'94 K2500 4wd 4L80E RPM Question

[Big T]

Just bought a '94 K2500 4wd Suburban with 4L80E automatic. It has 4.10 rear gears and the tires are LT 245s. Speedometer is broken, but we were monitoring speed with a GPS. I ran though all 4 gears to make sure that Overdrive was indeed working. The engine rpms were high (than I'm used to) for the speed. We clocked 70 mph at 2650 rpm. Wife and I both noticed the engine sound (rpm) to be much higher.

My '99 with 3.73 rears will do 70 mph at 2150 rpm with BFG A/T 265s. Our now totaled '95 did about the same with 265s and the 4.10 rears.

Is there something different with the gearing on the 4L80E in the '94 vs later model trucks?

 

[Burning oil]

4.10 vs 3.73?

 

[tanman_2006]

My 3.73s run right at 2k @60 on 245's. Now you have me wondering if I've had the wrong pulley all along.

 

[Big T]

Nope, the '95 with 4.10s was down around 2200 rpm at 70 mph. Someone suggested (in an e-mail) gasser alternator pulley plus the 245 tires and it does look to have a new alternator, autozone look to it. So it could be a gasser pulley on there. We'll put the GMTD Scan on it to check the read out.

 

[THEFERMANATOR]

Gasser pulley on mine with 4.10's and 245 tires read right around 2500 on the tach.

 

[handcannon]

I have a 94 K2500 ext cab pickup, 4l80E, 4.10 gears, 265/75/16 tires. At 60 MPH the motor is at 2000 RPM. I've been forcing myself to keep the RPMs down so I don't remember what RPMs would be at 65 or 70 MPH.

Also, what diameter is the alt pulley? The diesel pulley measures 2 3/8ths across the face. It is a common thing for replacement alts to have a gasser pulley on them, making the tach read too high.

Don

BigT and Ferm, you both posted while I was slowly tapping out my post, so some of mine is irrelevant.

 

[Big T]

Well we have a fresh 170 amp alternator with diesel pulley on the '95, so guess what's getting pulled first? Having this wrecked '95 in the backyard will be interesting. Nothing will go to waste.

 

[tanman_2006]

How do you change the pulley? I don't want to screw up my new alt, I have a spare alt to pull the pulley from.

This could explain my good mpg, I keep my rpms below 2k but if the tach is reading high then I am staying at more like 1700-1800.

 

[Big T]

Air wrench will get the nut off. Then the pulley will usually pry off pretty easily. Worst case you need to get a puller.

 

[tanman_2006]

Thanks for the info. Glad I didn't have to throw a thread too far off track lol.

 

[WarWagon]

245's? Look up the tire diameter and see if there is a big difference in diameter vs. your other 4.10 truck/burb. Tirerack or the manufacturer website will give you this info. Recall the squished diameter may be different as you have a specific amount of rubber on the ground at a specific PSI to hold the burb up. More PSI = less rubber needed (less sq in). Thinner tire means more squish.

Different exaust (louder?)

 

[Big T]

Stock exhaust, but that 4" Diamond Eye, Downpipe and Crossover are some of the first things coming off the '95 and going on the '94.

Front tires show wear (not feathering) on outside and the steering has some play and wandering, but about what you'd expect for a truck this age with everything original. So new Pitman, Idler & Bracket, ball joints, upper A-arm bushings, adjust steering box, etc.

What's amazing on this '94 is that there are no door rattles. After much off roading, my '99 has rattles everywhere, so I'm due to go through hinge pins and bushings.

 

[Cali6.5]

Could someone please explain how the diameter of the altenator pulley can throw off the tachometer? Is the tach reading coming from somewhere in the serpentine path? My girls Grandfather was a GM mechanic for 30 yrs and he couldnt think of what would make the RPMs read higher due to the pulley. Is 2 3/8" the correct diameter for the pulley? If mine is from a gasser, can I get a diesel pulley or do I have to buy another alt?

 

[Big T]

I measured the alternator pulleys on the '94, '95 and '99 today and they all were 2 1/2".

 

[ak diesel driver]

on the 6.5s the tach source is the alt. check with an alt rebuilder to see if you can get the correct one

 

[DieselCash]

Could someone please explain how the diameter of the altenator pulley can throw off the tachometer? Is the tach reading coming from somewhere in the serpentine path? My girls Grandfather was a GM mechanic for 30 yrs and he couldnt think of what would make the RPMs read higher due to the pulley. Is 2 3/8" the correct diameter for the pulley? If mine is from a gasser, can I get a diesel pulley or do I have to buy another alt?

The tach reading come from the alt. How fast the alt spins is what the reading will be on the tach. If the pulley is larger, the rpms go up. If the pulley is smaller the rpms go down.

 

[Burning oil]

The tach reading come from the alt. How fast the alt spins is what the reading will be on the tach. If the pulley is larger, the rpms go up. If the pulley is smaller the rpms go down.

Other way around

 

[DieselCash]

Other way around

Too many beers, going to bed.

 

[Burning oil]

Too many beers, going to bed.

:drunk1: Sleep well :hihi:

 

[Cali6.5]

That makes ense becuase I am running 245 tires with 4.10 and 2700 RPM at about 65 mph. Bill Heath says it should be about 2100-2200 RPM.