overdrive?

kenzoil

Active Member
Have a 07 trailblazer, L6 motor. Does anyone know how to tell if their Overdrive is working or not? Sounds stupid, but the truck shifts through all 4 gears, but doesn't seem to go into overdrive gear. It appears the rpms are all correct at shifting intervals, but on acceleration the engine seems loud , and once on the highway it seems loud also, and I am still at 1800 to 2200 rpms. When I got the truck a few years back, you could hear it dog out for a second, and I knew overdrive was engaging. Now all I hear is loud engine sounds in 4th gear, even on deceleration. Thanks for any help.
 

Robbabob

Well-Known Member
kenzoil said:
Does anyone know how to tell if their Overdrive is working or not? Sounds stupid...
You don't know stupid until you don't ask the question!

I might think a scan tool would tell you, but just hang on and someone will be here shortly that can help....

Welcome to the Nation!
 

Hypnotoad

Well-Known Member
I don't believe these trucks have overdrive, unless 4th is considered OD. Anyway, sounds like you're having an issue with the torque converter not locking. It should feel like your truck is shifting into another gear when it locks up.

Edit: Ah nevermind me, I pretty much just said what OP said. Need more sleep.
 

IllogicTC

Well-Known Member
Third gear is direct drive, fourth is overdrive.

Do you know which axle you have? You can find out by looking in the glovebox, there will be a list of codes on the inside of the lid. You'll either have GU6, GT4, or GT5. That'll give us an idea of where the RPM SHOULD be at a specific speed, such as 55MPH. My GT4 code 3.73 axle seems to hit about dead on 1700 RPM when TCC is engaged at 55 MPH.

The way I can tell is that you'll feel it going through the gears, and see an appreciable drop in the tachometer. When the TCC engages, it'll drop down maybe another 200 RPM or so. It's usually not sudden but rather gradual across the space of a second or so, but that's long enough to not classify it as instant. On my '05 with the 3.73 at least, TCC engagement appears to occur at a minimum of 40MPH. You may try setting cruise for 39MPH, then bumping it up a notch and letting it settle and see if it drops into engagement.

If you're on a level road and the tachometer is consantly wandering, something's not right.
 

CaptainXL

Well-Known Member
kenzoil said:
Have a 07 trailblazer, L6 motor. Does anyone know how to tell if their Overdrive is working or not? .
Very simple. Put it in third gear and when going about 50mph manually shift it into 4th. The rpms should drop. You now know you are in 4th.

TCC lockup is usually the item in question. Perhaps that is what you meant?
 

IllogicTC

Well-Known Member
CaptainXL said:
Very simple. Put it in third gear and when going about 50mph manually shift it into 4th. The rpms should drop. You now know you are in 4th.

TCC lockup is usually the item in question. Perhaps that is what you meant?
That's what I'm thinking it's about. I would assume that after shifting to fourth it wouldn't immediately engage the TCC, but may do so with a few seconds or less after shifting, so you should see the shift point from 3-4, then the TCC lockup drop maybe 1-5 seconds later. Good thinking on the shift, made me think of that as a test idea :rotfl:
 

dmanns67

Well-Known Member
I assumed the TB had OD. When watching my TorquePro app on my phone, there is a screen that shows my current gear. If I am in 4th gear and start to go up a slight incline, I will apply more pressure to the accelerator to maintain speed and the RPMs will jump up about 800-1000, but my app still shows that I am in 4th gear. If I press on the accelerator hard and the RPMs almost double, it will show that I am now in 3rd gear.

On the other hand, it does say I am 1st gear when it is in park, neutral, or reverse :undecided:
 

IllogicTC

Well-Known Member
dmanns67 said:
I assumed the TB had OD. When watching my TorquePro app on my phone, there is a screen that shows my current gear. If I am in 4th gear and start to go up a slight incline, I will apply more pressure to the accelerator to maintain speed and the RPMs will jump up about 800-1000, but my app still shows that I am in 4th gear. If I press on the accelerator hard and the RPMs almost double, it will show that I am now in 3rd gear.

On the other hand, it does say I am 1st gear when it is in park, neutral, or reverse :undecided:
What it needs to do is show when TCC lockup is commanded :rotfl: Overdrive isn't lockup, it's literally just having a gear ratio where the output shaft spins faster than the input shaft. Which makes first and second gear "underdrive" technically. 4L60E has a final drive of 0.696:1, which is definitely OD.
 

Hypnotoad

Well-Known Member
IllogicTC said:
What it needs to do is show when TCC lockup is commanded :rotfl: Overdrive isn't lockup, it's literally just having a gear ratio where the output shaft spins faster than the input shaft. Which makes first and second gear "underdrive" technically. 4L60E has a final drive of 0.696:1, which is definitely OD.
Lockup is mechanical, and not commanded.
 

