to small to supply the voltage/current needed and you are experiencing voltage drop via internal resistances in wire.... Always use hand held digital meter and do not rely on after market meter in dash for measurements...


Re: Voltage issues...

Posted: Mon Dec 07, 2015 12:35 pm
by Volky

Thanks for the response. I forgot to mention that I changed the alternator about a year back to a 75amp.

I will look in Google on wire testing, etc.

Re: Voltage issues...

Posted: Mon Dec 07, 2015 12:36 pm
by Dale M.
Would think if its a true 75 amp alternator, you have wiring issue (voltage drop) ...


Re: Voltage issues...

Posted: Mon Dec 07, 2015 2:11 pm
by Ol'fogasaurus
Volky, I agree pretty much with Dale but do yourself a favor and check your grounds.

Your talking about old metal here and if there has been any work (welding that it) you can always be sure of it unless you do it yourself. Also, if you still have, and depend on, the braided ground strap either replace it or bi-pass it. They have a bad reputation of fraying and losing potential so they aren't doing their job as well as you might think. If I remember correctly, the real work is done by the ground side.

Dale talked about voltage drop and there are web sites you can search for that will give you the lengths of wire run before you need to upgade wire ga..

VW, as I remember uses some small diameter battery cables. Try for an upgrade there.

Opinion: I like AMP measurements to show current flow and draw. Not everyone agrees with their use now days but if there is concern on how much draw is being done that is one way to know very quicklly and yes, there are meters that will show around 50 or 60 (I think that is the most I have seen on a standardmeter) amps. Once you learn how to read them, them then combined with a volt meter and you should have a pretty good idea of the electrical system you are working with.

My two-bits on this


Re: Voltage issues...

Posted: Mon Dec 07, 2015 9:47 pm
by Piledriver
As noted, the voltmeter is probably reading voltage at the fuse box.

I ran a 12 ga line for my gas heater (has a glow plug for startup), another for my AC condensor fan, ANOTHER for my headlights.
The relay-controlled headlights are controlled by the factory circuits but not powered through the original fusebox.
Saying the headlights are brighter is a severe understatement, esp true as they are H4s.

The old single run of 10 gauge headed up front from the battery was never really enough to begin with and the cheap crimps and wire VW used age badly.

The MS setup lives under the back seat on drivers side, gets power right off the battery via 2 x 12 ga run through ferrite EMI filters.
(ignition power is isolated there and has its own relay)
Ground connected via a woven strap.
There are two ground cables to the transmission. (battery cable type, sealed up and with painted ends)

The factory fusebox with the gimpy pointed end fuses doesn't age well either, didn't work terribly well new...
I have read those fuses were outlawed in some locales.

The Porsche guys can get a nice direct replacement fusebox for 911s and 914s that takes modern ATO fuses.
The only downside is it still uses push connectors for everything rather than screw terminals.
(There are reasons to use those, but you should probably replace all the 1/4" spade ends with best quality HD ones from 3M or such)

I'm surprised no one sells something similar for the far more common ACVWs.

Re: Voltage issues...

Posted: Tue Dec 08, 2015 10:27 am
by Dale M.
Another thing you can try is disconnect the battery (FIRST) and get small brass or stainless steel brush (tooth brush size) and polish all the push on contacts on fuse box and also terminals on wire ends (inside the curled over section) ... Apply a light coat of dielectric grease as corrosion preventive....

I also spin the stupid pointy end fuses a couple of times to break the corrosion bond between fuse and holder.... Like Piledriver said, sure wish somebody made a ATO replacement panel.... On my DIY wiring jobs I always use ATO fuses and panels and dielectric grease


Re: Voltage issues...

Posted: Fri Dec 11, 2015 7:23 am
by Volky
Good day,

I think I solved the dim light issue. I tested the wires and polished the contacts, the voltage stays at about 13.x volts. It doesn't light on very dim anymore.

Thanks to all. Hopefully thats the end of it. :D

Re: Voltage issues...

Posted: Fri Jan 22, 2016 12:00 pm
by Clonebug
I found on my buggy after about 10 years that the positive battery cable was giving a poor connection after many years of cleaning and scraping the post and clamp.
I went and bought a new positive cable and my start problems disappeared.

Re: Voltage issues...

Posted: Fri Apr 22, 2016 6:57 pm
by Marc
Dale M. wrote:Would think if its a true 75 amp alternator, you have wiring issue (voltage drop) ...Dale
Agreed, when my `69 still had a stock harness in it the addition of a 75W alternator revealed its limitations. Adding another B+ wire from the alternator to the battery cable at the solenoid is an easy "fix". Just be sure it's well routed and grommeted where it leaves the engine compartment since it'll be "hot" all the time.

Don't overlook that the ground path has to be able to handle the same current. The little brown groundwire is only there to provide reference for an external voltage regulator and can't do the job, there must be good contact between the alt housing and the pedestal...normally not a problem, but if the pedestal is painted/powder-coated it needs to be scraped clean where the alternator sits.