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error code P0740

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Some of you may know that my Boxster had a problem of not having any speed signal with both the ABS and Spoiler warning light on.

The CEL light is NOT on. The transmission 4th Gear light is NOT on, either.

But, the car has been shifting poorly lately especially when cold. It lurches and slips and the gears don't engage or downshift properly. It is still driveable, but only if carefully shifting. The problem is more marked when driving gently. It seems to be masked when driving aggressively (the delay in shifting and slip between gears become less noticeable to me).

Having spent heaps of money on my Boxster last year alone, I have decided to trouble shoot this one myself. So I bought a OBD2 scanner.

It didn't return any speedo signal related codes!

It did return a P0740 which is "torque converter solenoid circuit open".

I am guessing that this means the torque converter is not engaging properly due to this solenoid not switching it right.

I have two questions for those in the know:

1. What is the most likely cause/fix to this P0740? Is replacing the solenoid a start and can that be DIY?

2. Can this P0740 also relate to the lack of speedo signal problem which surprisingly doesn't seem to show any related codes at the moment?

The scanner I used is from http://www.obd-2.com/

Please help. The last thing I need now is a transmission replacement. It's a 1997 Boxster with 120kkm on it. No tiptronic service yet as far as I know.

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1. Potential fault P0740 causes are:

- ATF level not OK - check ATF supply

- Torque converter clutch worn. - check torque converter

- Mechanical damage to pressure regulator 4. - check pressure regulator 4 (hydraulic)

- Open circuit, short circuit to ground, short circuit - check electrical triggering to pressure regulator 4 to B+.

- Hydraulic control unit - check for sticking valves.

2. Not all pcodes cause a CEL light.

If the speedo signal problem is ABS related then you may have an ABS code. Unfortunately Porsche has not shared those codes with many companies so ABS can not be read with scan tools other than a PST2 or PIWIS. Your car can also generate code for Body faults - B code, C drivetrain faults - C codes, and U faults - for undefined (by SAE) faults. Porsche only publishes P codes.

Some independent shops have invested in PST2 tools so perhaps one could read your full code faults for you (for a nominal fee). Either way you should try to identify and fix the P040 cause - which could be as simple as low ATF level.

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Thanks Loren.

After some further checks on the speed sensors and revealed no obvious problems, I decided to take the car into the workshop.

Where I was told that the ABS controller was faulty and the tiptronic may need a rebuild. I was quoted:

1. Around $2500AUD to replace the ABS controller unit with a 2nd hand item

2. Around $4000AUD to rebuild the tiptronic transmission.

I was wondering if it's possible to replace the ABS controller myself? Has anyone attempted it or has any info on it?

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