420G auto gearbox speed vs revs

Saloons
1

Hi there,

I took out the 420G today after it has been sitting for 6 months and it was great.

However, I noticed it only changed gear once every time I pulled away. If it is a three speed box, that makes sense as it starts in second and then goes into third/top.

BUT… on the motorway I got to 70mph and according to the rev counter I was doing 5500 revs!

I thought these cars could do 100mph when new, but in my car as it stands now that would have meant doing revs off the scale.

The service manual has a table on pages A-5 an A-6 with more ‘normal’ revs for speed on the manual gearboxes (depending on the ratio just over 3000 rpm for 70mph in top gear - not overdrive), but there is no table with numbers for the auto box.

Have I a gearbox issue, or is my rev counter dodgy, or is this normal for auto gearboxes?

Cheers
Paul

P.s, I also cannot seem to get the kick down to work…

2

Sounds like the tranny has gone to sleep, if the engine sounds busy then yes there is a problem.
Perhaps the valve body has become gummed up from standing?
Can you force it to start in first?

3

420G should be equipped with a Borg-Warner type 8

if you start out in D2, it will always pull away in 2nd gear…stupid idea.

you need to start (and should leave it) in D1 for an auto 1-2-3 upshift

if started in Low, it will hold 1st

I have the table for autos in the Handbook, 2780 @60mph, 3240 @70, my tach & speedo both read high

I doubt your tacho is reading correctly…the engine would be screaming (an allow no further acceleration)…3400rpm is loud, but you would have plenty of power left…is there?

verify speed with your mobile phone GPS

420G will easily do 100mph

no 420G has been proven to me to kickdown,
those I ask say theirs will not either…or they dont know!

my 420G gearbox performs all changes as per FSM,
to achieve “kickdown”, I move the lever to Low…at any speed,

it will then change down one gear (unless it exceeds max speed for that gear),
and hold it till I change back to D1

the gears change down automatically upon reducing speed & throttle

I have had the gearbox apart

its a good idea to obtain the A5 sized handbook of Ebay

verify yours has not been fitted with a BW12 from an XJ6

4

I guess one way to identify a BW12 would be to look for the microswitch on the end of the outer of the accelerator cable - if it works a-la XJ6?

5

My speedo also reads high so I used my phone to see actual speed. At phone = 70, the speedo was hovering around 90.

As it was its inaugural trip I did not push it any further than that, one because I didn’t actually know where the engine was and two I was running out of road! I shall try again next week.

I suspect the rev counter tbh. How do I make sure that works/doesn’t work correctly?

I am not too bothered about kick down as it picks up fast enough and is not a racing car.

I’ll check to see if the box is original, but I suspect so… there is nothing in the historic documents to suggest it has been changed, and I seem to have every bill.

Thanks al.

Paul