Need to borrow TCU to swap out for diagnosis (Boston)

[AudHaus]

I need to find a kind soul near Boston that would let me borrow the Transmission Control Unit from their '98 or similar A8 to swap into my car for an hour or so. My transmission has been entering limp mode and is no longer doing so intermittently, but fairly reliably once warmed up. The transmission shop thinks it is the TCU, but I'd like to verify that before I spend the big bucks that even a used one costs. I think that if I drive in city traffic for an hour without entering limp mode, that would prove the diagnosis.

It is easy to swap, I've had mine out. I believe it is the same as later model years as well, but don't know for sure. We could verify by the numbers on it. (Mine: 4D0 927 156 BH. Lots more numbers on it, too.)

Car is in Waltham area nights/weekends; Chelmsford by day. Can travel.

Thanks.

[PaulW]

[AudHaus]

[PaulW]

When you do an output test of the transmission, it cycles all the solenoids, either a short to plus (which means it is energizing it with 12 VDC) and then when you're done with the whole output tests, you check fault codes.

If none are found, then the solenoid checks good. Basically the computer is checking current flow through the solenoid.

So were there any fault codes?

Any particular reason you think it's the TCU?

What is the symptoms with the tranny?

[AudHaus]

Last summer it was throwing 2 codes, sometimes short to ground, and sometimes short to plus, but when it restarted recently, it is always short to plus. (I don't have the exact number of the code with me.) These codes are read after a failure, not from the test I think you're talking about. Last summer it would sometimes run through the tests without problem, but occasionally would enter limp mode during the test on #5 solenoid, with one of the resulting codes.

Because the transmission shop can find nothing wrong with the transmission, and because they went ahead and replaced the #5 solenoid last summer, they now believe it is symptomatic of a TCU failure, possibly from overheating. If that's not it (ouch$!) then it is probably the wiring harness. I'd like to diagnose it more rationally than throwing expensive parts at it.

It never (well, once recently just as I was entering the highway and going from 2nd to 3rd at 65) goes into limp mode at highway speeds. It can be driven long distances on the highway. Low speeds, once warmed up, throw it into limp mode. There is no consistency with the gear it is in, or trying to shift to, other than lower ones. I've never seen it limp from 5th gear.

After the high-speed limp I mentioned above, there was also an "incorrect gear ratio" code. There is also an occasional overheat code, which the transmission shop thinks is because it would be likely to overheat being driven locked in 4th gear. Engine very occasionally (once every few months or so) throws an "implausible RPM signal" code, which I wonder if it could have something to do with it, but I don't want to spend money chasing an engine problem until I get the transmission working.

There is no evidence of water entry into the computer area, and the TCU didn't seem hot, just normally warm for the area it is in. The wiring harness is what runs by the exhaust, so I'd be more inclined to blame it.

I'd love to find it is the TCU via a swap fixing the problem, because at least I'd have something to spend money on knowing it would be fixed. Wiring harness will be even more expensive and difficult to prove without replacement.

[PaulW]

Okay.....now you're getting somewhere.

You've got a code after limp, that's good.

You've had the incorrect gear ratio which means that a clutch could not be holding or working properly (not good).

The clutches require to operate correctly to keep the tranny from going into limp mode.

I have a spare TCU that I could loan you, but you'd have to give me a deposit via PayPal and pay for the shipping.

I have one that came with my 97 and a 98 TCU, I think the 98 TCU is in my car right now. The 97 TCU has a different suffix of a lower letter or something.

If you post the exact code, we could maybe troubleshoot it. If it's electrical, it could be the TCU, or it could be in the wiring, but most shops are not good at troubleshooting the wiring.

[AudHaus]

Searching the forum turns up that last summer it initally gave:
18242 - Pressure Control Valve 5 (N233): Open or short to ground P1834 - 35 - 00 - -

It sometimes gave the adjacently numbered code, which was short to plus. It only and always gives that now. I'll verify tonight when I have my laptop. I've saved all the codes on it.

[tozoM8]

How the tranny shop know, that the tranny is good without disassembling it? The tranny would never overheat because it is locked in 4th gear.( only if you drive it over 90 mph for a long distance) It will overheat if you drive it in limp mode on the hwy. In limp mode the TC is open, so it generates a lot of heat.
We might be able to help you more if you get those codes.

[ttuling]

There could be corrosion from water in the 16-pin connector where the wiring from the TCU enters the transmission. It could be an easy fix if the connections can be cleaned.

If you don't want to crawl under the car but have an ohmmeter, the following tests from the Robert Bentley service manual (useful for these things) are done with the ignition off, TCU unplugged, and by measuring in the wiring harness plug that's normally plugged into the TCU. The manual also specifies 10 to 40 C ambient temperature, which presumably applies to the transmission itself and might affect solenoid resistance. However, you might want to check it when hot since that's when it acts up.

For pressure regulating valve 5 (corresponding to code 18242):
6 to 10 ohms between pins 52 and 51 (across solenoid valve),
infinite ohms between pins 51 and 34 (no short to ground),
less than 1 ohm between pins 52 and 53 (both solenoid common wires, tied together at the transmission).

For the transmission fluid temperature sensor:
Between pins 21 and 22:
approximately 0.83 k ohms at 20 C,
1.28 k ohms at 60 C
1.88 k ohms at 120 C
infinite ohms between pins 21 and 34,
infinite ohms between pins 22 and 28,
infinite ohms between pins 21 and 54,
infinite ohms between pins 22 and 54.

Tom

[ttuling]

There used to be cars made with manual synchromesh transmissions behind torque converters, and automatic clutch disengagement when you touch the gear lever (e.g. NSU Ro 80, Citroëns GS and CX with C-Matic. Strangely, the only one of these so equipped that I've driven was a green Ro 80 in Seattle).

On at least some of these cars, there was a converter fluid overheat warning light. It was there not because top gear was inappropriate for highway driving with a (non-lockup) torque converter, but because drivers might be tempted to use top gear in congested traffic or to climb hills at low speeds.

Tom