Battery problems

So I’m new to this forum so I apologize in advanced if this is a common problem but I have a 98 jetta tdi and the car will start off of a jump but once I drive it around for a bit and then shut it off it won’t have any power to start it back up, now my first instinct was the replace the battery, so I did that and it work for about a day and then the same problem was occurring, next thought was the alternator and the alternator is reading at about 13.5 to 13.7v so it is charging and I used a test light in series from the positive terminal on the battery to the positive connection and got no light so there doesn’t seem to be any draw. I think something is killing it as I am driving, also don’t know if this is related but my cluster lights cut in and out sometimes too, any help would be appreciated- thanks

What battery is it? needs to be a 600cca or larger 660 or 680 if im not mistaken, i have a 850cca in mine. What amp rating is the alternator, should be the 120 amp, not the 70 or 90.

Get the voltmeter in the car or other device that shows you volts so you can watch it as you drive, if they keep dropping no matter what you turn on (headlights etc...) than there is an issue with the carging system. fisrt thing to start with since you have checked the output of the alt and the bat, make sure your ground cables and the main cable from the alternator are in good shape. If there is an issue with them jsut do "the big three" google it, in a nut shell, 4 to 6 Gauge wire added to the main from the alternator to the battery, ground to engine, and ground to body.

Check the output of the alternator with the battery unhooked from it. honestly 13.5 to 13.7 is a bit low for the 98 mk3 tdi. the 3 cars i had all charges at 14.2v for some reason, maybe i just had 3 odd cars. Someone else can chime in on that one. 13.5 is ideal on most cars and my escape is 13.3v so go figure. You could just have 2 defective batteries. what brand, what is the date stamp on it, and CCA?
my car has a drain on both the amp and a few things when i run and its no where near choking out the alternator i have. i run 6 sets of HID's a 2400 watt amp, an 80 amp 12v 3/4 HP dc Air zenith air compressor, electric fans on full blast 100% of the time and a 1200 watt power inverter for various things, i had everything running , air compressor runs for 4 minutes at a time but i had it going non stop for about 20 minutes while blasting my train horns and never had an issue with draining the battery. im sure im putting a hurt on my alternator.

Point is, its not a electrical leak or short or draw doing this, if it was a short, a fuse would blow. Its got to be between the charging system and the battery. Is it the oem alternator?

So the battery is brand new and it’s an 800cca autocraft battery, don’t know if the alternator has been changed, I just bought the car about a week ago. What I have noticed though is there are no markings on it like the bocsh ones have so maybe it has been changed but I couldn’t verify that, the charging voltage does seem a little low, but would a loose connection on the starter draw anything?

Because on the starter- the main wire that is connected on the stud of the solenoid is slightly loose, reason being the nut is rusted on the stud and the whole stud spins and doesn’t tighten but the starter still works fine and gets a decent connection, so I wasn’t sure if that would contribute to anything

The charging rate is a bit low, you should really see 14+v with no lights on, just under 14 v with the headlights on etc. I regularly see 14.3v from my alternator.

Your battery is sufficient but there is some kind of problem if your dash cluster is doing odd things.

Best to start looking at your wiring for any 'custom' work that was done by a previous owner. Often times alien wiring will cause unintended consequences.

Also, for cluster issues check the ground wires near the round connector at the engine.

Steve

So the ground by where the transmission bolts up or a different location?

Ethan636 said:

So the ground by where the transmission bolts up or a different location?

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The round engine connector is attached to a metal bracket on the side of the cylinder head. That bracket has a stud where three small wires are attached, that ground causes a lot of problems with the cluster.

There are a lot more grounds than just the big cable that connects from the trans bolt to the battery.

You should consider getting a Mk3 Bentley manual if you intend to keep the car.

Steve

Ok thank you for the help

oem (bosch) alternator will have markings right at the top. you probably have an aftermarket POS, replace it. 100% sure. do not buy anything other than a OE. had this issue on the escape i have, its a PITA to work on and worth the extra $$$ for OE. the problem with remans is that the quality is junk and the rebuild process leaves sand and crap left from the cleaning process and messes up the bearings or something. thats why CV axles, water pumps, alternators, and brake calipers are never to be purchased as a reman (remanufactured.) i literally had to take a brake caliper apart to find 3 bits of sand in them that was making the piston stick.

Do the big 3 upgrade, repair or replace that starter nut issue, and replace the alternator. and you wont have any issues for the rest of the cars life.

also alien wires dont cause a car to die, they will drain the battery over a weekend or overnight. when running the alternator can put out more than enough power to solve any issues with a leaking wire or a stuck on dome light or something.
your issue is the charging system. The big 3 kits are cheep, like $40-60 so just do that and your OE alternator NEW not reman

Ok yeah I think I might just gonna end up spending the money, thanks

If anyone has done one of the big three kits on the mk3 tdi could you explain where the grounds are that are necessary for that installation

trans ground, battery to alternator + red, and battery to body anywhere at all. Just sand down or wire wheel off the paint and use a nut and bolt to ground the the body and then paint over it to seal so no rust. I did this all in 4 gauge K wire i got from capitol tristate, you can get it from any welding shop or commercial electrical distributor and a hand full of ring end terminators, some shrink wrap. Solder all connections, no crimp. watch some youtube videos on it and how to solder big copper ring terminals. you need a butane mini torch at least. You will see a noticeable difference in the brightness of your interior lights, no dimming issues when low rmp or sub woofer or amp draw.

Ok great thank you

So I’ve put in An alternator and I’m getting 15 volts at the alternator and 11.3 at the battery and I have a loose connection at the stud of the starter and the nut is rusted on so the whole stud spins and doesn’t tighten down, I know there shouldn’t be any loose connections and I have another starter that’s getting shipped to me but I noticed that I’m getting 15v at the starter which is the same as the alternator but only 11.3 at the battery, anybody know why that would be happening?

You have bad cabling somewhere if you're only seeing 11.3v at the battery, the charging at the alternator isn't getting to the battery...

This would be the reason your battery is going dead.

Steve

So I replaced the starter today and so that insured a good connection at the starter, now I’m getting 14.3 at the battery so it definitely helped but I just hope it solved the problem

Ethan636 said:

So I replaced the starter today and so that insured a good connection at the starter, now I’m getting 14.3 at the battery so it definitely helped but I just hope it solved the problem

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I hope it does too. No one likes to swallow the price tag of a new starter (or alternator for that matter) but if it gets you back to good running condition then sometimes we have to do it.

I need to have my Bosch starter repaired locally. I don't like the idea of not having a backup for critical stuff.

Steve