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New Alternator & battery, techo still jump up and down ...

Ed Cheung

Accelerating Away
Folks, I my Disco need some help.
Few months ago I replaced the alternator and the battery, it ran fine but after a while, the tacho started to jump up and down, and from the head lights you can see the lights got brighter, then dimmer the brighter..., and at idle with gear selector at D, you can here the tone of the engine goes up and down as will.
First, i thought it was a comes and goes thing, as sometimes it didn't happens. But as I started the Disco this morning, I saw the jumping needle of the technometer, I was thinking to myself, well let's see how long it last this time, and as the engine warmed up I turned up the heater fan to 3, SUPRISINGLY the needle on the techo act normal! I turned off the heater, needle up and down again, switch on the lights, needle stay right on it's spot, off the headlights and the needle start jumping again.
So whenever I had some electric power hunger device switched on, the needle of the techo doesn't jump.
BTW, when the engine is idle I do notices a whoop whoop ....sound come up at the belt, and I first suspect it was the belt tensioner, but seems the belt is nice and tight around those pully. Are they related? Any ideas? And is this affecting the electricity to the spark plugs as well as I can hear the tone of engine goes up and down as well,( don't tell me the techo was wire wrong so the needle's movement depended on the tone of the engine).
Thanks in advance.
It could be the tensioner spring is weakening. We change quite a few on other types of vehicles, ED. When the engine is running, the tensioner bobs about, instead of putting a constant tension on the belt.

JONV8 is probably the Man to give you a better indication of the electrical problem.

We have just started to get into the realms of the ECU controlled Alternator on Jaguars, and they aren't much fun.:(

Thanks for the your help, Chris. I will see how it goes with a new tension then.

ECU controlled Alternator on Jaguars??? simple device can be really complicated nowadays.
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