Throwing in towel on tranny limp mode - A8 for sale
07-21-2005, 12:48 PM
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Throwing in towel on tranny limp mode - A8 for sale
Here's the latest update on my tranny problem I've been chasing for months. Regulars will recall I was getting into limp mode with codes for pressure control valve 5 short to plus, and sometimes short to ground. Both a transmission shop and Auto Europa (http://aeuropa.com/) have verified the transmission is sound. The transmission has a new internal wiring harness (which had been bad), a new PC valve 5 (which had not been bad, but wtf), new filter and fluid, and new gasket, and is healthy as a horse.
There is some kind of wiring problem which is causing the J271 (I think I got that right) relay in the E-box to overheat one of the pins, causing a voltage difference between the ECU and TCU, and the TCU shortly thereafter throws its code and limps the transmission. The mechanic cleaned up various ground contacts in the main wiring harness to no avail. Since the J271 feeds power to the ECU, I tried disconnecting all non-essential outputs from the ECU (disconnecting oxygen sensor heaters and idle air control valve, leaving only fuel injectors and spark) to see if those are drawing too much current. Both me and the mechanic at Auto Europa, who did most of this work gratis just because he was curious as to why there was a problem he couldn't solve, have thrown in the towel. The mechanic suspects it might need a new wiring harness, but that gets into expensive parts and we don't know wtf is really wrong.
The mechanic let me use his Mercedes E320 for the 6 weeks or more he had my car. It had many redeeming qualities, but the A8 felt like a rocket-powered small sports car when I got it back. Too bad it only made it 3.3 miles before going into limp mode again.
I'm selling this otherwise fine POS. This is the perfect car for someone who thinks they can diagnose and fix an electrical problem. Kelly Blue Book is $9800, so I'm looking for best reasonable offer lower than that. The bad part is, I think I'm going to replace it with another A8. Someone talk some sense into me.
There is some kind of wiring problem which is causing the J271 (I think I got that right) relay in the E-box to overheat one of the pins, causing a voltage difference between the ECU and TCU, and the TCU shortly thereafter throws its code and limps the transmission. The mechanic cleaned up various ground contacts in the main wiring harness to no avail. Since the J271 feeds power to the ECU, I tried disconnecting all non-essential outputs from the ECU (disconnecting oxygen sensor heaters and idle air control valve, leaving only fuel injectors and spark) to see if those are drawing too much current. Both me and the mechanic at Auto Europa, who did most of this work gratis just because he was curious as to why there was a problem he couldn't solve, have thrown in the towel. The mechanic suspects it might need a new wiring harness, but that gets into expensive parts and we don't know wtf is really wrong.
The mechanic let me use his Mercedes E320 for the 6 weeks or more he had my car. It had many redeeming qualities, but the A8 felt like a rocket-powered small sports car when I got it back. Too bad it only made it 3.3 miles before going into limp mode again.
I'm selling this otherwise fine POS. This is the perfect car for someone who thinks they can diagnose and fix an electrical problem. Kelly Blue Book is $9800, so I'm looking for best reasonable offer lower than that. The bad part is, I think I'm going to replace it with another A8. Someone talk some sense into me.
07-21-2005, 03:21 PM
#2
Has your pressure control valve 5 been replaced once or twice?
If only once, my guess is that they changed the wrong valve. You measured significantly lower resistance across that valve than the others, right? And it's still the only code the TCM stores when going into limp mode, yes?
You have a burnt pin on the J271 relay? That's a good catch -- it's an important relay. Did they actually remove the relay, put an ammeter across the wires it connects when energized, and measure too much current with the car running? If not, I'd suspect the relay itself has resistance on the contact associated with pin, change the relay, and see how it goes. (I had an older Audi on which I noticed the fuel pump relay getting hot to the touch. The replacement relay stayed cool.)
But I don't think the J271 has anything to do with your limp mode unless you're also getting a low voltage supply code in the TCM, which you haven't mentioned.
Good luck!
Tom
You have a burnt pin on the J271 relay? That's a good catch -- it's an important relay. Did they actually remove the relay, put an ammeter across the wires it connects when energized, and measure too much current with the car running? If not, I'd suspect the relay itself has resistance on the contact associated with pin, change the relay, and see how it goes. (I had an older Audi on which I noticed the fuel pump relay getting hot to the touch. The replacement relay stayed cool.)
But I don't think the J271 has anything to do with your limp mode unless you're also getting a low voltage supply code in the TCM, which you haven't mentioned.
Good luck!
Tom
07-21-2005, 04:44 PM
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Yup. Yup. Yes.
Yes, the control valve was replaced. Although the transmission shop claimed to have replaced it, Auto Europa found that it had not been previously replaced according to its appearance. Low readings were from the bad harness. All readings are now in spec.
