RPM's

retiredcopper

Original poster
Member
Nov 16, 2013
59
Morning,09 Trailblazer with the 4.2....when doing approx. 60MPH the RPM's sit around 2000...is that about right?...seems a bit high...can anyone advise?....Thanks
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
Morning,09 Trailblazer with the 4.2....when doing approx. 60MPH the RPM's sit around 2000...is that about right?...seems a bit high...can anyone advise?....Thanks


What is your gear ratio?? (See the glovebox sticker for the rpo code)

Edit: 60 mph at around 2000 engine rpm sounds close to mine with rpo GT4, 3.73 gearing. Maybe a tad high on the engine rpm though.

Just checked and my 2002 with 3.73 gears is 65 mph at 1965 rpm +/- a fuzz.
 
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TJBaker57

Member
Aug 16, 2015
2,897
Colorado
Here is an example of the available gear ratios as applied to my 2002 with Michelin LTX A/T 2 245/70R16 tires at 60 MPH:

4.10 gearing = 2208.9 rpm
3.73 gearing = 1827.6 rpm
3.42 gearing = 1675.7 rpm

Edit: above rpm arrived at by the following:

Revs/Mile of my particular tire size/brand = 704 revs per mile.

Revs/mile x mph / 60 minutes per hour = axle rpm (revs per minute).

Axle revs per minute x gear ratio = driveshaft rpm.

Driveshaft rpm x 4l60E 4th gear ratio (0.696) = engine rpm (assuming full TCC lockup).
 
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littleblazer

Member
Jul 6, 2014
9,265
Sounds a little high depending on gear ratio. But could very well be a stepper motor not reading correct.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Try to compare the speed with a GPS. It's possible that the PCM was replaced with one with the wrong gear ratio and thus, the wrong speedo spec. And to check if the speedo itself is off, you could use a scanner with live data, or a bluetooth OBD adapter with the Torque app where you can get what the PCM is actually seeing and compare to the cluster.

If you have 4.10 gears and 245/65R17 tire 29.5" height (usual stock size), I got an RPM of 2014 at 60 MPH using this calculator so yours sounds about right if you have this gear ratio.
 
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gmcman

Member
Dec 12, 2011
4,656
Old Torque screenshot when looking at fuel trims.
 

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TJBaker57

Member
Aug 16, 2015
2,897
Colorado
Stock.

245/65/17


I guessed something like that.

Mine are stock at 16". I find the online calculators base their numbers on a tire being a perfect circle, which is not the case when the tire is carrying a load. That's why I use the revs/mile as given by the tire manufacturer for the specific model of tire. An example is my Michelin AT/2 tires. They have a tire diameter of 29.5 which if the tires were made of iron and didn't deflect under load would yield a revs/mile of 684. But Michelin states revs/mile for this tire as 704, nearly a 3% increase. I have found that figure to be dead on.
 
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retiredcopper

Original poster
Member
Nov 16, 2013
59
What is your gear ratio?? (See the glovebox sticker for the rpo code)

Edit: 60 mph at around 2000 engine rpm sounds close to mine with rpo GT4, 3.73 gearing. Maybe a tad high on the engine rpm though.

Just checked and my 2002 with 3.73 gears is 65 mph at 1965 rpm +/- a fuzz.
TJ,I looked at the codes on the glove box sticker and no GT4,might you be able to advise the RPO code for the other 2 gearing ratios
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
Sounds a little high depending on gear ratio. But could very well be a stepper motor not reading correct.
Blazer,I was having problems with the stepper motor at first,but after taking apart the dash and a good cleaning it seemed to be working fine....thanks for the input!
 

