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‘95 After automatic to manual trans swap tachometer is not accurate


Resbum
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This 95 pathfinder is new to me. It came to me with an automatic to manual transmission swap. Whoever has wrenched on this vehicle in the past should never again be allowed to approach a vehicle with a tool bag in their hand, let alone do something as major as a transmission conversion. That’s a totally different story so I won’t get into it here, but I’ve had to fix and correct sooooo many things since buying it.

 

Here’s the issue I’m posting about. The tachometer always works, it’s nice and smooth in its operation, but it’s constantly running high.

 

At idle it’s above 1,000rpm. At 60mph in 5th gear it runs dead on 3,000rpm and in 4th gear it runs approx. 3475rpm. That spread between 4th and 5th comes out to a .866:1 5th gear, if 4th is 1:1. That ratio combined with the “low” oil filler port leads me to believe the manual transmission came from an 87-95 Pathfinder, or similar.

 

Next, I put the rear axle up on jack stands, and with the both tires off the ground, counted the drive shaft to tire rotations. This came out to 4.6:1, which matches the info plate in the engine compartment. i.e. H46.

 

With the above info figured out, and confirmed, and knowing I’m running 33” tires, I headed to some online rpm calculators and plugged in these values:

 

60 mph, 1:1 (or .862:1) trans gears, 33” tires, 4.625:1 differential gears

 

The results came out to:

 

4th gear @ 60mph should be 2825rpm, not 3475rpm

5th gear @ 60mph should be 2435rpm, not 3000rpm

 

Oh, and one more thing, the speedo/odometer are dead nuts on per GPS and some radar boards, which surprised me with the 33” tires I put on it. I just did a 365 mile (per Google maps) road trip and the odometer read 362 miles.

 

One final bit of info, I don’t know if it matters, but the previous owner gave me the manual trans. instrument cluster and ECU that came with the donor transmission for the swap. He said the Pathfinder is still running the original automatic transmission ECU and instrument cluster. He said this doesn't affect anything so he didn't switch them out.

 

After a couple days of searches I've seen a bunch of posts about not working tachos and erratic tachos, but nothing about tachos that work smoothly all the time except for running high. Any help from more knowledgeable members will be greatly appreciated.

 

Resbum

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  • 5 years later...

My 95 XE V6 4x4 does 3,250rpm in 4th gear @ 40 mph. 

8D5F1D9B-EDDC-4E2B-82B6-508551B5CE1C.jpeg.23ced69d31194de090e5211466fcb4c5.jpeg

That's 5th gear at almost 55 and 4358E349-79A2-4B58-AC59-7EC8FD98289A.jpeg.8f64d499a89eb6a53209845af4ac6635.jpeg

That's 4th gear at 60... I'm running 235/75r15, same H46 rear axle... I'm redlining when I get on the interstate?? can't imagine my mpg likes it 

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I can't help but wonder if the tach originally came from a 4 cyl truck. Just because the guy you got the truck from said it was original doesn't mean that it is. 

 

Just noticed that this is a 6 year old post..

Edited by Mr_Reverse
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I am no expert, just putting my small amount of knowledge out.....
Manual Transmissions are coupled with a 4.3 rear end, while Automatic Transmissions are coupled with a 4.6 rear end. Also taking into account SE usually came with 31's (tire size) vs XE 235's (tire size). This affects the speedometer and odometer gearing in the transmission.  Remember that we need to understand that each tire brand within the same size will give you a slightly different rotation distance to the measure of a mile (this is where the +/- error variation comes in). So if the conversion was an XE transmission with gearing for 235's and a SE automatic 4.6 rear end ..... these issues can really throw your numbers far off. 
I am sorry this is all I can add to this conversation, I hope it helps. If not then please just ignore all my efforts. :) .

 

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