Automatics and RPM

[Stefan11]

I finally have my 89 automatic 325i on the road. All four gears work but at 120km/hr my tach reads 4200rpm???? The engine doesn't sound stressed so I am thinking that there might be an issue with the tach. What should I expect for RPM in fourth at highway speed?

[E36is]

hmm thats pretty high, i have auto, and at 120km/h im sitting at 2200 rpm.... tho mine is an eta

[Dissembler]

^^anywhere in the 2200 range should be about right for all E30 autos i think. My 88 would sit right around there as well although that was a long time ago so I am not 100% sure on the exact number.

Do you have another tach you could swap in?

[Stefan11]

I have another tach in a car. I guess I will start by removing the instrument cluster from that car.......

[InfiniteDice]

Automatic.... I don't like thinking back to those days.

Rpm is a simple gauge to wire, comes from the coil. Unless someone has messed around with the gauge cluster or any adjustments on the throttle body including the kickdown cable... I'd suspect the transmission isn't shifting into overdrive.

On my old auto the first thing to die was the overdrive. Over the course of a year it became progressively worse: slipping in first, then it started bouncing back and forth between 2nd and 3rd. Finally at the end of a long trip back to northern Quebec all forward gears died and I drove it backwards 30km until the reverse blew. (I was up north in the middle of nowhere - long story).

Check the Fluid level and colour. You do that with the car off.

[Stefan11]

The car shifts smoothly through all the gears, I have fourth..... I'm convinced that there is something wrong with the tach.

[InfiniteDice]

So rev the car in neutral, does the tach work normally? What does your economy gauge read? In overdrive at 100 - 120 cruising it should be around 8L to 9L /100 or something around that.

How long have you had the car? It is possible someone changed the diff or screwed up and swapped to a cluster that has the wrong code chip in it.

[Stefan11]

Saturday was the first day on the road with this car. It is a southern convertible. The speedo works, the odometer doesn't..... The tach reads over 1grand at idle even though it sounds lower..... the car has issues which I plan to slowly resolve. I don't really care about the tach, yet looking at 4200rpm at 110km/hr feels rather uncomfortable.

[supernaught]

Auto trans has a 4th gear of .73, so 4200rpm becomes 5915.5 rpm at the driveshaft.

Varying diffs are possible, but even with the highest reduction diff(4.10), the diff output shafts are turning at 1442.8 rpm.
Multiply that by the stock tire circumference (1915mm) and you get 2763mm/m. Multiply by 60 mins and you get 165.8km/h

I would say something ain't right.

My 89 auto ix rolls at around 3100 rpm at 120kmh

[InfiniteDice]

Someone probably swapped the cluster and didn't think of the code pin.