Pardon My Ignorance

kbswear · December 2, 2009

I have a 1500 HD CC Sierra 4x4. I recently bought the truck. Just this morning on my way home from work i was paying attention to the engine speed and mph. I was on a flat road for @ 3 miles. My speedo is registering 48 mph and engine rpms is @ 2k. I looked this up using a calculator and that engine speed and mph calls for 4.10 gears. The RPO code says its 3.73. The guy i bought it from was the original owner and he said they've never been changed so i'm just wondering. The truck has 33" tires.

Excuse the auto tranny ignorance please. I race mustangs and i'm more familiar with the TKO 600 and T 45.

kbswear · December 2, 2009

I'm not thinking very straight. I would guess one to do it is hook up my boat and while under load dis-engage the OD to see if the tranny kicks down.

Shaners · December 2, 2009

I see you live in GA so this may not be the case since the weather isn't cold there yet. But GM computer programming won't allow the tranny to shift into OD until certain operating temps are reached. The colder the temps outside, the longer it keeps the truck from shifting into OD.

kbswear · December 2, 2009

That makes sense. It has gotten some what cold here. It was 34 yesterday morning when i noticed this. Operating temps? Are you referring to transmission temp? I did notice it barely got above 100 degree's while i was driving home.

salden77 · December 2, 2009

I see you live in GA so this may not be the case since the weather isn't cold there yet. But GM computer programming won't allow the tranny to shift into OD until certain operating temps are reached. The colder the temps outside, the longer it keeps the truck from shifting into OD.

+1 to this, in the winter months my truck will not shift into overdrive for several miles until the truck is warmed up, which can take a while with this northeast weather lately.

kbswear · December 2, 2009

I believe you guys are right. This morning on the way home it was 59....so about a 25 degree difference and it was shifting like it should. I played with the OD off button a couple of times and everytime i did it did downshift.

thanks

gmc_todd · December 3, 2009

I believe you guys are right. This morning on the way home it was 59....so about a 25 degree difference and it was shifting like it should. I played with the OD off button a couple of times and everytime i did it did downshift.

Are the 1500HD's set up like the 2500HD's? I'm thinking the button is the tow/haul mode and to keep it out of OD you would shift to 3 instead of D. Is that correct?

kbswear · December 3, 2009

I believe you guys are right. This morning on the way home it was 59....so about a 25 degree difference and it was shifting like it should. I played with the OD off button a couple of times and everytime i did it did downshift.

Are the 1500HD's set up like the 2500HD's? I'm thinking the button is the tow/haul mode and to keep it out of OD you would shift to 3 instead of D. Is that correct?

Yes i was reffering to the tow/haul button. As far as the other i'm not sure.

brims · December 3, 2009

Ford has the O/D off button on the shifter not Chevy.

I don't ever recall my truck downshifting when I hit the tow/haul button, stock or tuned, to do that I'd have to do something with the throttle.

builderpaul · December 5, 2009

I have a 1500 HD CC Sierra 4x4. I recently bought the truck. Just this morning on my way home from work i was paying attention to the engine speed and mph. I was on a flat road for @ 3 miles. My speedo is registering 48 mph and engine rpms is @ 2k. I looked this up using a calculator and that engine speed and mph calls for 4.10 gears. The RPO code says its 3.73. The guy i bought it from was the original owner and he said they've never been changed so i'm just wondering. The truck has 33" tires.

Excuse the auto tranny ignorance please. I race mustangs and i'm more familiar with the TKO 600 and T 45.

Hi

Your calculator will only be correct if the correct info is entered. Your truck came with 31" tires so unless you reprogrammed your speed is not correct. Also did you calculate using gearing of 1:1 (3rd gear) or .75 (OD)?

FargoZ71 · December 7, 2009

Cold starts take for ever for my OD to kick in, usually only drops about 200-250rpms..

With 31's and 3.73 @ 2k rpm im running almost 70mph.. Check to see if you were in 3rd... be my guess you never made it to 4th cause I drive around 1500rpm @ 50.

FargoZ71 · December 8, 2009

Suposivly having the same gearing as you.. I tested today on my way to school and my way to work.. In 3rd gear at 2krpm im at your 48 mph..

In 4th gear before over drive kicked in this morning( Cold ), at 2k rpm I was at 65mph. Once it dropped down 2k rpm took me to 70mph..

My opinion is your trucks not even making it to 4th.

kbswear · December 8, 2009

Suposivly having the same gearing as you.. I tested today on my way to school and my way to work.. In 3rd gear at 2krpm im at your 48 mph..

In 4th gear before over drive kicked in this morning( Cold ), at 2k rpm I was at 65mph. Once it dropped down 2k rpm took me to 70mph..

My opinion is your trucks not even making it to 4th.

I've found that it does make it to 4th, it just takes a while from a cold start. I'm thinking longer that it should on occasion. From work to home is @ 6 miles. WHen it is cold sometimes it will never see 4th gear, but there is usually at least on stop for breakfast before i get home. If i'm on a long stretch of road and the truck is up to running temp then it will go into 4th. Is this normal or is there a problem somewhere. The truck has 169,xxx miles and the previous owner did not say he had serviced the tranny. So i'm wondering if a flush or just a part drain and filter change would be better???

FargoZ71 · December 8, 2009

far far from normal..

It should take time for the tranny to shift from 4th to OD.. not from 3rd to 4th..

Might wanna have it scanned. Mines got a BB Performance tune which increased some line pressure for better shifts, but even before that in Minnesota winters ( not uncommon for -20 in the morning) It always shifted into 4th right away.

Could be a Servo.. Could just be alot of junk. We have tranny experts on here, I just havnt needed there advise so i cannot quote there names.

jro909 · December 8, 2009

2k isen't overdrive at that speed my 5.3 runs at around 1500 rpms at about that speed thats in 4th gear...if the tranny is cold it will ride 3rd to heat itself up