I am doing an AWD transmission change and the replacement transmission which is a later model has a blanking plate instead of an electrical socket next to the firewall engine mount. This is next to the diff. Can anyone tell me what it is for and what I should do about it?

If you have a TJ/KJ and have acquired a gearbox from an L or W series, you have a problem as the speed sensor arrangement (including the ECU support) is quite different. From what I understand, that blanking plate is where the output shaft sensor would be in a J series box (hence the connector). Your replacement gearbox won't have the toothed wheel used for the OSS so just installing the sensor won't help. This issue affects FWDs as well as AWDs.

MadMagna might have figured out a way round this otherwise the only other alternatives would seem to involve either rebuilding your old box from the internals of the replacement box making sure the OSS wheel is carried over (if it isn't damaged), or having the replacement box modified by fitting the OS sensor and wheel. Or you might be able to trade your L/W gearbox for a J gearbox...

Thanks for your reply WytWun, I didn't want to find this out after it was in! The Magna I'm talking about is a dirt circuit race car. I'm guessing without the speed sensor it wont change gears even when driven using the tiptronic?. Would it be possible to control the solenoids directly to change gears bypassing the ECU? It can be done with other cars such as a Prado - http://www.offroadexpress.co.nz/Forums/viewtopic.php?f=30&t=25229

Or, How easy would it be to move over the toothed wheel? Alternatively I could change the ECU & loom but this would be a pain and where does it get speed info from? I have the complete vehicle that the "new" transmission came from. Its a stat write off. The old transmission is 03/2003 the new transmission is 02/05.