Gear Ratio

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  #1  
Old 11-05-17, 06:52 AM
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Gear Ratio

I recently purchased a new work truck. 2012 GMC Sierra 2500 HD with a work cab on the back, not a traditional truck cab. I regularly visit our mountain townhouse which involved a steep road to get down the mountain. In my previous work vehicle, 2002 Chevy Suburban Z71, I always put it in second gear to have the engine assist going down the mountain.

My question is, with this new truck, second gear doesn't seem the same. Granted, have only tested it once on a short trip to the general store up on the mountain. Should I put it in tow mode first before dropping down to second gear or see what happens using just second gear? I will be heading down the mountain later this morning and want to spare the brakes as much as possible. Never driven a 2500 before.
 
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  #2  
Old 11-05-17, 07:07 AM
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What is the engine and vehicle speed when you go down the mountain? I doubt tow mode would make much difference as those generally just change the shifting rate/speed.
 
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Old 11-05-17, 07:57 AM
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Agreed, tow mode primarily affects the shift points in order to keep the torque up and eliminates overdrive, so it's effective for starting out and pulling a heavier load, but I don't see any benefit to it in the situation you are describing. Your best bet is doing what yo have been, selecting "2" rather than "D" on your transmission. Or it's possible that you may even need to select "1" if this truck has a lower rear axle ratio. The difference in rear axle ratios between say a 3.55:1 and 3.73:1 or a 3.73:1 and 4.10:1 are going to be relatively negligible to many if not most light truck owners, except that a lower number will equate to better fuel economy, particularly for longer drives on flatter surfaces, whereas a higher number is going to give you more torque and pulling power, as well as perform better in a situation like yours where you want the drivetrain to provide some of the braking power. So even if the new one is comparable to your old one, and assuming the same engine, a change in rear axle ratio may be the reason you detect a difference.
 
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Old 11-05-17, 08:43 AM
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Im pretty sure the 2012's have the six speed trans and grade braking assist with tow haul on.

I dont know what you weigh but I would run through the mountains with tow haul on. Coming down the mountain a light tap on breaks should activate grade assist and hold your speed...

Unfortunately my 2006 does not have that feature and I am forced to down shift.
 
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Old 11-05-17, 08:48 AM
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tow/haul mode grade brakingtow/haul mode grade braking isonly enabled while the tow/haulmode is selected and the vehicle isnot in the range selection mode.see tow/haul mode listedpreviously and manual mode onpage 9-32. Tow/haul mode gradebraking assists in maintainingdesired vehicle speeds when drivingon downhill grades by using theengine and transmission to slow the vehicle.to disable or enable tow/haulgrade braking within the currentignition key cycle, press and holdthe tow/haul button forfive seconds. When the button is released, the requested modechange is made. A dic message isdisplayed. See transmissionmessages on page 5-46
. .
 
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Old 11-05-17, 08:52 AM
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Video.....................................................


https://www.youtube.com/watch?v=1MLuOBHfsbw

 
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Old 11-05-17, 09:27 AM
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Well there you go, learn something new ever day! My work truck is a Chevy, and use tow/haul frequently, but we're not in the mountains so have never had need for it and did not know that they had brake assist. My first pass too I read your post as it being more of very low speed secondary road with a maximum speed of something like 10-20 mph, sort of like Mark Sr's driveway, and I wonder how it works in a situation like that, but see that now that you mentioned a trip to the store on this road so maybe it's more than that. Regardless, it sounds like that might be your answer, definitely worth trying.
 
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Old 11-05-17, 10:48 AM
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Awesome, thank you guys. Of course the truck came with no manual to check, just like my last one. I will be experimenting with the tow hold button going down the hill in an hour or so. Will report back later this evening.

Hold tow button 5 seconds and see what happens. Will probably still put it in 2nd gear unless the engine takes over. Its a pretty steep grade. Went down once in my suburban without using the gears and all 4 brakes were smoking when I got to the bottom of the hill.
 
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Old 11-05-17, 06:17 PM
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Well, the 5 second rule on the tow button yielded no results. The video was for a 2013 model and mine is a 2012. However, the truck held its own well in 2nd gear down the mountain. Much to the dismay of the two cars following me. LOL. Found it actually performed better than my previous at keeping forward momentum from taking over 30 mph.
 
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Old 11-05-17, 06:25 PM
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Do you have the 6 speed trans? Is there a manual mode on the tow haul lever?
 
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Old 11-05-17, 06:30 PM
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Heres your manual... If you only have the 4 speed then you dont have grade assist...

2nd gears the best youll do..

https://my.gmc.com/content/dam/gmown...rra_owners.pdf
 
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Old 11-05-17, 06:32 PM
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I would still run in tow haul in the mountains as it changes shift points and keeps it in gear longer between shifts.. It also trys to lock the torque converter for as long as it can.. locked converter = cooler trans temps.

Additionally it turns off the alternater fuel saving feature and runs full power to the battery...

These GM's are funny and voltages fluctuate.. Sometimes 15.5 volts, when in sulfation mode..