Yes, the relay was replaced, I think twice, after continued problems stressed the first replacement. Various inexpensive relays and switches (such as ignition switch) were replaced as anything came up that suggested they might have something to do with it. Lots of places in the wiring harnesses have had their connectors replaced and/or cleaned up.
Sounds like you think you can fix it. Such a deal!
Yes, the relay was replaced, I think twice, after continued problems stressed the first replacement. Various inexpensive relays and switches (such as ignition switch) were replaced as anything came up that suggested they might have something to do with it. Lots of places in the wiring harnesses have had their connectors replaced and/or cleaned up.
Sounds like you think you can fix it. Such a deal!
07-21-2005, 05:17 PM
#4
AudiWorld Super User
I'm sure it could be fixed
However, I understand you are at the end of your rope.
Electrical problems can't hide, testing will find them. What year is your car?
Good luck!
pw
Electrical problems can't hide, testing will find them. What year is your car?
Good luck!
pw
07-21-2005, 06:14 PM
#6
I thought that was pressure control valve 1, N215 ...
and that pressure control valve 5, N233, next to a solenoid valve, the fifth one in, was like pressure control valves 2-4 (N216, N217, and N218).
Tom
Tom
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07-21-2005, 07:38 PM
#8
Okay, that's weird.
J271 feeds two 15 A fuses, one for the injectors and some ECM power (obviously not all ECM power, since the ECM decides when to turn on J271), and the other for various solenoid valves and the leak detection pump. That's all. The oxygen sensor heaters' power comes from the fuel pump relay, J17, and the ignition coils' from the ignition switch via a fuse. The ignition coil power also goes to the ECM via a thin wire, presumably to wake up the ECM when you turn on the key.
If the ECM has enough voltage to run the engine and not log any low voltage code, how is this supposed to cause the TCM (which is not fed by J271) to enter limp home mode and complain about pressure control valve 5? It seems pretty unlikely.
And the J271 relay is burning out quickly from passing not more than the 30 A total of the two fuses it feeds? Really? Sure it's not heat from a bad connection that it plugs into? Or alternatively, do they know which branch -- injectors or solenoids -- is taking too many amps, and how many that is? Or with the two fuses pulled, is there a short between the switched contact of the relay and something else, which really could draw more than 30 A and implicate the wiring between the relay and those nearby fuses (yet, suspiciously, always leave enough voltage for the injectors and solenoids to operate)?
And they replaced the ignition switch??? Why? And connectors have been replaced (hopefully with good workmanship) or cleaned in the search for something which uses too much, not too little, current? That sounds like guessing not testing, and makes me think the shop is not the last word in electrical diagnostics. Either that, or changing the switch was motivated by an unrevealed clue, which could be interesting.
Just in case the J271 relay has some kind of overvoltage protection in it, is your alternator output voltage always reasonable? No propensity to burn out lights? Channel 36 on the climate control display (see URL) shows something in the 13s (or maybe 14s after a cold start)? Eh, it's probably not that -- the instrument cluster is supposed to show a yellow battery symbol in the central display if there's too much or too little voltage.
Still have the old transmission wiring harness? What was wrong with it to produce too-low resistance readings? Melted insulation? Conductive transmission fluid? I can't think of anything else it could be, and either one is worth digging into further. And maybe the TCM failed to protect itself when used with a shorted transmission wiring harness or valve, and its pressure control valve 5 driver is now faulty. Which bring us back to your original inquiry about borrowing a substitute to test with -- has that happened? Though it would be good to figure out the J217 overheating and those purported ECM/TCM voltage differences first.
Sorry this isn't helping you sell the car and that I'm about 2500 miles too far away to look into it in person. My personal feeling is that something doesn't add up and I'd look into changing shops before changing cars. It just shouldn't be a six week mystery at the shop, and I'm not sure how they'd handle an unusual problem like this if one occurred with your next A8.
Tom<ul><li><a href="https://forums.audiworld.com/a8/msgs/99716.phtml">https://forums.audiworld.com/a8/msgs/99716.phtml</a</li></ul>
If the ECM has enough voltage to run the engine and not log any low voltage code, how is this supposed to cause the TCM (which is not fed by J271) to enter limp home mode and complain about pressure control valve 5? It seems pretty unlikely.
And the J271 relay is burning out quickly from passing not more than the 30 A total of the two fuses it feeds? Really? Sure it's not heat from a bad connection that it plugs into? Or alternatively, do they know which branch -- injectors or solenoids -- is taking too many amps, and how many that is? Or with the two fuses pulled, is there a short between the switched contact of the relay and something else, which really could draw more than 30 A and implicate the wiring between the relay and those nearby fuses (yet, suspiciously, always leave enough voltage for the injectors and solenoids to operate)?
And they replaced the ignition switch??? Why? And connectors have been replaced (hopefully with good workmanship) or cleaned in the search for something which uses too much, not too little, current? That sounds like guessing not testing, and makes me think the shop is not the last word in electrical diagnostics. Either that, or changing the switch was motivated by an unrevealed clue, which could be interesting.
