Major Electrical/Electronic fault (?)

I was driving about 40-45mph and accelerating yesterday when my instrument panel went dark, the tach dropped to zero (even though the engine was still running), and I felt a large thud from the transmission anytime I went from neutral to drive or back. I let it sit awhile and restarted it and had the same problem. Where do I start? Fuses, bad ECM, something else? The car is all OEM except RocketChip Stage 1 about 22,000 miles ago. I'm presently at 137,500 miles. I don't have a VAG-COM and don't live close to anyone that does, as far as I know (Tifton, GA).

Check for the presence of water under the carpet in both the driver's and front passenger's footwell. Sounds like the tranny is in safe mode. As for the IC, need to start checking fuses.

vwztips said:

Check for the presence of water under the carpet in both the driver's and front passenger's footwell. Sounds like the tranny is in safe mode. As for the IC, need to start checking fuses.

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When checking for signs of water in those areas it is more than just lifting the carpet to feel for water or looking for signs of water. The water could have been there 5 years ago and show no signs of it ever being present... You may need to open a harness, skin a wire or 2. From your description, I would suspect the brown wire at the TCM with the bang felt. There are, I believe 3 brown wires that go to/come from the transmission control module and are "stamped" together - find that spot and observe for signs of corrosion. If present, the corrosion will tend to go towards the tcm and the weakest point is the thinnest brown wire.

the relay power distribution panels as well as cluster wiring is right underneath the plenum drain under the mastervac. it might sound basic, but other than obvious faults, its really start at the battery and check every connection, both b+ and grounds.

Can a BEW run (at least at idle) with no alternator or battery output? Since thepharmacy injection and last fuel pump are mechanical, it seems plausible that a running BEW would continue to run even without electrical input, rght? Loss of alternator (and then battery) seem like a possibility worth checking out.

no it cannot

@thundershorts: I don't doubt you, but can you please tell me what prevents this engine from running without alternator output?

thundershorts said:

no it cannot

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I drove my Passat about 20 miles with a completely fried alternator. Battery was good though. As long as the battery can supply voltage to the ECU, engine function and transmission function will continue. I don't know how low battery voltage can go before things deteriorate, though.

The question was:

Can a BEW run (at least at idle) with no alternator or battery output?

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Thundershorts didn't say it wouldn't run without an alternator. He said it would not run if it had no battery or alternator output. This means there is nothing to run the electronics. Big difference. As Aja pointed out, a battery will let you limp home...

thanks Moose for helping clarify. even mechanical injected older engines need some electrical power to hold the stop solenoid open to run even if they don't have an ecu.

So the battery has been completely dead (0 volts) for the past two weeks. It took 8 hours at 20 amps to get enough juice in it to be able to start. With the car off, I get 12.8-12.9 volts. With the car on and no electrical load, I get 17.1 volts. With full load (hi-beams, A/C fan on max, radio on, etc), it won't drops below 14.3-14.4 volts. Four on-off cycles confirmed these readings. I guess this means the problem isn't the alternator.
When I put the car in either reverse or drive, it engages with a very hard thud. Am I possibly looking at a computer issue? Does anyone have a VAG-COM for loan or rent so I can pull the Check Engine codes?

If you are really getting 17.1 volts, the alternator is defective (voltage regulator circuit).

Is that 17.1 v at the battery? I think that's too high. Bad alternator possibly frying other electronics including the battery.

Is your gear display on the dash showing a reverse image? That would indicate that your transmission is in safe mode and the thump may mean the tranny is toast.

I agree with Aja, if the voltage regulator circuit is allowing that much voltage to the battery, that is bad - cook the battery (and other electronics) bad.

!@#&*

Yes, that is 17.1 volts at the battery. And yes, the gear display is showing a reverse image. Is it likely an alternator overvoltage has damaged my computer and/or transmission?

formerPDfan said:

Yes, that is 17.1 volts at the battery. And yes, the gear display is showing a reverse image. Is it likely an alternator overvoltage has damaged my computer and/or transmission?

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School is out on what happened. The first thing I would do is get that alternator off and out of the circuit. Over 17 volts may do a lot of damage to solid state devices located in the control modules. Put on a rebuilt alternator on and see what happens when you start the car up. With the shift display color-reversed, the transmission is in limp mode.

your alternator is attempting to charge a bad battery. the regulator is raising the voltage to overcome the high internal resistance from the bad, sulfated battery. If you want to take my advice, replace the battery before you go any further, otherwise you may indeed fry the alternator and other things. Why people operate vehicles with batteries that are trash, makes no sense to me. Batteries are cheap compared to the damage and trouble caused by not replacing them when they are worn out. It if futile to ascertain electrical or other faults in the system without a stable, functioning battery and charging system.

I hadn't considered the battery being defective because it was replaced summer 2010, which was just 22,000 miles ago. The 80ah was installed by Oilhammer. I will take it out and have it tested, though, along with the alternator. If it happens to be the alternator, is this correct part # and a good price? I am definitely not willing to go with a cheap Ebay Chinese knockoff.
http://www.worldimpex.com/parts/valeo-alternator-120a---new_2625426.html

Good plan to get both tested.

whatever happened to the battery, its most likely scrap. when charging a battery, the rule of thumb is charge rate no more than 10% of rated capacity. After you replace the battery, then do a system test. your alternator may or may not have been severely damaged. My guess is that its still functional, as long as you don't run it till the new battery is installed. leave the alt in the car, most places won't be able to test it properly anyway, as they won't even be able to mount it on the test stand.

