battery disconnect

I will soon be doing some airbag work and will have to disconnet my battery. What adverse effects will this have? Specifically, I'm concerned about the stereo: I know tht on the Hondas if you disconnect the battery, the stereo goes into safe mode and won't work. The delaerships have a little battery pack that plugs into the OBD port and keeps power on the stereo onl. Could I do that by applying 12vdc to the K line in the VW? If the stereo goes into safe mode, can I get it out with a vag-com?

I heard the same thing and without thinking the other day took off my battery connectors to clean them. I had them off about 10 minutes and it did nothing to my Monsoon stereo. I was supprised it didn't need reset.
2000 GLS Jetta 5 SP manual.

If the stereo is coded, there should be instructions in the handbook on how to unlock it after power is removed. The code should be on a sticker/card in the vehicle documentation wallet along with the manuals. If it's not, contact VW for the code before you do any work.

You should be fine, I think. Nothing will happen to the stereo, there again, I think. I recently had the battery out of my '04 for 2-1/2 days replacing my clutch. I got my radio code handy because I thought the radio would be in "safe" mode after I put the battery back in. It was not. It worked fine. Perplexed, I read my owner's manual and it said that now VW stereos have the capability to detect the car they are installed in originally and will not go into "safe" mode if they are in the same car when the power is connected again. I do not know when this change occurred, so you may want to check your OM.

I think on the older VWs where it will go into "safe" mode, you have about two hours without power before this occurrs.

Also, follow whatever Bentley says about deactivating the air bag. I know on some makes, the air bags stay active after the battery is disconnected for a certain time to prevent deployment failures in serious accidents where the battery gets taken out.

--Nate

The newer immo2 cars will lose the immobilizer/key programming if the battery is disconnected too long. Might be days, might be weeks, not sure. A month I know for sure will do it.

What are you doing specifically to the SRS system? You really do not need to disconnect the battery for many things.

oilhammer said:

The newer immo2 cars will lose the immobilizer/key programming if the battery is disconnected too long. Might be days, might be weeks, not sure. A month I know for sure will do it.

What are you doing specifically to the SRS system? You really do not need to disconnect the battery for many things.

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I'm going to disconncect the control module and troubleshoot a "low B+ voltage" code with my DMM. I'm expecting to have to disconnect the battery before disconnecting/reconnecting the controller to guard against accidental detonation of the bags.

I have a bentley CD rom en route that UPS says will be here tomorrow. I intend to fully read that and the wiring diagrams before I begin.

PDJetta said:

Also, follow whatever Bentley says about deactivating the air bag. I know on some makes, the air bags stay active after the battery is disconnected for a certain time to prevent deployment failures in serious accidents where the battery gets taken out.--Nate

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Yes, I would not be at all suprised to find several capacitors in a system of that nature.

I would check your battery first. Low voltage is usually caused by a weak battery or a bad ground under the battery.

Use a good (Fluke, etc.) DVOM and do a 'min-max' voltage reading at the battery when you start the engine. It should NEVER drop below 9.6v as that is the threshold for the ECU KAM. Most good batteries will only drop to about 10.5v during cranking.

I'd also check that ground point under the battery box.

Well, I can't get the code to go away, even with the engine running and Vag-Com showing 14 vdc. I've also got that 65xxx dead controller code as well but Uwe said, a long time ago, that even with the 65 code, the low voltage would reset once corrected.

But I can do a load test before I start.(I'm also changing trans fluid filter and engine oil and repalcing a broken mirror switch and programming new keys that weekend-so load testing the battery won't be out of the way.) I'll be suprised (and pissed) if it's a battery issue-the car starts fine and the battery is only 3 years old. (Interstate)

I have a Blue Poiunt 503a (I WISH I had a bought a Fluke-but this has served me well in aircraft electronics for-wow it's 10 years old lol) It's been out of cal for a while now. (I don't use it for anything other than reference voltages now.) I'm thinking about sending it out for cal before I begin. There is a company-owned, current cal Fluke here as well.

Check that, it's like $75 for recal-maybe it's time for a new Fluke LOL!

I have a Fluke 88, great automotive meter. Not cheap, of course. It has the min-max function which is great.

I am not talking about a load-test of the battery, I'm talking about the voltage during cranking. Load tester is something that has a big resistor or something in it that puts a massive load on the battery...typically people who use these do not even know how to properly use them anyways, but I digress.

If you do not have a DVOM with a min-max function, one that has a fast refresh rate can be used so long as you can read it while the engine is cranking. Problem is most [healthy] TDIs start instantly so sometimes the refresh rate, especially on cheapo DVOMs, is just not quick enough for you to see the voltage accurately.

I'd still check that pesky ground point under the battery box.

oilhammer said:

I have a Fluke 88, great automotive meter. Not cheap, of course. It has the min-max function which is great.

I am not talking about a load-test of the battery, I'm talking about the voltage during cranking. Load tester is something that has a big resistor or something in it that puts a massive load on the battery...typically people who use these do not even know how to properly use them anyways, but I digress.

If you do not have a DVOM with a min-max function, one that has a fast refresh rate can be used so long as you can read it while the engine is cranking. Problem is most [healthy] TDIs start instantly so sometimes the refresh rate, especially on cheapo DVOMs, is just not quick enough for you to see the voltage accurately.

