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very high rpms, bad tach??

kridgley

Well-known member
Joined
Jun 17, 2004
Messages
83
Location
Bear, DE
Corvette
1975 Corvette, White T-Top
hello all,

I just had my trans rebuilt (turbo 400) and this was the first time driving it to work (mostly down I-95). I noticed the engine rpm to be very high for the speeds. I was reading almost 4000 at only 70mph. I have basically stock 75 with 3.08 gears i believe. Last summer I recall them being a lot lower at 70mph ... down near 3500, 4000 rpms was at 80mph. Could it be the circuit board on the tach, is that common? Or is it a more serious problem. Im hoping its just a bad reading and not actually high rpm.

Any help would be awesome.

Kev
 

kridgley

Well-known member
Joined
Jun 17, 2004
Messages
83
Location
Bear, DE
Corvette
1975 Corvette, White T-Top
looking into stuck in second gear ... it was just rebuilt and looking at the bill - they put on an adjustable modulator ... no kickdown on this trans? Would it need to be adjusted?

unfortunately i cant drive just this minute ... but will confirm that in a few hours.

Kev
 

kridgley

Well-known member
Joined
Jun 17, 2004
Messages
83
Location
Bear, DE
Corvette
1975 Corvette, White T-Top
ok, it def shifts into third, even made sure my shifting 1-2-d. so now could it maybe be the converter not locking up? thoughts ...

Going to give the trans shop a call and let them know whats up - good thing theres a warranty.

Kev
 

Crimson Thunder

Well-known member
Joined
Dec 16, 2001
Messages
814
Location
Orange Park FL USA
Corvette
1970 Marboro Maroon Convertible
Did the transmission shop change the speedometer gear at the tranny by accident?
Did you get your trans. rebuilt or did they exchange it for another one?(it has your vin on it)
It could be that your tach is correct and your speedometer is off.
 

kridgley

Well-known member
Joined
Jun 17, 2004
Messages
83
Location
Bear, DE
Corvette
1975 Corvette, White T-Top
guy at trans shop seems to think the tach is off by 1500-2000 - makes sense because under full throttle from 1-2 and 2-3 the shift point is about 7000, which used to be around 4200-4700 and the tach doesnt settle back to zero anymore.
so now if it is off by 2000, cruise rpm at 70mph would be about 2500.
way to reset or does it need a new circuit?
ways to 'check' the tach for an accurate reading?
 
B

bmotojoe

Guest
When the car is running at an idle and you turn your ignition switch off your tachometer needle will most likely stay or climb just a little from the idle RPM speed, this is normal. With the ignition switch in the off position turn the ignition switch back to the on position. The tachometer needle should now reset itself back to 0.
Here are a few things I would first check before ripping the dash cluster out.
Make sure that the tachometer filter has a nice clean ground connection when bolted to the intake manifold as this is your filters ground source and must be clean of any engine paint color.
The tachometer filter has a wire and connector coming out of each end, one connected to the distributor cap (NEXT TO THE BATT WIRE CONNECTION) and the other end connecting to the tachometer via harness. If a replacement filter has been installed they are not directional and must be installed correctly. Make sure both of these connections are nice and clean.
The easiest way to check your actual engine RPM speed is with a timing light with a digital readout.
Hope this helps,
Brian
 

stingraymax

Member
Joined
Sep 17, 2013
Messages
7
Location
Belgium
I know it's an old post but......Did you ever found what the problem was ? Mine does the exact same thing.

Thanks
 

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