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Old 01-12-2010, 03:11 PM   #1
Tony
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Default Can I Overcome P0500 (VSS low/malfunction) by pulling ABS fuse?

Background info: '05 Saab 9-2X Aero (WRX Wagon) with a shredded speedo gear in tranmission. I'm getting a P0500 code since the VSS is working and reading zero while the ECU is seeing wheel speed from the ABS module (I assume).

I am working on getting the tranmission dropped to get to/replace the speedo gear but in the mean time I'm in limp-home mode due to the discrepancy between the VSS and ABS. Since I'd like to drive this for a week or two without running in limp-home mode and potentially melting a cat due to being rich my question is: Do you think I could pull the ABS fuse and then reset the ECU thereby removing any discrepancy between sensors (at the cost of functional ABS) and get out of limp-home mode?

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Old 01-12-2010, 05:08 PM   #2
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no....and do NOT drive the car....you WILL **** it up...BAD
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Old 01-12-2010, 05:34 PM   #3
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Not to seem ungrateful for some input on this but care to share the tech behind your comment?

Tony
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Old 01-12-2010, 06:11 PM   #4
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Auto or 5 speed? If auto, don't drive it. 5 speed you might be ok doing what you describe.
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Old 01-12-2010, 09:14 PM   #5
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Quote:
Originally Posted by 69subaru360 View Post
Auto or 5 speed? If auto, don't drive it. 5 speed you might be ok doing what you describe.
5-speed. Well I tried it and while I did not "**** it up...BAD" it didn't make a difference. I cleared the CEL with the AP but after 5 minutes of driving the CEL came back and the car was back into limp-home mode :/ I suppose all the means is that the ECU gets the wheels sensor data directly and is not dependent on a functional ABS. Any other ideas on getting out of limp-home mode with a non-functional VSS? I'll park it in the mean time and drive my old Miata (no snow please!).

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Old 01-12-2010, 09:51 PM   #6
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Quote:
Originally Posted by Tony View Post
5-speed. Well I tried it and while I did not "**** it up...BAD"

Tony



what is it with d000000000ds here, and total lack of understanding

simple
direct
and
straight
foreward

posts

if you continue to drive the car in 'limp' mode

Quote:
you WILL **** it up...BAD
questions????
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Old 01-12-2010, 11:02 PM   #7
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Quote:
Originally Posted by Uncle Scotty View Post
what is it with d000000000ds here, and total lack of understanding
Asking a technical question in a technical forum and getting "**** it up...BAD" as an answer is less than helpful.

Quote:
simple
direct
and
straight
foreward [sic]

posts
You only managed simple.

Back on topic, is there any other method to avoid the limp-home mode with a functional VSS but one that's constantly reading zero due to the bad speedo gear? I'm guessing it's unlikely. I suppose I could disconnect the ABS wheel sensors themselves if that is actually what's causing the discrepancy. It would also depend on if simply leaving the ABS wheel sensor connections open would even work or if the ECU needs to see some specific resistance in the circuit for each sensor. Anyone know that?

Tony
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Old 01-12-2010, 11:59 PM   #8
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If the VSS is not reading any valid data it severely limits the operability of the vehicle. The ECU won't ignore the condition and try to read from the ABS sensors. That information is tied into a separate ABS computer, which to my knowledge does not share wheel speeds with the ECU. On newer vehicles this may be the case, but not the 2.0 WRX's.

Driving it around in limp mode for lengthy periods of time can cause engine detonation and damage. The ECU fail safes are meant to get you safely off the road until the condition can be repaired.
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Old 01-13-2010, 12:01 AM   #9
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Quote:
Originally Posted by Tony View Post
It would also depend on if simply leaving the ABS wheel sensor connections open would even work or if the ECU needs to see some specific resistance in the circuit for each sensor. Anyone know that?

Tony
I'm only guessing that there would have to be some sort of fail safe that if you unplugged the sensor, you would get a CEL for a bad abs sensor.

Aside from the gear for the VSS, what is it external of the trans case? EG is there a connection you could have jarred while doing exhaust work?

