Okay, this is weird - it happened again

My Sentra doesn't get much highway use, just back and forth to work each day. The only time it sees on the freeway is back and forth to autocrosses. While returning from one back in April, the engine skipped a beat, dropped out of cruise control, and the tach dropped to zero and stayed there. The engine was still running fine, and hitting the resume button reactivated the cruise. The tach needle stayed at zero for the rest of the trip home, but resumed normal function on the next restart. I've had no issues with it since, so I kind of forgot about until yesterday. I went autocrossing again at the same site (did pretty well, too - first in class and fifth overall on the PAX index), and the same exact problem occurred on the return trip. This time the tach started working again as soon as I got off of the highway.

This seems like a heat issue, but to what? If it was the tach itself, that shouldn't have made the cruise drop out. If it was the coil, you'd think the engine would quit along with the tach. A wire to the tach pickup? Wouldn't that prevent the cruise from re-engaging? A real puzzler.

Slow down on the access road

My Toyota Camri (plural for Camry?)used to do that when the batteries started to die, usually due to boiling the water out of the battery. Yes, even on maintenance free batteries. Check the water level, or have the battery tested. And YMMV.

I checked the battery Saturday night before the race. It looks like it has plenty of water, and it cranks the engine over just fine.

In reply to Entropyman:

I tried taking the access road at 15 on the way out and saw two full inches of cowl shake in spots. It's way smoother at 25.

CRX's are notorious for weird tach fluctuation problems... usually traceable to the ground (from the distributor) at the thermostat ... gets corroded and makes the tack all kinds of flaky... (also sometimes the connection behind the dash)

I was going to suggest an electrical issue. With the extreme turning required in an autox.. either something is flexing, or a wire is getting pulled taut

I don't think either of those is an issue with this car. It's garaged at night, so there's zero corrosion anywhere on the car. If it were an autocrossing stress issue, you'd think it would show up during a run; but no, it performs flawlessly during the event. It's about 60 miles from home to the autocross site, and I was about half way home when the problem occurred. It seems to me that something is getting heat soaked after extended running, but I can't quite figure out what that might be.

Electrical gremlins LOOVVE to find a little bit of rust or corrosion to drive people nuts. If your area uses salt or chemical deicer on the roads, and you've driven through the resulting slush, you've got some corrosion somewhere that's affecting a ground. I had a Renault R-12 that gave me lots of grief before I replaced several of the chassis ground connections. It's definitly worth looking further.

I had a Nissan 4x4 which would do something similar, except the tach would keep working. That turned out to be a crappy connection in the fuse block; someone had used one of those adapters that goes on a 'leg' of the fuse to add a power tap. It had bent the terminals in the fuse block enough so that if you hit a bump, the fuse would shake, break contact and the electrical weirdness would begin. Once I made the 'hit a bump' connection (I am slow sometimes ) I was able to duplicate it by slapping the dash in the area of the fuse block, then wiggling the fuses one at a time. The fun part was that the truck had to be moving at the time.

Back in the stone age, there was a billboard that used to shut VW's off. The lighting transformer leaked some weird frequency, and it would suppress the EFI computer, and the car would shut off. We have a lighting transformer at work that will kill FM radio reception for about two blocks.