Tach pinned full CW

I was breaking the Jag our from winter storage yesterday. As usual the ATF had drained on the ground so I filled it back up and was working the shifter into Rev - N - Drive, etc. to get the new fluid circulating when suddenly the tach pinned full CW (i.e. beyond redline). Engine was still at idle but seemed to be running a little rough. (I don’t think it was rough before the tach pinned, but not sure.) Engine revs OK but tach doesn’t budge. It does fall full CCW with switch off. Today the tach seems to be working normally, but I know it is only a matter of time…
What could cause this? Is there anything I can investigate now or do I wait for it to happen again and then what to look at/for?
Thanks for any thoughts

Maybe the infamous bad contacts in the back of the instrument cluster - what I’m dealing with now except with the speedo and a couple of the tumbler gauges. Don

Yup. Poor ground or poor voltage supply; almost certainly bad contacts/connections somewhere

Cheers
DD

What year is your Jaguar and what engine does it have?

Paul

1992 v12 convertible

Well, of course, it happened again. While driving today, the tach jumped around alot, pinned CW and then dropped to 0. At the same time, the engine became very weak, no power and put up the check engine light with code 44 (O2 sensor, I think R). I limped home. Now I don’t believe in coincidences so something is connected here, but I have no idea what. In the FWIW dept., the temp gauge is not working so that adds weight to the loose connection theory, but what about the other things?
I’ll try to crawl under the dash tomorrow and see if I can get at the connectors. I’m getting pretty good at Jaguar Yoga.

Before getting under the dash check the battery connections and main ground. And check the “+” voltage junction posts on the firewall. You may have a voltage drop out problem.

If no joy then the contortionist act can begin!

Cheers
DD

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Well, low voltage may indeed be the issue. I have been having an issue with my alternator. When I checked, the battery was at 11.9V. I charged the battery overnight (to 12.8) and everything was fine. I took it for a drive and the alternator was sort of working off and on. Eventually the tach went crazy again, pegging high then dropping to zero. When the tach hit 0, the engine lost power dramatically and started backfiring. When it pegged high, the engine ran normally. At least this time it didn’t throw a ‘check engine’ code. When I got home, the battery read 12.3 with the engine running and rose to 12.45 when I shut it down. Clearly the alternator was not working, or at least not working well.The weird thing is that this alternator has <100 miles on it and it was replaced because the old one was doing the exact same thing (but without the tach stuff).
Any and all ideas welcome!!

Well, since it doing same as previous, I wonder if there is a loose wire causing these issues

Start checking at the alternator.

It happened to me too. In my case, the fault was false contacts in the positive and negative poles located in the bulkhead of the engine compartment, near the hood openings.
I also replaced the alternator (to no avail), before finding the problem.

Looking at the firewall from the engine side, I see two good sized studs with rubber covers on the passengers side and one on the drivers. Are these the ones you are talking about? If not, can you (or anyone) send a pic or a diagram of where I should be looking?

Yes, these are both +12v.
It’s good practice to check them and clean them.
Be very careful the are unfused and permanently connected to the battery.

Grounds there are many, the big ones are Alt to Chassis, Bat to Chassis, Engine to Chassis.

Yes, those are them