Check your blinker fluid... maybe thats low.
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Old 01-13-2010, 10:19 AM   #10
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Quote:
Originally Posted by Haelan View Post
If the VSS is not reading any valid data it severely limits the operability of the vehicle. The ECU won't ignore the condition and try to read from the ABS sensors. That information is tied into a separate ABS computer, which to my knowledge does not share wheel speeds with the ECU. On newer vehicles this may be the case, but not the 2.0 WRX's.
Thanks Haelan. I agree mostly but the VSS is giving a "valid" signal, zero. It's fully functional but will always read no vehicle speed since the gear that drives it is toast. The car should be able to run the engine with the car at a standstill so what causes the CEL? (can't you rev the engine indefinitely while parked?) My guess was that it also looked at the ABS to compare the two to know that the car was indeed moving but the VSS signal didn't agree and hence the CEL. What else would trip the CEL if it's not the ABS? I can't imagine the VSS is critical to engine operation on a 5-speed so I'm hoping there's a relatively simple way to get it to ignore or at least not see the conflict with that sensor. I know driving it in limp-home mode is bad, I don't intend to, my question is how to get it out of limp-home mode with a VSS that reads zero.

Quote:
Originally Posted by Mike2000GT
Aside from the gear for the VSS, what is it external of the trans case? EG is there a connection you could have jarred while doing exhaust work?
I've verified the VSS is fully functional, it's just the gear that drives it that's the issue. Also, I topped off the (HID) blinker fluid

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Old 01-13-2010, 11:03 AM   #11
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When you're parked you either have the clutch in or the shifter is in neutral position. If neither of those inputs are reading then the ECU assumes you are in gear at whatever RPM you're currently cruising at. The ECU also calculates engine load and when in neutral and revving--there is basically none.

There are a bunch of factors that determine the car is being driven, but like I said, the ABS sensors don't provide that sort of feedback.
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Old 01-13-2010, 11:18 AM   #12
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Ah, that's what I was missing. I didn't consider that the clutch and neutral position switches would be ECU inputs to determine if the engine should or shouldn't be under load.

Thanks Haelan for bringing some actual tech to this thread. There doesn't seem to be any easy work around to this so it'll be parked until I get around to dropping the transmission.

Tony
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Old 01-13-2010, 12:52 PM   #13
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Your VSS being out throws the car into limp mode, period. VSS doesn't come into the picture at all.

Logic is suggesting that limp mode = base timing, and from reading around, no timing + subaru motor = unhappy motor... So, just don't drive it. Unless there's multiple failsafe modes and I'm thinking of a different one. I am now going to school to learn about automatic transmissions, and to put the turbo 350 back together that I ripped apart yesterday.
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Old 01-13-2010, 01:30 PM   #14
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Again, I don't plan to drive the car in limp-home mode. I want to see if there's a relatively simple way to get it out of limp-home mode with a VSS that thinks the car is stationary. So far the answer is definitely no.

But while we're talking limp-home mode and since I'd prefer to drive it than tow it to where I'll do the transmission work. Is limp-home mode open or closed loop? I'm assuming/hoping closed. Also, there's another issue, it has a custom AP map setup to use an aftermarket BCS. To confuse matters more I only have the realtime map loaded, not the base map (which is currently the AP stage 2 93 oct map). Are you telling me the limp-home mode is also using the timing for the base map rather than the realtime map too?

Tony
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Old 01-13-2010, 01:38 PM   #15
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'Limp mode' is when the DAM goes to 0. This effectively turns off the boost control and will not used advanced timing. This is via ROMRaider's website.

There is a lot of confusion when someone says they have a CEL and claim they are in 'limp mode'. The truth is the behavior of the vehicle will depend on the problem and associated CEL. Each CEL has different 'fail safes'--some have an entire list of fail safes while some CEL's only have one associated fail safe. These are all listed in the DTC manual for each problem code.

Having a CEL does not automatically throw the vehicle into 'limp mode'. Limp mode happens when fail safes kick in and based on the adjustments the normal method of determining the DAM drops to 0.

In the case of the VSS I was not able to find P0500, but the other codes associated with the VSS contain a rather large list of fail safes including fuel cut well before redline.
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Old 01-13-2010, 02:02 PM   #16
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Sorry, I believe the code is P0500 or P0502. And there is definitely a fuel cut at 4500 rpm.

Can you point me to your source of the DTC manual? I don't have it in the PDF manuals I have. Only a description of the codes and the troubleshooting guide to fix each. No mention of the fail safes assosiated with each code.

Thanks,
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Old 01-13-2010, 02:43 PM   #17
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It's part of the service manual. I believe it's listed under 'DTC Criteria' under the Engine section, but I can double check on that later for ya.
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Old 01-13-2010, 05:15 PM   #18
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You will be in Limp mode until the VSS is fixed.