My original battery lasted 11 years...



grade braking (6-speed
automatic transmission)
the grade braking shift modes can
be activated by pressing the button
on the end of the shift control lever.
While in range selection mode,
grade braking is deactivated
allowing the driver to select a
desired range of gears.
Grade braking is only active while
the tow/haul mode is selected and
you are not in the range selection
mode. See “tow/haul mode” listed
previously and manual mode on
page 9‑38 for more information on
the range selection mode. Grade
braking assists in maintaining
desired vehicle speeds when driving
on downhill grades by automatically
implementing a shift schedule that
utilizes the engine and transmission
to slow the vehicle. This reduces
wear on the braking system and
increases control of the vehicle.
Grade braking monitors vehicle
speed, acceleration, engine torque
and brake pedal usage. Using this
information, it detects when the
truck is on a downhill grade and the
driver desires to slow the vehicle by
pressing the brake.
This feature is active when the
exhaust brake is enabled (if
equipped).
Also see towing equipment on
page 9‑94 for more information.
. .
 
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Old 11-05-17, 06:46 PM
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while going downhill:
. Vehicles with a four speed
automatic transmission may
need to have the brakes applied
or the transmission shifted
to a lower gear to keep the
speed down.
. Vehicles with a six speed
automatic transmission have a
cruise grade braking feature
that may automatically shift to a
lower gear to keep the speed
down. It may be necessary to
apply the brake or manually
shift the transmission to a lower
gear. The vehicle may shift to a
higher gear when grade braking
assistance is no longer required.
Cruise grade braking is not
available while in range
selection mode. This feature is
active when the exhaust brake
is enabled (if equipped). See
"range selection mode" under
manual mode on page 9‑38.
When the brakes are manually
applied the cruise control is
disengaged.
. .
 
  #14  
Old 11-06-17, 03:12 AM
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very low speed secondary road with a maximum speed of something like 10-20 mph, sort of like Mark Sr's driveway,
Not sure I'd want to be in a vehicle going down my driveway at 20 mph I'm not that brave more like 5-10 mph down hill and 8-15 mph uphill.

However, the truck held its own well in 2nd gear down the mountain. Much to the dismay of the two cars following me
While it's never nice to hold up traffic, the number one concern is being comfortable with your speed down the mountain ..... and the more weight you have the more it takes to slow down. Glad 2nd gear worked well for you.
 
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Old 11-06-17, 05:05 AM
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And by the way, Mark, I wasn't picking on your driveway, and have obviously never seen it firsthand, but you've mentioned it in various posts so figured it was somewhat common ground to use as a reference point.
 
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Old 11-06-17, 10:23 AM
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Id still like to know 4 speed or 6 speed trans????
 
  #17  
Old 11-06-17, 05:13 PM
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Id still like to know 4 speed or 6 speed trans????
.

At this stage, based on the info you have provided, I'm gonna have to say 4 speed. I searched the vin number this morning to see if that would provide the answer with not clear results other than the point to the previous listing of my vehicle that was for sale.

On the column, it has PRNDM with M being manual at which time there is a plus and minus toggle on the column. It only gives me options of 1, 2 or 3rd gear. This is where I selected 2nd gear to make the trip down the mountain.

Thank you for the manual by the way, I have saved the link to my favorites and will continue to refer to it as I get adjusted to the new truck.

It performed surprisingly well on the trip. I have to use all mirrors which is new to me. Backing up the trailer has been a learning curve to say the least. Was a major cluster the first time, but I am getting better. Despite having a backup camera, It is useless with a trailer. With the previous suburban I would turn and visualize the trailer and always never used the mirrors. Got 14 years of bad habit to change before I get good at it. To boot, my current job requires me to back the trailer down a long narrow driveway that is curved in an "S" shape. Heaven help me, I've made it twice, but it was ugly both times.
 
  #18  
Old 11-06-17, 05:50 PM
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When backing, try holding the bottom of the wheel rather than the top, and the trailer will go the same direction as your hand. I did not learn this way so it's not normal to me at this point in my life, but I read it someplace over the last 10 years or so, tried it a couple of times and it works, so have shared it with a couple of our younger guys who have difficulty backing. Another tip, which I do with my enclosed tandem is to disconnect the trailer harness when I remember and when it's convenient. It's not as long from the axles to the hitch as my flatbeds, so when it starts to turn too short of course the tendency is to touch the brakes and when they engage on the trailer it only makes things worse. And shorter trailers are more difficult to back than longer ones because they want to jackknife easier. I'll guarantee that there are more than a few long haulers who can't back a snowmobile or small boat trailer worth a darn. And do forget the backup camera, as well as the rearview mirror on the windshield. Make yourself rely solely on your side mirrors and it will get a lot easier. This is my first truck with a rear camera, and it is really nice for backing up to the coupler, first shot every time, without having to get out of the truck, but I have turned the screen off a couple of times when backing a trailer into a tight spot because you of course want to glance at it and, in my opinion, it will only get you into trouble.
 
  #19  
Old 11-06-17, 06:19 PM
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Another comment about backing a trailer.. AKA Pedro has great advice and I will say basically the same thing with different words.