Matt

Member
Dec 2, 2011
4,019
TJ,I looked at the codes on the glove box sticker and no GT4,might you be able to advise the RPO code for the other 2 gearing ratios

You can find them using the search forums feature, it's a hand device. Here they are anyway:

G80 Differential, locking, heavy-duty, rear
GT4 Rear axle, 3.73 ratio
GT5 Rear axle, 4.10 ratio
GU6 Rear axle, 3.42 ratio
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
You can find them using the search forums feature, it's a hand device. Here they are anyway:

G80 Differential, locking, heavy-duty, rear
GT4 Rear axle, 3.73 ratio
GT5 Rear axle, 4.10 ratio
GU6 Rear axle, 3.42 ratio
Winner,winner...chicken dinner!!....GU6!...Thanks Matt!!
 
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retiredcopper

Original poster
Member
Nov 16, 2013
59
I can't say it enough....Thanks all for the help!!....apparently RPM's are a bit high....on to the next problem...WHY?
 

gmcman

Member
Dec 12, 2011
4,656
Blazer,I was having problems with the stepper motor at first,but after taking apart the dash and a good cleaning it seemed to be working fine....thanks for the input!

This is where your problem could be.

Are you sure you replaced the tachometer dial back in the correct position?

Can you take a pic of where it rests when the key is off?

Also, take a pic of the tach when the engine is at a hot idle, AC off.

Should be right at 600 RPM.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Getting a Bluetooth OBD adapter and the Torque app will tell all. Maybe your speedo is off so check with a GPS. Torque also can display the read vehicle speed and GPS speed to compare. Has the PCM ever been replaced?
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
This is where your problem could be.

Are you sure you replaced the tachometer dial back in the correct position?

Can you take a pic of where it rests when the key is off?

Also, take a pic of the tach when the engine is at a hot idle, AC off.

Should be right at 600 RPM.
Cman,I never removed the needle,just gave the contacts a good cleaning and at hot idle,a/c off 650rpm's is the magic number.
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
Getting a Bluetooth OBD adapter and the Torque app will tell all. Maybe your speedo is off so check with a GPS. Torque also can display the read vehicle speed and GPS speed to compare. Has the PCM ever been replaced?
Moose,I bought the vehicle a few months ago from the son of the original owner,who is no longer among the living and the son wasn't much help with repair records. I don't know...
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
OK but have you tried any of the other suggestions?
 

Chickenhawk

Member
Dec 6, 2011
779
Convertor not locking? Stepper motors replaced in the past and not marked properly before prying them off? Cat plugged? There really aren't too many other ideas we can think of.
 

mrrsm

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For an “At-A-Glance” Reference Chart of the relationships between Tire Sizes and Rear Differential Gear Ratios… THIS Image is Hard To Beat:

TIRESVSGEARS.jpg
Source for the underlying "Tire Size vs. Gear Ratios” Formulas:

https://www.alloyusa.com/gear-ratio-calculator/
 
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retiredcopper

Original poster
Member
Nov 16, 2013
59
I'm going to get on it this weekend if the weather complies,I don't have a garage,I work in the driveway and its been coooold here in NYC
Morning,did the comparison reading from the speedometer and GPS as you suggested...did it at 30,50,and 70mph and according to the GPS the vehicle is traveling at 1mph slower than what the speedometer says. Moose for a 13 year old vehicle I don't think that's bad....Thoughts?
 

Matt

Member
Dec 2, 2011
4,019
Morning,did the comparison reading from the speedometer and GPS as you suggested...did it at 30,50,and 70mph and according to the GPS the vehicle is traveling at 1mph slower than what the speedometer says. Moose for a 13 year old vehicle I don't think that's bad....Thoughts?
It's farking good. My brand new 22 Sportage is slightly out compared to GPS speed.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Yep, pretty good accuracy so something with the PCM is not in question.

Possibly the torque converter isn't locking. If you have an OBD adapter and torque, you could verify the RPM being reported by the cluster and also if the torque converter is locking. A quick way to verify this is to hold the speed at 60mph with the right foot for a while and with the left, lightly tap the brake. You should see the RPM jump up.

With 3.42 gears, using the same calculator, at 60mph, you should have an RPM of 1680. Check to see what RPM you also get in 3rd gear. There, you should have an RPM of about 2400.

It is possible that your gears were changed at some point in its life. Might have to verify what gearing you actually have using a manual method.

