Incorrect Tach Reading (Possible Causes?)

[cwestra]

My tach is reading incorrectly.  What are the possible causes and fixes?

[moparmaniac59]

Is it a mechanical tach or an electrical tach?

[cwestra]

Is it a mechanical tach or an electrical tach?

Sorry, it is the factory electronic tach on a 1970.

[BS CUDA]

My tach is reading incorrectly.  What are the possible causes and fixes?

Mine was reading very sporadic - roughly 1200rpm high at cruising speed. I traced my tach wires end to end and found 2 "twist tie" connections. I soldered them up and voila - tach is bang on. Perhaps you have a bad connection.

[Chryco Psycho]

modern electronics works too , RTE sells a simple conversion kit

[DAYLEY/CHALLENGER]

Lets get a few more causes.........mine used to read to 8000 when curising........I know that is wrong.......so it is unhooked until..............

[Chryco Psycho]

the electronics have changed a little in 42 years , remember what a computer looked like ?

[Jamiez]

Sorry for the newbie question but, how do you know it's not accurate and what the correct number is?
 

[moparmaniac59]

I think you should be able to roughly estimate what your engine rpm's are at certain cruising speeds based on your miles per hour that you are cruising at, your rear end gear ratio and tire size. The variations will be based on those factors. Most old Mopars cruising at 60 mph with and average street gear such as a 3:23 and an average 15" street tire would not see more than 3,000 rpm on their tach and I would expect it would run in the 2,200-2,400 rpm range with the above given example. That's just ballpark, so if you are crusing at 60mph with the same example I gave above and your tach reads 5,000 rpm, well then you know something is wrong. As been stated before with these old cars, a lot of the electrical problems can simply be from a loose connections, bad grounds, broken wires ect. & doesn't take an electrical degree to figure out, but a good eye and multi-meter will git-r-dun!! 

                                                                        Matt B.

[BS CUDA]

I think you should be able to roughly estimate what your engine rpm's are at certain cruising speeds based on your miles per hour that you are cruising at, your rear end gear ratio and tire size. The variations will be based on those factors. Most old Mopars cruising at 60 mph with and average street gear such as a 3:23 and an average 15" street tire would not see more than 3,000 rpm on their tach and I would expect it would run in the 2,200-2,400 rpm range with the above given example. That's just ballpark, so if you are crusing at 60mph with the same example I gave above and your tach reads 5,000 rpm, well then you know something is wrong. As been stated before with these old cars, a lot of the electrical problems can simply be from a loose connections, bad grounds, broken wires ect. & doesn't take an electrical degree to figure out, but a good eye and multi-meter will git-r-dun!! 

                                                                        Matt B.

     Mine was doing exactly that - showing 5300 RPM @ 100kmh with 4.10 but still too high. Even at idle my tac was showing 2000ish RPM but my timing gun was showing 800. Usually your ear will tell you something is off unless it is only out a small amount.

[cwestra]

Sorry for the newbie question but, how do you know it's not accurate and what the correct number is?
 

I used the RPM, speed, gear ratio calculator at vexer.com to verify accuracy.  It's been referenced on this site before.  Mine was pretty easy to diagnose since it changed abruptly recently and I could compare the difference. 

[MEK-Dangerfield]

I had an aftermarket tach on my steering column since my Rally tach was dead when I first bought the car. I have lent the aftermarket tach out a few times to friends. You connect it under the opened hood and face it towards the driver's seat. That way you can compare the two tachs. I just know the originals always read higher after time. I had my original rebuilt and now it agrees with the aftermarket tach. 

[1of1]

Sorry for the newbie question but, how do you know it's not accurate and what the correct number is?
 

Use a engine anaylizer with a tach in it....Start the engine rev to 2500 rpm, compare.... the test will let you know if you are off or not...if you are and have the newer electronic convertion you will need a kit as mentioned in this thread..if its way off its mostlikly a short, the electronic system will make the tach "Lazy" and not very responsive..but normally will not show large variations...........

[Jamiez]

Thanks for the info.  Unfortunately a lot of those methods require you to know the speed the car is going and I'm pretty sure that's way off.  I'm guessing the speedo gear was never changed to match the increased tire size.  I'm just waiting on some parts before I finish correcting that.

I could use a GPS, but I was sort of hoping for some easy way to tell while the car was in the garage without needing extra equipment (second tach, engine analyzer, timing gun, etc).

I'm pretty sure my dad has an old school timing gun I can borrow, so I'll try that route

[Charles]

Our '69 Charger's tach on the dash is about 1500 rpm higher then what the rpm should actually be. The previous owner installed an aftermarket tach on the column, so it doesn't bother me too much.

I know that the AAR is about 3000-3200 rpms for 60 mph.