Hi Everyone.
i have a new condition that I wanted to see if it will cause a problem. I just drive my 1986 XJ6 Vaden Plas to Phoniex. It’s a bit hotter here.

I just noticed the rpm needle idles at 600rpm (it was 1000 before)
and if I speed up to 30mpg the needle goes from 2000 rpm and drops to 1500 with a little bit of bouncing.

This only started to happen. Is there something I should be aware of? New transmission oil?
does it need to be serviced.

 

John, to clarify, is the car running and driving normally while these unusual tachometer readings are presenting themselves?

I'm not sure if you have a tachometer problem or a running problem which is reflected in the tachometer reading

Cheers
DD

 

everything seems to be running fine

 

The tachometer has little if anything to do with the transmission.

it relies on ignition pulses for data to move the needle.

1. Colder at the former location. 1000 RPM as a cold idle, on warm up drop to 600? PHx plenty warm! lived there a spell in early 60's!!

2. Aye, it might relate a transmission issue!!! Slips on the upshift!! Fluid level? Nice clean red fluid or dirty brown!!! Good flow through the trans cooler? Bad line?

Aye inspection by a shop familiar with an old BW transmission.

 

if nothing else bad happens when the tach does that, then you have a tach going bad.
Replace it with a known good one to confirm.