Just in case the J271 relay has some kind of overvoltage protection in it, is your alternator output voltage always reasonable? No propensity to burn out lights? Channel 36 on the climate control display (see URL) shows something in the 13s (or maybe 14s after a cold start)? Eh, it's probably not that -- the instrument cluster is supposed to show a yellow battery symbol in the central display if there's too much or too little voltage.
Still have the old transmission wiring harness? What was wrong with it to produce too-low resistance readings? Melted insulation? Conductive transmission fluid? I can't think of anything else it could be, and either one is worth digging into further. And maybe the TCM failed to protect itself when used with a shorted transmission wiring harness or valve, and its pressure control valve 5 driver is now faulty. Which bring us back to your original inquiry about borrowing a substitute to test with -- has that happened? Though it would be good to figure out the J217 overheating and those purported ECM/TCM voltage differences first.
Sorry this isn't helping you sell the car and that I'm about 2500 miles too far away to look into it in person. My personal feeling is that something doesn't add up and I'd look into changing shops before changing cars. It just shouldn't be a six week mystery at the shop, and I'm not sure how they'd handle an unusual problem like this if one occurred with your next A8.
Tom<ul><li><a href="https://forums.audiworld.com/a8/msgs/99716.phtml">https://forums.audiworld.com/a8/msgs/99716.phtml</a</li></ul>
07-21-2005, 11:59 PM
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I'm sure it can be fixed too...
But were it working, I'd probably sell it and buy a newer one, so I've been trying to fix it to sell a known-good car in good conscience and am giving up and selling a known-bad one. It's a '98.
07-22-2005, 12:23 AM
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Re: Okay, that's weird.
The transmission harness has lots of diodes in it. These were dead and dying. PCV5 diode was burnt to a crisp.
Ignition switch was replaced when mechanic accidentally discovered it threw the code when he turned off the ignition slowly. He later realized this was a red herring.
Contacts started being cleaned up (I have the corroded copper originals) when he noticed the voltage difference between the ECU and TCU, which indicates a ground loop problem. Many bad ground points were found and fixed. There is a service bulletin regarding ground point 43.
I disconnected the sensors and valve because that was all I could see on the wiring diagram that would draw current from the ECU, and it appeared to me the relay is only connected to the ECU, so I figured it was worth a shot.
He was at the point of thinking it needs a new wiring harness, but these are $1900 and there were no used ones he could locate. He recommended getting rid of the damn thing. He's the one in that shop stuck working with Audis because he has the experience and can fix them, but he said that is why he doesn't drive them.
He also thought it could be the TCU, though he doubts it because he has had it apart and it is very clean. No smoke has escaped from any of the components. He said, "But where are you going to get a spare TCU to test with?" and I thought, "I could try to borrow one on Audiworld again." But then he said, "And if you did, whatever the problem is could blow that one, so it would be risky." And I left it at that. However, a faulty driver sure sounds like the problem given that it had been driving a near-short wiring harness. (Which would be odd that the incompetent transmission shop would ultimately have the correct diagnosis once the overlooked obvious problem with the harness was fixed.) Do you know any way to test this?
I told him his Mercedes was nice, but that when I got back into the Audi, it felt wonderful. He said, "If you like Audis or BMWs, you won't be happy with anything else."
Ignition switch was replaced when mechanic accidentally discovered it threw the code when he turned off the ignition slowly. He later realized this was a red herring.
Contacts started being cleaned up (I have the corroded copper originals) when he noticed the voltage difference between the ECU and TCU, which indicates a ground loop problem. Many bad ground points were found and fixed. There is a service bulletin regarding ground point 43.
I disconnected the sensors and valve because that was all I could see on the wiring diagram that would draw current from the ECU, and it appeared to me the relay is only connected to the ECU, so I figured it was worth a shot.
He was at the point of thinking it needs a new wiring harness, but these are $1900 and there were no used ones he could locate. He recommended getting rid of the damn thing. He's the one in that shop stuck working with Audis because he has the experience and can fix them, but he said that is why he doesn't drive them.
He also thought it could be the TCU, though he doubts it because he has had it apart and it is very clean. No smoke has escaped from any of the components. He said, "But where are you going to get a spare TCU to test with?" and I thought, "I could try to borrow one on Audiworld again." But then he said, "And if you did, whatever the problem is could blow that one, so it would be risky." And I left it at that. However, a faulty driver sure sounds like the problem given that it had been driving a near-short wiring harness. (Which would be odd that the incompetent transmission shop would ultimately have the correct diagnosis once the overlooked obvious problem with the harness was fixed.) Do you know any way to test this?
I told him his Mercedes was nice, but that when I got back into the Audi, it felt wonderful. He said, "If you like Audis or BMWs, you won't be happy with anything else."