Tach stopped working after hose down

[aussie paul]

Guys any ideas where to start looking? My stock 94 XJ 4 litre sports has a full guage pack. For the last few years the tach, and only the tach has stopped working for a few days after a lot of rain (car parks outside) then it drys out and all OK. Last week I lightly hosed out the engine bay and tach has not worked since.
Any suggestions as the where faulty connection might be.
Thanks for any replies.

 

[MJR]

PCM connector possibly but I would suspect it's in the bulkhead connector (firewall driverside below booster/behind under dash fuse box.

 

[aussie paul]

Thanks MJR I'll check that bulkhead connection this weekend.

 

[Rob Mayercik]

Could also be the computer itself - that's what happened to my '92.

Had the "no tach for a day or so" bit after crossing water on the trail. Then, I started getting lots of crank-and-no-fire when trying to start. End result for me was a reman PCM.

My guess is that either the water shorted something in the connector or (more likely) the 14-year-old silicone-like material used to "pot" the internals of the computer had started to fail and water got into the electronics.

You can "shore up" the potting in your PCM by taking it apart and appyling a liberal coating of RTV over the existing material. Just be sure to use the kind that doesn't release ammonia or acetic acid. I did this to repair the potting material in the spark control computer of an '84 dodge (whose computer IS the intake duct), and it worked nicely.

Good luck.