New Differential on my 99 sport

[mhill73]

Just had to have my rear diff replaced on my 99 sport after 238,000 miles. Before my RPMs at 70 were about 2200 to 2500 and after the swap, they are now between 2700 and 3000. the mechanic assured me that they matched the tags when getting the replacement. I was told my sport uses a 3.73 gear. Any thoughts on what could cause the RPM change and ideas on how to resolve? Thanks in advance.

 

[boominXplorer]

Sure your truck is going into overdrive? Even if he swapped in a 4.10 axle they still won't be that high.

 

[mhill73]

Thought about that but the mechanic seems to think it's okay. May try and get them to put a diagnostic on it Monday. I'll reply back with an update. Thankss.

 

[imp]

That difference in rpm (about 500) could very well be due to no overdrive gear, but why would this have coincidentally happened right after a diff. change? In the event the gears are numerically higher, a 4.11 would only account for about half the increase in rpm. But, a 4.56 would fit the numbers almost perfectly.

In either case, the PCM should be detecting what it would think is a transmission failure or a speed discrepancy between front and rear wheels, calling out a trouble code as "incorrect gear ratio detected. Look for a code generated. If none, I would certainly check the gear ratio by properly counting turns. imp