Transmission light and limp mode after hard rain

Been working on this one for a while, finally making my way here. I have a 1991 XJ40. I’m already aware of rain getting into the car, but I forgot the cover one night and a big rain hit. The day after, transmission started acting up, but only after warming up for a bit.

Felt like Limp mode. Transmission light didn’t come on neither did error codes. The Transmission light eventually did come on after battery drained out / was charged again.

Checked the TPS and checked the purple cube relay under the hood (I believe that’s the transmission relay).

Water enters the gearbox computer through the wiring harness. Seal the gland under the vacuum accumulator under the battery. There was a jaguar service bulletin for that. Check if the gearbox computer inside is not wet.

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44-24.pdf (31.3 KB)

From reading up on tech bulletin 04-18, my 91 has the battery in the Boot (trunk) would be gearbox computer be located under the firewall?

Odd that a 91MY would have the battery in the trunk? I thought that was just 1993MY on?

…->92, the computer is located under the passenger compartment

Well I just learned my Jag is in fact, a 93 not a 91. The paperwork was wrong !

That’s why it’s best to put the vin in your profile, the year means very little usually.

And update on progress:

  1. Because it was inexpensive, I replaced the TCM. I noticed that the dashboard light for Sports mode now comes on when it didn’t before. But the transmission light still persists

  2. I know this is related to rain/water somehow. Changed out the Purple relay on the firewall with good ones, but no luck

  3. I checked the transmission decoder module, and everything looks on the new side and good there (though i didn’t do any electrical testing). I don’t think water disrupted this module at all, but it’s in the check list for transmission light

  4. I’m rebooting after all these steps (taking battery off waiting, and putting it back)

  5. Checked TCM battery supply fuse (good)

  6. Though i’ve inspected the TPS and it is very clean, as is it’s connector - i did not check the electrical readings (i was told that problems with the TP potentionmeter would throw a check engine light)

Next up is the Transmission Rotary Switch check,

You could use a dry napkin to probe connectors in car under dash etc etc and if any water drop or damp spot… on napkin get in there and blast it with WD40 water displacement spray… and hopefully you can figure out the leak point and seal it.

You’d think with all the rain in England they’d be experts at sealing cars…

Finally getting around to an update here.

Found the major leak and plugged it up. Big hole on the passenger side where the windshield meets the body (under the small grill).

Lots of water hit the car in the initial flood. All dried out after water fans and a few months drying.

Lights still on - i’m not feeling the car in limp mode anymore - just a major Jerk when changing gears.

Transmission Rotary switch is all clean, and not the issue. For the TCM, i just did a swap for one unit to the other. Anything special when swapping the TCM that I should know about ?

Other thoughts on the transmission light of doom?