With no load other than what the car requires when completely off, the battery was down to 10.8 volts last night and further to 4.8 volts this morning. It seems pretty safe to say the battery is done. I still think the voltage regulator in the alternator is bad. The odds seem that a 137K mi alternator would fail before a 22K mi battery. I wouldn't think a properly functioning voltage regulator would let the alternator go overvolt regardless of the battery condition because the overvolt would damage other items. It would simply put out more amps, right?

The battery is toast unless you have a huge current draw with the car off. The alternator can be tested with a good battery in the car. It should not put out more than 14.5 volts at low RPM and about 12.5 volts at idle. Anything greater than 14.5 volts is not acceptable. Get a good battery test first and go from there.

Alternators will overcharge (voltage wise) a battery with a bad cell(s). Amps (or current) is drawn from the battery as it is a storage device and has the ability to provide high amperage (power) @ 12 volts to things like the starter motor.

you seem still reluctant to just buy a battery and have it done with. Just out of my own curiosity, was it a Johnson controls battery? Did your car have any high current aftermarket add ons?

Yes, it is an Interstate Battery (made by Johnson Controls). The model is MTP-H7 80AH. I have zero aftermarket electrical add-ons. I am reluctant to buy *only* a battery and not check out the alternator, too. As I said above, it's less than 2 years old and it showed no signs whatsoever of going bad before this incident. I'd hate to put a new battery in and have a potentially bad alternator fry it, too, so that's why I'm checking both.

your alternator will not fry your new battery unless its really bad and you run it for weeks. You will most likely find the alternator puts out proper voltage with the new battery. The battery you have, MTP-H7 is no longer available according to the interstate website perhaps there is a reason for that. 640cca isn't particularly high for a battery of that size. try either a Varta or one of the top of the line ones and you might have greater longevity and less risk of damaging other components. Interstate megatron's were available when you bought this battery, but would have cost 10-20 more.

Acording to their website http://www.varta.com/eng/ Varta is made by Johnson Controls also. Is that a problem? I thought it was pretty much impossible to get a non-Johnson Controls battery anyway. Also, I am unable to find a place to buy Varta batteries in the U.S. If I can't get one, I may just get my Interstate replaced under warranty.

I would just get another Interstate if you have a warranty on the old one.

I have seen alot of batteries with bad cells, but never had the system voltage rise to over 17 volts because of a bad cell. I think your alternator is your problem.

Varta has been owned by Johnson controls since 2002 for the automotive division but may still be building batteries in Germany, rather than Mexico. Exide is still the largest battery manufacturer in the world. Bosch labeled batteries are built by Exide. All battery manufacturers build batteries to a price, the top of line ones are built differently internally. There is more wrong with this battery than just a bad cell. The regulator will keep increasing increasing the voltage to maintain system voltage of 14-14.5 till either battery is charged or the alternator burns out. In this case, the battery cannot be charged. If you wish, by all means replace the alternator as well as the battery, but it may not be necessary. As I've repeatedly stated in other threads, defective, worn out batteries overstress alternators to the point of failure. The solid state regulator is unlikely to cause overvoltage in itself,unlike electromechanical regulators. generally when they fail, they do nothing, no field output.

Has anyone checked to see what batteries are used in the new passat? here is a post from someone who worked for Johnson...

posted: Jan. 24, 2011 @ 11:15a

I worked for Johnson in the 80's... True, they are one of the 3 big battery makers. However, they manufacture batteries to different specs for certain high volume buyers, like Sears/WalMart/etc.
That being said, I always use Die Hard. Sears is easy if you have to return/exchange for any reason.
I would recommend a few things:
1- Get the largest capacity battery that can fit in your vehicle. This is meaasured by 3 factors: CCA, reserve capacity, and discharge rating. Many companies do not provide discharge rating. The bigger numbers are better.
2- Maintenance Free batteries are usually a little bit more expensive, but worth the cost. Unless you overcharge them, they never need water added which is one of the biggest reason batteries fail. (Even if you add water later, damage has already been done to a battery that has had plates exposed.
3- If you make a lot of short trips, or the car sits for days at a time without being started, I suggest looking at the added cost to go to an AGM battery (DieHard Platinum, I think). These do not have liquid electrolyte... the have an electrolyte embedded into a fiberglass matting. They lose about 1-2% charge per MONTH compared to regular batteries which lose 1-3% per DAY. When a battery falls below 80% charge, it starts to lose lifespan. These batteries are also much better at handling vibration and have a lifespan of 8-10 years if treated properly!
4- If you run your battery low often (ie: listen to cd's at a campsite, use an inverter from your car, etc.) look into the AGM Marine battery (Sears PM-1 or PM-2 I think). Same as the AGM auto battery, but with thicker plates so it can handle being run down then recharged with limited effect. Note that the warranty is lower on these batteries, but that is because they think you will be using it for a trolling motor... it is a slightly better constructed battery than the auto version (I think the die-hard marine batteries are made by exide, not Johnson).
Good Luck!
SteveG