Every Subaru I've had from 1995 until 2003 went into limp mode when the VSS went out. The VSS is a necessary component for timing advance. Trying to jerry rig some crap with your ABS or disabling your wheel speed sensors is a horrible idea, and will most likely cause more problem in the future.

The simple, straight forward idea is to just fix the ROOT of the problem, and the ROOT CAUSE of LIMP MODE (fuel cut at 4,500 RPM is a failsafe/limp mode) which is your defective VSS. Mucking about on the forums for "limp mode defeat" is pointless and a waste of time. If your speedometer is not working, and your odometer is not working, then you have a VSS problem, or a gauge cluster problem. If your VSS is intact and not the problem, you will need a new cluster (or to figure out what is causing the lack of VSS signal to the ECU causing limp mode).
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Old 01-13-2010, 07:38 PM   #19
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Chi_San... you are NOT listening to the entire situation.

The VSS is WORKING. It is sending a VALID signal based on it's input. The problem is that the signal is static because the transmission gear is broken. The solution that is being looked for is to TRICK the car into thinking that it is not rolling, so that the OP can drive the car to the repair facility. He's not looking at this hack as a permanent fix.

Tony, what Haelan said regarding the neutral switch and clutch make sense. Rigging the neutral switch might give you a shot a temporarily drivable car.
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Old 01-13-2010, 08:06 PM   #20
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flatbed / tow or trailer the car..
$80 <<< $3000
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Old 01-13-2010, 08:08 PM   #21
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Not to start a flamebait war, but actually he wanted to drive it for a week or two in limp mode per the original post, which several members here have recommended against.

It it was a one time trip to get it repaired I'd recommend going for it--I love a good hack.
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Old 01-13-2010, 09:00 PM   #22
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No flame war intended Haelan (you seem to be the only one bringing good tech here) but my original post stated that I wanted to drive it for a week or two without being in limp-home mode. All these hacks/work-arounds were to fool the ECU into thinking the VSS reading zero vehicle speed were not a cause to go into "limp-home" mode.

I didn't believe the VSS had a meaningful input on anything related to engine's operating parameters though now Chi_San is saying the VSS has a meaningful input on timing advance. I have no proof otherwise but I'd like to hear if there's any evidence to that effect. Seems odd to me. Speed might have an effect on the charge temp but the ECU measures this directly I believe. Backing into charge temp via speed and making guesses at the intercoolers efficiency seems a very bizarre and error-prone way to get any meaningful engine operating parameter.

Tony
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Old 01-13-2010, 11:02 PM   #23
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IAT is measured at the MAF, so it is pre-charge. The ecu then uses forward velocity to predict airspeed and then heat transfer through the intercooler to estimate manifold inlet temperature.

Vehicle speed also has a direct control over the radiator fans. Speed also helps the ecu determine and estimate load conditions, which controls timing, and open and closed loop fueling.

The car is going into limp mode for a reason - the ecu is missing a piece of CRITICAL operational data and there is no way to bypass it or override it

If you want proof of any of this stuff, just take a look at the ecu control tables in romraider, and you can find several tables where vehicle speed has an input. If you have no input or conflicting input, the table values are rendered null, and then no control can go over that device, which again is why the car is going into limp mode.
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Old 01-13-2010, 11:30 PM   #24
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Quote:
Originally Posted by Uncanny View Post
IAT is measured at the MAF, so it is pre-charge. The ecu then uses forward velocity to predict airspeed and then heat transfer through the intercooler to estimate manifold inlet temperature.

Vehicle speed also has a direct control over the radiator fans. Speed also helps the ecu determine and estimate load conditions, which controls timing, and open and closed loop fueling.

The car is going into limp mode for a reason - the ecu is missing a piece of CRITICAL operational data and there is no way to bypass it or override it

If you want proof of any of this stuff, just take a look at the ecu control tables in romraider, and you can find several tables where vehicle speed has an input. If you have no input or conflicting input, the table values are rendered null, and then no control can go over that device, which again is why the car is going into limp mode.
Good stuff Uncanny, that is exactly the type of info I was looking for. Much appreciated. Doesn't solve my initial issue but that's my problem and now I know why I can't solve it.

Thanks,
Tony

P.S. Still find it an odd way to get the charge temp. I'm guessing they have their reasons but logically I'd want a primary measurement for IAT not some estimate that could vary from something as simple as a few leaves blocking some air flow over the intercooler. But that's another discussion and I'd likely be wrong anyway

Last edited by Tony; 01-13-2010 at 11:33 PM. Reason: Added post script
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