 
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retiredcopper

Original poster
Member
Nov 16, 2013
59
Yep, pretty good accuracy so something with the PCM is not in question.

Possibly the torque converter isn't locking. If you have an OBD adapter and torque, you could verify the RPM being reported by the cluster and also if the torque converter is locking. A quick way to verify this is to hold the speed at 60mph with the right foot for a while and with the left, lightly tap the brake. You should see the RPM jump up.

With 3.42 gears, using the same calculator, at 60mph, you should have an RPM of 1680. Check to see what RPM you also get in 3rd gear. There, you should have an RPM of about 2400.

It is possible that your gears were changed at some point in its life. Might have to verify what gearing you actually have using a manual method.

Moose,I don't have an OBD adapter...I'll try the speed/brake method....thanks!
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
Moose,I don't have an OBD adapter...I'll try the speed/brake method....thanks!
Moose,I tried the speed/brake method and the RPM's didn't flinch...as someone else suggested,even though there is no code it might be a blockage in the Cat. converter...I looked at some youtube videos and am going to borrow a temp gun and check it next. Thanks!
 
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Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
If the rpm isn't changing while cruising at maybe 60 mph, then this indicates that the torque converter isn't locking, which would explain the higher rpm. Either the solenoid for it isn't working or the torque converter ckutch is defective.

Cat clogging would manifest itself with low power. A poorly performing cat would trigger a CEL and the lower output temperature. A clogged cat could still be working and still show a higher output temp but not necessarily trigger a CEL.
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
If the rpm isn't changing while cruising at maybe 60 mph, then this indicates that the torque converter isn't locking, which would explain the higher rpm. Either the solenoid for it isn't working or the torque converter ckutch is defective.

Cat clogging would manifest itself with low power. A poorly performing cat would trigger a CEL and the lower output temperature. A clogged cat could still be working and still show a higher output temp but not necessarily trigger a CEL.
AHH!!....Okay,I had it backwards...I read where the TCC is located inside the trans pan,is the solenoid located there as well...?.....Power?....dude,she runs circles around most newer vehicles on the road!!
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
The TCC is in the torque converter, hence the name torque converter clutch. The solenoid that controls it is located in the valve body accessible from the pan.

4l60e-tcc-solenoid.gif

Surprising it didn't trigger a code for the slippage.

I doubt very much the cat is at issue here.
 
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gmcman

Member
Dec 12, 2011
4,656
Keep in mind you have to check the converter lock up when the trans is hot. Not sure of the exact temp but if you drive it away cold, the trans won't lockup right away.

The converter also locks in 3rd, try that also.

If the converter is not locking, you will need to address that sooner than later to prolong the life of the trans.
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
The TCC is in the torque converter, hence the name torque converter clutch. The solenoid that controls it is located in the valve body accessible from the pan.

4l60e-tcc-solenoid.gif

Surprising it didn't trigger a code for the slippage.

I doubt very much the cat is at issue here.
Moose,I have had that problem since day 1...the vehicle won't trigger codes,but if a tire is low on pressure,THEN the dash lights up like a X-mas tree...go figure...I'm going to retry the speed/brake method...maybe I didn'r let the trans get hot enough...as always...Thanks!!
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
Keep in mind you have to check the converter lock up when the trans is hot. Not sure of the exact temp but if you drive it away cold, the trans won't lockup right away.

The converter also locks in 3rd, try that also.

If the converter is not locking, you will need to address that sooner than later to prolong the life of the trans.
Thanks for the input...I'm going to retry the speed/brake method moose mention again,this time I'll run it longer,get the temp on the trans up
 

retiredcopper

Original poster
Member
Nov 16, 2013
59
UPDATE:....swapped out the TCC solenoid along with the A & B ones since I had the trans pan off and PROBLEM SOLVED!!......RPM"s are back in range and even picked up about 2 miles per gallon on the highway.....What a messy job to do especially on your back!......I can't say it enough....Thanks to all for the help!!.....stay safe!!
 

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