How accurate is the Tach in a ATX

[shoguy3.2]

ok guys, i need to know , how accurate is the tach in a ATX car....i was on the highway today and i noticed something weird....my car usualy does 140 KPH at 3000 RPM. today going home down the highway my car was doing 120 KPH at 3000 RPM ....the car runs great and the tranny was just rebuilt.

i also notcied seomthing....when i first started on the highway, i was doing 120 KPH at about 2700 RPM. 3000 RPM was at 130 KPH ...and as i continued ( i had my cruise set ) the RPMS on ther tach gradually started to climb...but it didnt speed up over 120KPH. I also notcied when i tapped the break to slow the car down , and then hit resume on the cruise the car would go back up to the speed i had it set at (120 kph) but the RPMS climbed every time i hit resume.

When i pulled off the highway, the tranny still shifted amazing, soo i dont think its a problem with my tranny ......

soo guys tell me shoudl i be worried or do i have a very very inaccurate tach ??

io cant see the engine working harder everytime i hit resume button on the cruise.

any ideas what the **** is wrong with my car ??

i think the reason my RPM werent 3 grand at 140 KPH is cause i had the car loaded full and full tank of gas... sooo what do you think im lookin at guys, ****** car or innacurate tach. ??

any ideas ?? anythign at all

 

[Mr Anonymous]

Tach accuracy likely has nothing to do with what you're describing. If your tach is off, say, 150 RPM, then that's what it's off. The error isn't going to change as you drive along.

As long as you are traveling on a straight and level road and not accelerating or decelerating, your RPM and speed in a given gear should correspond regardless of the weight of the car, etc. When you're accelerating or decelerating, the RPMs and speed won't correlate until you have stabilized at a speed.

Are you sure you're not just seeing the higher RPMs as a result of the torque converter not locking until the tranny fluid is up to temp? This will normally cause the engine to turn around 300-500 revs higher, and in really cold weather it can take 10-15 minutes of driving for the fluid to get warm enough for the PCM to command the TC to lock.

 

[shoguy3.2]

well this happened to my car the whole way home, and i was on the highway for 2 hours . it is a cold day, and ihave a huge tranny cooler on my car 1/2 the size of the rad, could that be it ?? its not heating up enough ?? ther tranny shop installed it ...

i let it warm up for 15 min before i even left for the highway....

 

[Mr Anonymous]

well this happened to my car the whole way home, and i was on the highway for 2 hours . it is a cold day, and ihave a huge tranny cooler on my car 1/2 the size of the rad, could that be it ?? its not heating up enough ?? ther tranny shop installed it ...

i let it warm up for 15 min before i even left for the highway....

Yup, you may have too much cooling going on resulting in the converter not locking. This would be exacerbated if they eliminated the integral radiator tranny cooler part of the circuit when installing the larger aux cooler. You would see a decrease in fuel economy as well.

I have a bigass cooler on my V8, BUT I have a thermal bypass valve (basically a fancy thermostat) that bypasses the cooler and returns the fluid directly to the tranny until the fluid is up to temp.

 

[shoguy3.2]

if i ziptied a piece of cardboard across half of the cooler...would this help my little RPM problem ?? if i can dfo that as a easy fix ....i will ..

woudl it work ??

 

[Mr Anonymous]

if i ziptied a piece of cardboard across half of the cooler...would this help my little RPM problem ?? if i can dfo that as a easy fix ....i will ..

woudl it work ??

Yes, but only as a temporary experiment to determine if the ATF temp is being kept too low to allow the converter to lock. If that's the result, then you'll need to modify the cooling setup to allow the ATF to reach and maintain temperature. But, depending on the size of the cooler and the ambient air temp, you may still not get the fluid warm enough and in that case you might need to completely bypass the cooler for a day.