Electrical Drain Mystery

Technical questions and answers concerning all models of VW diesel vehicles.

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StevenPH
Diesel Freak
Posts: 150
Joined: Wed May 11, 2011 10:31 pm

Electrical Drain Mystery

Post by StevenPH »

Hello all,

Thanks for all the help with my Jetta over the years, it's still my daily driver and running (mostly) great.

I have had an ongoing mystery for several years now that i'm trying once again to solve. I thought i'd throw it to you all to see if fresh eyes gave any new clues.

Here goes, i'll try keep it brief as i can:

--------

Something is amiss in my battery charging system in my 89 Jetta, 1.6 NA diesel (250k).

It started about 4 years ago when i was on a long drive, and realize my battery was completely dead (my radio started to drop out, i pulled over and couldn't restart, battery was drained almost entirely.)

I got a jump, had the alternator checked (good), had the battery recharged and kept an eye on it. I had to recharge the battery because it had drained so low that even with a jump and an hours drive it was still too low to start the car again.

Over the next year or so this kept happening. Not all the time, but everytime it happened it was on a long drive. I would notice the radio go out, or the headlights start to dim, and the battery would be drained.

In trying to solve this i replaced the starter, the voltage regulator, then the whole alternator, got a new battery, checked all wire connections, belt tensions, nothing changed it.

I hooked up a digital volt-meter to the battery, and found that when i initially started the car, it was only reading about 13.9v, and the battery light in the dash stayed red. With some throttle, the charge would get up to about 14.10, and the dash light would go off. But when i add more throttle, say highway speed rpms, the charge actually goes back down to 14.2 or 14.5ish. When i release the throttle it goes back up to 14.10ish. My electrical knowledge is low (obviously), but this seems counterintuitive. I took it to couple mechanics (including respected vw diesel specialists) and they couldn't figure it out.

So then i hooked up a bypass directly from the battery + terminal to the alternator post (see yellow wire in attached pic). The charge jumped up to 14.22ish, and stayed put even with throttle.

So me being me, rather than dig deeper, i just ran that bypass through to the dash, put a switch on it, and whenever i go for a long drive i flip it on and have had no issues since. That was 3 years ago.

So now, over and above just wanting to figure out what is going on in there, i'm getting sick of all the times i forget to turn the switch off and come back to my car and find the battery completely drained because it was hooked up to the alternator all night...

-------

This ring a bell for anyone? I've read some threads on this forum that have mentioned something like a switch or relay in the steering column and could potential be the weak link in a charging system?

And if not - any ideas for how i could DIY my bypass system so that it automatically turns on and off with the car? I tried attaching the alternator post to the fuel solenoid thinking i was clever, but then of course the car just didn't turn off...

Thanks for reading this far, sorry for the novel :)
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'89 Jetta 1.6d
Fatmobile
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Location: north central Iowa

Re: Electrical Drain Mystery

Post by Fatmobile »

So you put a jumper in parallel with the blue wire.
I'd guess that the blue wire is becoming corroded under the battery.
I think there is a plug there that corrodes.

You know 14.5 is more than 14.1 right?
"the charge would get up to about 14.10, and the dash light would go off. But when i add more throttle, say highway speed rpms, the charge actually goes back down to 14.2 or 14.5ish. When i release the throttle it goes back up to 14.10ish."

More throttle gave you more volts.
'91 Golf gasser converted to a 12mm pump, M-TDI.
'84 1.6TD Rabbit with a VNT-15 turbo, still setup to run on vegetable oil.
'84 GTI with 1.7TD pistons and intercooled.
2003 TDI wagon
2000 TDI Jetta.
StevenPH
Diesel Freak
Posts: 150
Joined: Wed May 11, 2011 10:31 pm

Re: Electrical Drain Mystery

Post by StevenPH »

Right sorry, forgot a zero in there. Meant to say "goes down to about 14.02 or 14.05ish."

The only time it ever gets past 14.1 is when the jumper is connected, when it gets to 14.22 and stays there.

I'll check that blue wire on the alternator end and also see where it goes on the other end (ignition switch area?). I checked the connection at the alternator last time but didn't dig into the steering column. I'll do that this weekend.
'89 Jetta 1.6d
Fatmobile
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Re: Electrical Drain Mystery

Post by Fatmobile »

Under the battery.
'91 Golf gasser converted to a 12mm pump, M-TDI.
'84 1.6TD Rabbit with a VNT-15 turbo, still setup to run on vegetable oil.
'84 GTI with 1.7TD pistons and intercooled.
2003 TDI wagon
2000 TDI Jetta.
greg lousy
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Location: Rhode Island

Re: Electrical Drain Mystery

Post by greg lousy »

I've driven 2 of my cars for times with bad exciter wire connections. The alternator can still kick on but it takes a shot of high rpms. I would guess that on some drives you didnt hit enough rpms to excite the field and get the alternator charging..