The way I learned to back a trailer using only mirrors is to place both hands at the bottom of the steering wheel. If you lift the wheel with your left hand your trailer will get bigger in your left mirror. Lift with the right, bigger in the right mirror. At a certain point you need to decide that you have the right angle (between truck and trailer) to "arc" the trailer to your target. At that point you drive the truck as if you didn't have a trailer attached - it's called following the trailer. Minor adjustments as necessary.

Good luck!
 
  #20  
Old 11-06-17, 10:58 PM
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Hey, try backing a hay wagon! That will drive you nuts. There's a reason many barns have front and back doors.
 
  #21  
Old 11-07-17, 12:12 AM
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Looks like its a 4wd work truck ext cab..

The good thing , and im jelous is you have the 6.0 liter engine.. Not that the 5.3 I have is bad but I would prefer the 6.0..

Plus a 2500hd is nice. Mines just a grocery getter 1500...lol

You have

Horsepower
360.0 @ 5400
Fuel Capacity
36.0 gal
Net Torque
380.0 @ 4200

These engines like to be revved so do be afraid cranking on it to 4000 rpms. Although me pulling 6000 lbs up NC mountains 3500 rpms was a good spot to be in 1st gear 35 mph... ha ha ha...

Look in your glove box. The RPO codes tell you all about what you have.

You have a 6 speed trans in there from my findings..

Trans Order Code MYD

GM 6-Speed 6L90 MYD Hydra-Matic Automatic Transmission

GM 6-Speed 6L90 MYD Transmission Info, Specs, Wiki | GM Authority

Tranfercase Magna MP1225 RPO NQG ( Manual shift... very nice..)

Rear you either have 3;73 or 4;10s. RPO will tell you.. GT5 is 4.10, and GT4 is 3.73.



Now what I suggest..

Thats a G80 RPO locker rear.. Get that fluid changed.. As well as the fron diff.. Rear takes synthetic front takes regular...

Tranfer case. Most dont check.. when I buy a new truck I always change fluids. All trucks I have owned transfer cases were empty/ dry. about a pint I got out of my 2006....

Usually they take about 2 qts trans fluid. But I read info as why they go dry. So I added 3 qts 5w30 syn mobil one..

Read here..

http://tcase.rsgear.com/articles/2007_03.pdf

I also added bigger trans cooler as I tow heavy... Coolers cheap, transmissions are expensive.. I recently flushed my trans and filter change with full synthetic..

Thats all I can think of... Im bored and I am a chevy guy so I figure ill help ya out...

You have a good truck there....

6000 lbs pulling to 3400 ft... 1st gear 35 mph video... lol 230 coolant temp... ( But trans was cool 160f... he he )

Video

https://flic.kr/p/WztwfL

Trans cooler below and you see how sludgy the g80's rears get...
 
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  #22  
Old 11-07-17, 03:51 PM
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Yep, MYD for a 6 speed transmission and GT4 for the rear end. Thanks Mike, I see you are as excited about my new truck as I am. Hell, you did all that research at 3 am in the morning.

Thank you others for the trailer backing help. I will try steering at 6 o'clock with my hands in the morning.

On another point - have a check engine light, code 455. Have changed the gas cap with no luck. Had the evap solonoid replaced which tested as defective when we smoked the gas tank. Now the light is back, same code. Tech said that the engine was saying that it is not getting sufficient vacuum when I went by today to have them scan while the check engine light was on. Any thoughts so I can have an intelligent conversation with the garage on next steps while it is still under warranty.
 
  #23  
Old 11-07-17, 08:42 PM
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When the code comes up does the DIC on dash say gas cap?

If you changed the whole solenoid that part was toward the fron of truck? How about the whole vapor canister? Did he change that?

Although I am going by my truck but I am sure they are all similar..

Need to blow line out too or replace. easy to check if debris in it but possibly the line can be leaking...

Example ACDelco 214-1680 GM Original Equipment Vapor Canister Purge Valve




Let me find the other part
 
  #24  
Old 11-07-17, 09:02 PM
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What this video for helpful tips on operation...

https://www.youtube.com/watch?v=Pg4AEe_xWo4
 
  #25  
Old 11-07-17, 09:24 PM
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001 CANISTER. Fuel Tank Evaporator/Purge Control

002 BRACKET. Fuel Tank Evaporator/Purge Control


003 BRACKET. Fuel Tank Evaporator/Purge Control

004 SOLENOID. Fuel Tank Evaporator/Purge Control

004 SOLENOID. Fuel Tank Evaporator/Purge Control


005 CLIP. Early Fuel Evaporation.


007 BOLT; NUT; RETAINER; WASHER. NUT. BOLT


015 HOSE. Fuel Tank Evaporator/Purge Control
 
  #26  
Old 11-07-17, 09:35 PM
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GMC/Silverado

 
  #27  
Old 11-08-17, 02:28 PM
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They changed the solenoid that goes from the tank (I believe) and ends with a filter that is fitted just behind the gas fill cap and zip tied to the fill hose. When we did a smoke test on the gas tank, this is where the smoke exited. So the solenoid was stuck in the open position. Replaced it and thought they fixed it and off I went. Then yesterday, the same code pops up. 455 evap leak (large).
 
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