RPM s Still Reading Low

[boxathey]

Hello, I was wondering if there are any of you that can verify your rpms on your guage. I have a zf5 5 speed with 3.55 gears. can you tell me roughly what rpm your reading in 3rd gear going 35mph. 4th gear going 50 mph , I just purchased a new tach sensor and it still seems low to me. If i can verify its low ill take out my inst. cluster. I think i read on here somewhere that a bad ground wire going to the inst. cluster will cause funky tach readings.

Thanks For The Help

 

[gdhillon]

I wish I could help you out but my tach either doesn't work at all/acts weird or doesn't really go past 1000rpm. So I just go by the sound of the truck

But I do know in the owners manual it tells the what speed you should shift at, I can get you those numbers tmo if youd like

 

[TWeatherford]

I would, but I've been dealing the same issue for a few years now. I don't really know what I'll do with a tach when I actually have one I've been doing without so long.

 

[Shadetreemechanic]

I don't remember where the truck is at the rpms you asked about, but I cross the 2K mark in 5th gear at about 68 mph. That is with zf5 and 3.55 gears. Tires are 285/75/16, but it seems like the PO has corrected the speedo for the larger tires.

 

[Jake_IN]

can you tell me roughly what rpm your reading in 3rd gear going 35mph. 4th gear going 50 mph , I just purchased a new tach sensor and it still seems low to me.

I made a spreadsheet for this awhile back but never confirmed that my math was correct. However according to the spreadsheet and just taking a wild guess at your tire size (33in dia) these are the RPM's you should be seeing.

3rd gear 35mph=2038.65 RPM's
4th gear 50mph= 1808.92 RPM's

I think i wrote the forumlas right on the spreadsheet, but im not 100% sure. So yeah those are approximates.

 

[Dave 001]

RPM = MPH x transmission gear ratio x rear axle ratio x 336.13 / tire diameter

RPM = 50 x 1 x 3.55 x 336.13 / 31

RPM = 1925 (at 40mph in 4th gear)

I'm assuming a 31" tall tire.

 

[boxathey]

Thanks for the info mine is definitely reading low it hasn't worked for probly four or five years I figured it was time to try to fix it. do you guys have any suggestions on what I should be looking for inside the instrument cluster. does the little servo motor ever go bad on the tacometer itself

 

[boxathey]

@ dave 001, Im definately going to save that formula. I think it will come in handy down the road

 

[Dave 001]

I "google'd" the formula as I was not at home when I posted the above message. At home, I have the book "HPBooks Auto Math Handbook". It's very handy and inexpensive.

http://www.amazon.com/Auto-Math-Handbook-John-Lawlor/dp/1557880204

Dave

 

[Agnem]

Low RPM's with a new sensor usually indicate voltage issues to the gauge. Either low alternator/battery or bad ground will cause a tach to read low.

 

[boxathey]

the battery and alternator check out good where should I be looking for a bad ground at