14.1 V is ok with me

At night start it and see if the lights get brighter when you give it a good rev.
Various vehicles in various states
TylerDurden
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Re: Electrical Drain Mystery

Post by TylerDurden »

I'd disconnect the blue wire from the alternator, get it up to voltage with the jumper, then take the jumper off and see if it continues to charge or not (with no wires on the d+ (exciter) terminal).

I'll wager if the charging is good with the blue wire off, the blue wire has a short to ground or some water got into the fusebox.
Have a nice day.


'91 Jetta ECOdiesel TD - clean & complete (less motor/tranny) for sale

'82 Westy Vanagon 1.9 N/A - 23.5mpg
'86 Jetta TD - 45-50mpg
'81 Dasher Wagon 1.6 N/A - 52mpg
'84 Wasserboxer - DOA, parts donor
'94 Passat wagon VR6
'03 Jetta TDI wagon 230K, 52.3mpg
'89 Jetta N/A - 51mpg
'82 Caddy 1.6 N/A - Sold
StevenPH
Diesel Freak
Posts: 150
Joined: Wed May 11, 2011 10:31 pm

Re: Electrical Drain Mystery

Post by StevenPH »

The lights definitely get brighter with high RPMs after start-up. At the same time the battery light turns off.

I'll run your test, Tyler. I've had issues with water leaks around my fusebox for years, the driver's side floor mats always get damp in the rainy season, and various efforts to stop it always come up short. It wouldn't surprise me if there was a water-related electrical issue in there.

Greg - I would think 14.1 would be fine, too. Which it part of what i don't understand about the complete loss of battery power on certain occasions. On a long drive with sustained high RPM's, headlights, radio, fan blowing, etc etc, could 14.1 not be enough to keep up and it slowly drains?

It seems silly to be quibbling over a tenth of a volt... but clearly there's more happening in there than i understand.

Thanks for the help and suggestions :)
'89 Jetta 1.6d
wmasscaddy
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Location: northampton,ma

Re: Electrical Drain Mystery

Post by wmasscaddy »

Crazy theory:
It used to be a 5 speed but a 4 speed was swapped in. There is a short in the upshift indicator light. All that highway driving in 4th when it "should" be in 5th lets the upshift short run down the battery.
82 pickup 1.6 n/a
350,000 + miles lost to broken odo
11mm block, 12mm head
Fatmobile
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Joined: Wed Oct 30, 2002 10:28 pm
Location: north central Iowa

Re: Electrical Drain Mystery

Post by Fatmobile »

Also a short brush in the alternator can lead to charging sometimes and not others.
Watching an analog voltmeter would show if the voltage is jumping around.
'91 Golf gasser converted to a 12mm pump, M-TDI.
'84 1.6TD Rabbit with a VNT-15 turbo, still setup to run on vegetable oil.
'84 GTI with 1.7TD pistons and intercooled.
2003 TDI wagon
2000 TDI Jetta.
StevenPH
Diesel Freak
Posts: 150
Joined: Wed May 11, 2011 10:31 pm

Re: Electrical Drain Mystery

Post by StevenPH »

Tyler your test is correct - I removed the blue wire, started with the jumper attached, and had a voltage of up to 14.25ish. With high rpms the voltage remained the same. When i removed the jumper (with no blue wire attached), voltage stayed the same. But with increased rpms the voltage went down to 14.1 or less.

I checked the blue wire connection at the battery - that plug is good. I'm poking around in my fusebox now - but i don't see where those wires connect. My diagrams don't show any fuses associated with the alternator or exciter wires, and i don't see a matching wire in or around the steering column.

No signs of water damage or other corrosion anywhere around the fusebox or steering columns.

Any advice on the next step of troubleshooting? If it is in fact a short to ground, can i just add a redundant ground wire at some stage of the circuit?

---

I like your theory WMASS, but sadly i still have a 5 speed transmission :)
'89 Jetta 1.6d
StevenPH
Diesel Freak
Posts: 150
Joined: Wed May 11, 2011 10:31 pm

Re: Electrical Drain Mystery

Post by StevenPH »

Some other readings i took that may or may not be meaningful -

With only the blue wire attached (no jumper), and the exciter wire engaged (I start the car and give it some revs to get the battery light to turn off) i get the following readings:

Alternator Positive terminal to engine block - 14.25
Battery Terminals - 14.1
Alternator positive terminal (blue) to alternator ground (red) - 7.0

I'm fairly sure i'm identifying the correct alternator ground wire - it's the red wire, same size as the blue, that rides in the same sleeve to the plug by the battery.
'89 Jetta 1.6d
TylerDurden
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Re: Electrical Drain Mystery

Post by TylerDurden »

IIRC, the small red+black wire is the tach/cruise wire. ( "W" terminal)

There are no fuses in the alternator excitation path. There is a harness connector in the engine bay.

The alternator ground is through the engine>negative cable. Those all must be in good contact.


