Voltage always low new battery and new ult?

Hello I have a 1982 Jaguar xj6 I just replaced the alternator and the battery brand new everything works! But I’m having a crazy rough idle when the voltage starts to drop to red it never reaches the 13 it should be at I know for a fact it’s not a vacuum issue Causing the idle because I just fixed it and changed every tub could it be the ignition coil ? Or something else ?

This is where the meter is at all the time

what does the voltage read at the back of the gauge?
that looks like my S1, i have 13.5v at the back of the gauge but gauge reads low

Is it at idle that the volt meter reading falls to the low level? Is the ignition light on? I have two cars that if I start them the light will not go out and the voltage will not rise until the engine is revved a little. Once this is done the excitation seems to “stick” and the light stays out at idle. My XJ is a Series 1, simpler than your car. Perhaps the electronic ignition or fuel system in yours needs the voltage maintained at idle? Paul.

My S3 did the same on hot starts. Rev it up to 1500 and it started to charge. Idle is a little slow for the alternator anyways. The gauge is not to be trusted, but it is very likely alright, with the alternator just not charging.

Put a volt meter to the battery direct or a plug in tester in the lighter
If it’s reading 9 or below the alt is bad
Sure battery is charged at the get go
Sure cables are clean at ground?
I forgot if your year had a seperate regulator?
Turn on head lights do they get brighter when you hit the gas?
Good luck
You just might have a low battery or loose belt to start
Don’t trust the gage

Any alt will charge better when revved up. But the other factor that may be at play here is that you have just started the car, which has discharged the battery a little bit. Right after starting the alt gets loaded up recharging the battery. That’s why, when the belt is loose, it squeals right after startup. The load of recharging the battery will pull the voltage down a little bit.

Be careful not to confuse ‘cause’ and ‘effect’, Josiah…

Voltage readings will drop if the idle drops - but the voltage drop has no effect on engine rpms. Ie, the voltage depends on rpms; at very low rpms the voltage will drop to battery voltage - which is perfectly adequate for running the engine.

While the charging voltage should be supervised with a multimeter to ensure the alternator performs as it should - you should pursue the idle. What is your hot and cold idle; the first should be around 800 rpms - the latter some 400+ rpms higher. If not; ‘why not’, and ‘when’ is material…

xj6 85 Sov Europe (UK/NZ)

The dash gauge is “approximate” to say the least.
It’s a thermal bi-metalic strip type and it’s very slow to react.
Measure with a real volt meter, on idle and on 1.500RPM and report back.

YUP!! And a tad slow to react. I made a jumper for my VOM. Allows it to connect to the car via the cigar lighter. I can read volts in real time at various loads and RPMs. My suspected alternator was innocent, my battery, not so. Big GM Delco, 140 amps, easy…

I thought i was a genius. Ugh, a simple plug in device exists!!!