I recently discovered that I had a dead battery in my XJS and hooked up a battery charger to the battery.{I didn't unhook the negative battery terminal as I probably should have} Once the battery was fully recharged, I removed the charger and started the XJS. I immediately discovered that the speedometer was pegged in a clockwise rotation indicating a speed in excess of 160MPH. What might of happened and can I somehow remedy the problem$? Any and all help is most appreciated. Thanks in advance, Fr.ick

 

The needle pegged purely as a result of using the battery charger?

FWIW, can't say I ever heard of that. Not sure how it would happen. Was the key was 'on' at the time of using the charger?

Was IS common, not just on old Jags, is for a speedo to react wildly when there is a large voltage drop. Poor connections, poor ground, or cranking the engine with a weak battery. If that's what happened I suspect the needle swung over and simply became stuck mechanically. remove the cluster (not a big deal, really) and then remove the speedo (or maybe just the lens?) a flop the needle back over and then see what happens

If you remove the cluster make sure to gently clean all the contact points. Lots a guys add an extra ground wire to the cluster while they're at it.

Cheers
DD

 

A lot of clusters do a needle sweep (0 to max) on some sort of reset / power up sequence. The needle's not meant to get stuck, though!

 

Doug, I just know that the speedometer was pegged after the battery recharge. It might be a big coincidence, but the fact remains that I have an inoperative speedometer. I notice that when I slowly turn the ignition to start that the needle moves back ever so slightly and then quickly returns to the extreme pegged position. This movement is about an eight of an inch. I probably need to remove the instrument cluster and see if I can free up the needle. I will let you know what happens. Finally, thank you for your quicp reply, Fr. Rick

 

I've moved your question from General Tech Help to XJS forum.

Graham

 

Here's something that has worked on some other makes of cars that have had a similar problem. With the key off, take a strong magnet, wrapped in a soft cloth and see if you cannot get the speedo needle to rotate back to the at rest position. Many gauges are moved by magnetism, and if that is the case with the XJS speedo, (I don't know) that should get you back to zero. Once at zero, ot may work OK again. I agree that voltage spikes or ground issues are likely to be the root cause of the needle jumping up to 160.

 

Hi Fr.Rick

Exactly the same thing happened to me!

So I'm guessing you have a 4.0L

Changed the Battery for a New one and as soon as I put the Terminals on, the Speedo Flew right over to 160 mph

Tried the Magnet idea and that didn't work!

At which point a slight trace of panic started to set in

Though not for long, as I managed to Fix it!

What you do is remove the Speedo Binnacle in one piece, while being very careful not to damage the printed circuit on the back and also after removing the Two electric Multiplugs which plug in the back

All of which is an absolute 'Walk in the Park'

Then once you've got the Binnacle out, hold it like you would a Steering Wheel and then revolve it, at which point the Speedo Needle will flop back into position at which point you can then replace it in the Car

The Full procedure including Photos, is all detailed here, on Page: 97 of my Cherry Blossom Restoration Thread

Speedo Fix when Needle flew over to 160 mph when Changing the Battery

The Thread contains lots of other stuff including Fitting New Injectors and Replacing Spark Plugs and generally almost every Trick in the Book to get her to Start

 

Thank you all for the help. I am so happy to be part of a group that truly cares for other folks and their problems. To all who answered, I am going to gently remove the instrument cluster and delicately see if I can move the needle back to where it should be. Again, thank you all. Blessings! Fr. Rick

 

If you can't easily get a voltmeter onto the starter motor wires, try checking the resting voltage at the bulkhead power points and then monitor that voltage as the starter turns. If you get a lower than resting voltage at the bulkhead compared to measuring straight at the battery, or an abnormal drop when the starter is turning, you may have an earth or cable problem that is causing excessive voltage drop between the battery and the front of the car.

Good luck

Paul

 

I know my oil pressure and temp gauges in my 93 convertible, peg full right on startup and they would get stuck on the fake wood surround on the cluster, partially because once they go so far, the internal spring wants to make it keep going that way. Cheap easy "fix" was to drill a small hole and install " pegs" made from cut trim nails. Now the needles hit the peg and go back home when shut-off. I really need to get those working properly at some point! Of course today I installed led bulbs and buttoned everything back up only to find that some of the warning lights don't work and also that my speedo decided to stop! Argh! Oh well I'll have to read it open again!