I'd still check that pesky ground point under the battery box.

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Is that ground for jus the srs or is it THE ground for the electrical system. Because if it is-and it's weak-wouldn't I have other electrical problems?

As far as the battery test:I canjust watch the meter while someone else starts the car.

My meter doesn't have min/max, but it is pretty sensitive. I can recall if being too sensitive, floating too much when working on VHF and FM coms; the display voltage woul float a lot when keying the transmiter-I often had to use an analog.

Evenn though this one is out of cal, I have no reason todout its accuracy. I can't justify buying a new meter. I'm getting out of aviation soon-I hope. No, I pray! It's time for a new career. lol

The ground I speak of is for the whole car, yes, but for some reason the SRS ECU seems to be a wee bit more sensative to voltage drops.

14v at idle does not really mean anything, because if the voltage dropped below 9.6v...just for a split second...that is enough for the SRS ECU to see a "low B+ voltage". And typically the only way the voltage would drop that low is during cranking.

Chances are the SRS ECU is shot, but before I bought a $900 part (or whatever it is) I'd want to make 100% certain all the voltage and ground supplies to that module are perfect.

You may even try unlpugging the SRS unit and plugging it back in, just to see if there is a suspect connection there.

It's not the only ground. You'll see that in the Bentley. IIRC, there is one behind the cluster, one down by both the right and left kick panels and yet another at near the center console. And more in the back.

weedeater said:

It's not the only ground. You'll see that in the Bentley. IIRC, there is one behind the cluster, one down by both the right and left kick panels and yet another at near the center console. And more in the back.

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Yes, that is true, but usually the one under the battery is the one that can get corroded and cause all sorts of goofy problems. That is a good point, though, to check ALL the ground points.

oilhammer said:

The ground I speak of is for the whole car, yes, but for some reason the SRS ECU seems to be a wee bit more sensative to voltage drops.

14v at idle does not really mean anything, because if the voltage dropped below 9.6v...just for a split second...that is enough for the SRS ECU to see a "low B+ voltage". And typically the only way the voltage would drop that low is during cranking.

Chances are the SRS ECU is shot, but before I bought a $900 part (or whatever it is) I'd want to make 100% certain all the voltage and ground supplies to that module are perfect.

You may even try unlpugging the SRS unit and plugging it back in, just to see if there is a suspect connection there.

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It makes sense that the SRS (and probably the ECU as well) would be the most sensitive to voltage drops. Aren't those the typical 5vdc (internally) that most other electronics are? Logic boards are typically sensitive to that. Hey-I just had a logic board issue in my 01M last month....hmmm. Incedentally, the full description of the fault code invag-com states low or intermittant voltage.

As fir the airbag controller, a new one with the "F" suffix at worlimpex is 585 but I've found used ones for ~300. I'm a little skeptical about installing a used airbag controller-but the come with a warranty and if vag-com says they check out, what more can you do? There's a company online that claimes to be able to wipe the controller and reinstall all the software/firmware for $75. I'm actually considereing it. Even it goes awry, with a used one, I'm till way cheaper than a new one.

Back to the comments about voltage sensitivity-Oilhammer, in your professional opinion, is a curently cal'd meter a necessity for this type of troubleshooting? (i.e not board work but source voltages) I can recall only one component on our planesthat truly require a cal'd meter-out of a LOT of electronics.

Had the battery out of my daughter's 2001, 2.0ltr gasser Jetta for 3 days (had all of the seats out & carpeting chasing a moonroof leak). No problems with radio or anything else after hooking it back up.

I would think if it was within 1v tolerance (pretty big gap) you should be fine. 9.6v is the typical threshold for automotive ECUs, with an internally regulated 5v sensor loop as you stated.

The norm they say is 10v, but somewhere I have read that 9.6v is closer to exact. In school we had an older Bosch L-jet system (donated by me ) and we connected a potentiometer to its main power feed. I seem to recall it would work just fine down to 9.6v, then it shut down completely. We tried it on a newer Ford EEC-3 system and we got the same results.

High end I have no idea, maybe 18v? But that is not your issue anyways.

pruzink said:

Had the battery out of my daughter's 2001, 2.0ltr gasser Jetta for 3 days (had all of the seats out & carpeting chasing a moonroof leak). No problems with radio or anything else after hooking it back up.

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I'm surprised you didn't have to do a throttle body alignment. Sure did on my 00 NB when I swapped the battery. Easy to do, just didn't know how to do it for a while.

rdkern said:

I'm surprised you didn't have to do a throttle body alignment. Sure did on my 00 NB when I swapped the battery. Easy to do, just didn't know how to do it for a while.

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You do. The throttle adaptation is not right if you don't, however if it starts and idles it will eventually relearn on its own. Usually the idle is kinda lumpy if you do not do the relearn.

There are 3 different 2.0L gas engines in 2001, the earlier non-drive-by-wire throttle setup is far less finicky, but it should still be performed for best performance.

Mine was way off - just felt like 3 cylanders and wouldn't hold cruise. Wasted $112.50 at the dealers, then searched the Ross Tech site for the answer.

Yep, if it is not done, they can really run crappy. Simple thing to do, though.