Well, we can say that the alternator will self excite after the initial excitation.

Since I haven't read that the tests were done with loads, I would continue the test by running the car with the blue wire OFF (after initial excitation) and with loads like the brights and cabin fan on high.
  • I'd measure for voltage between:
    Battery posts
    Alternator body to battery positive post
    Engine block to battery positive post
    D+ terminal to battery negative post
After those tests, I'd put the blue wire ON and repeat the measurements.

When I go out later, I'll duplicate the tests and post my results.
Have a nice day.


'91 Jetta ECOdiesel TD - clean & complete (less motor/tranny) for sale

'82 Westy Vanagon 1.9 N/A - 23.5mpg
'86 Jetta TD - 45-50mpg
'81 Dasher Wagon 1.6 N/A - 52mpg
'84 Wasserboxer - DOA, parts donor
'94 Passat wagon VR6
'03 Jetta TDI wagon 230K, 52.3mpg
'89 Jetta N/A - 51mpg
'82 Caddy 1.6 N/A - Sold
TylerDurden
Turbo Charger
Posts: 1285
Joined: Tue Dec 22, 2009 1:01 pm
Location: Michigami, USA

Re: Electrical Drain Mystery

Post by TylerDurden »

I have tested running my 86 TD @ 1200rpm with the blue wire ON and with loads: High-beams, fog lights and cabin fan on high:
Battery posts: 13.83V
Alternator body to battery positive post: 13.86V
Engine block to battery positive post: 13.86V
D+ terminal to battery negative post : 14.14V

Also measured the voltage between the D+ and Battery+... 0.3V

Keep in mind, I have LED lights all around, so the load test is not quite like incandescent lighting. I also have 2ga. cables to the starter and between the engine and Battery(-).

I don't have my Bentley in front of me, but the circuit is simple: switched positive feed goes to the dash led then exciter terminal. The engine bay connector is suspect...

Edit: My bentley says the connector in the engine bay is for the wiring to the D+ and W terminals. I wonder if your tach/cruise/upshift stuff is working.
Have a nice day.


'91 Jetta ECOdiesel TD - clean & complete (less motor/tranny) for sale

'82 Westy Vanagon 1.9 N/A - 23.5mpg
'86 Jetta TD - 45-50mpg
'81 Dasher Wagon 1.6 N/A - 52mpg
'84 Wasserboxer - DOA, parts donor
'94 Passat wagon VR6
'03 Jetta TDI wagon 230K, 52.3mpg
'89 Jetta N/A - 51mpg
'82 Caddy 1.6 N/A - Sold
StevenPH
Diesel Freak
Posts: 150
Joined: Wed May 11, 2011 10:31 pm

Re: Electrical Drain Mystery

Post by StevenPH »

Okay I got a minute to get out there do some readings -

With a medium push on the throttle (i don't have a tachometer, so my rpms are just a guess), and as much of a load as i could put on (high beams, high blower, radio, cabin lights), i got the following readings:

With the blue exciter wire ON:
Alternator Body to Battery Pos = 13.5
Block to Battery Pos = 13.5
D+ terminal to Battery Neg = 14
D+ terminal to Battery Pos = -.5
Battery pos to neg = 13.4

With the blue exciter wire OFF (and no jumper wire):
Alternator Body to Battery Pos = 13.5
Block to Battery Pos = 13.46
D+ terminal to Battery Neg = 14
D+ terminal to Battery Pos = -.62
Battery pos to neg = 13.34

So pretty much no difference. The numbers are average estimates as my throttle wasn't perfectly consistent, but in general the numbers were very similar, if not slightly lower when the blue wire was disconnected.

I have no tach, nor cruise or upshift signal.

When you refer to the engine bay connector, you are referring to the two wire connector located near the battery, correct?
'89 Jetta 1.6d
TylerDurden
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Location: Michigami, USA

Re: Electrical Drain Mystery

Post by TylerDurden »

StevenPH wrote: Mon Apr 01, 2019 4:57 pmWhen you refer to the engine bay connector, you are referring to the two wire connector located near the battery, correct?
Yes.
Do you have a wire on the alternator connected to the W terminal?

I was hoping there would be a reading significantly different from what my car presented, so we seem to be missing something.

The remark was that on long drives this issue would happen... any chance this happens when the engine is warm enough to kick-on the radiator fan? That motor can draw some major power even when it's new.
Have a nice day.


'91 Jetta ECOdiesel TD - clean & complete (less motor/tranny) for sale

'82 Westy Vanagon 1.9 N/A - 23.5mpg
'86 Jetta TD - 45-50mpg
'81 Dasher Wagon 1.6 N/A - 52mpg
'84 Wasserboxer - DOA, parts donor
'94 Passat wagon VR6
'03 Jetta TDI wagon 230K, 52.3mpg
'89 Jetta N/A - 51mpg
'82 Caddy 1.6 N/A - Sold
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