IllogicTC

Well-Known Member
Hypnotoad said:
Lockup is mechanical, and not commanded.
Not on 4L60E, which may or may not be a good or bad thing. TCC lockup is commanded by the PCM. All five shifting solenoids in the transmission receive power from a common source, through the IGN D fuse. The PCM provides a ground for solenoids in order to energize them. Two solenoids alone run just the TCC - The TCC solenoid and the TCC PWM solenoid. The idea is to facilitate a smooth lockup by gradual application of the clutch through this setup.
 

kickass audio

Well-Known Member
A way to see if you are in lockup is to go through the 4 shifts and then keep constant pressure on your accelerator then slightly tap your brake. You only need to tap the brake enough to get the brake lights to kick on for a second, you don't want to press it so hard that you actually begin grabbing the rotors with your pads, just a very soft tap on the brake. If your engine is hot enough and you are at a good speed, like thruway speed, you should notice the rpm shoot up about 200 and then go back down after about 2-3 seconds. If this happens then you are going through the 4 gears and are getting a good TCC lockup.

Some things to note: Lockup will not occur if you are at low speeds or if your engine has not warmed up enough. I am not sure how that plays a role in it but I saw that on another thread here about the TCC not allowing a lockup when it is cold even if you have enough speed. I guess it needs warm tranny fluid for it to work right? I really don't know for sure. Also something I noticed about the whole lockup thing, some have said that shifting into 3 instead of D will disengage the transmission from locking the TCC but I am somewhat going to disagree with that. I have noticed a good 350rpm increase if I shift from D to 3 where if I have it in D and do the brake test to get out of lockup it gains 200rpm. I also can't explain that behavior.
 
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kenzoil

Active Member
I can follow most of what you guys are saying. So I do the brake test and I'll see what happens. Any good in doing another fluid change? done about 40,000 Miles ago. Can anyone explain the loud engine rev noise? It is not the fan clutch, since I already had that problem and replaced everything.. Even going down a hill, with my foot off the gas pedal, coasting, it seems like the engine is loud reving, even though the rpms are low??






kickass audio said:
A way to see if you are in lockup is to go through the 4 shifts and then keep constant pressure on your accelerator then slightly tap your brake. You only need to tap the brake enough to get the brake lights to kick on for a second, you don't want to press it so hard that you actually begin grabbing the rotors with your pads, just a very soft tap on the brake. If your engine is hot enough and you are at a good speed, like thruway speed, you should notice the rpm shoot up about 200 and then go back down after about 2-3 seconds. If this happens then you are going through the 4 gears and are getting a good TCC lockup.

Some things to note: Lockup will not occur if you are at low speeds or if your engine has not warmed up enough. I am not sure how that plays a role in it but I saw that on another thread here about the TCC not allowing a lockup when it is cold even if you have enough speed. I guess it needs warm tranny fluid for it to work right? I really don't know for sure. Also something I noticed about the whole lockup thing, some have said that shifting into 3 instead of D will disengage the transmission from locking the TCC but I am somewhat going to disagree with that. I have noticed a good 350rpm increase if I shift from D to 3 where if I have it in D and do the brake test to get out of lockup it gains 200rpm. I also can't explain that behavior.
 

Envoy_04

Well-Known Member
IllogicTC said:
Not on 4L60E, which may or may not be a good or bad thing. TCC lockup is commanded by the PCM. All five shifting solenoids in the transmission receive power from a common source, through the IGN D fuse. The PCM provides a ground for solenoids in order to energize them. Two solenoids alone run just the TCC - The TCC solenoid and the TCC PWM solenoid. The idea is to facilitate a smooth lockup by gradual application of the clutch through this setup.
You are very correct. The 4L60E's predecessor, the 700R4, is an auxillary unit (non-electronic) and when it is in 4th and the TC locks you can most definitely feel it doing so immediately, it really feels like shifting into a 5th gear compared to the smooth lockup on the 4L60E.

kenzoil said:
I can follow most of what you guys are saying. So I do the brake test and I'll see what happens. Any good in doing another fluid change? done about 40,000 Miles ago. Can anyone explain the loud engine rev noise? It is not the fan clutch, since I already had that problem and replaced everything.. Even going down a hill, with my foot off the gas pedal, coasting, it seems like the engine is loud reving, even though the rpms are low??
Just a stupid question, have you checked your exhaust for leaks/rustouts? The very same thing you're describing happened to me in my S10 one day and I checked to see if somehow I'd bumped it out of OD into drive. Turned out the bottom of my old stock muffler had rusted out so badly that it literally fell out. :rotfl:
 

HARDTRAILZ

Moderator
At 40K, it won't hurt to change the fluid and would probably be a positive since you are suspecting issues.
 
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kenzoil

Active Member
I did check for exhaust leaks, and found none. I will change the filter and tranny fluid, since it is about due also. What I was thinking , and didn't check is maybe the exhaust manifold gasket has a small crack/ hole, between the header pipe and head. Sometimes hard to see, but that kind of a noise maker. How common do you see the exhaust manifold gasket going, in these engines?
 

HARDTRAILZ

Moderator
The manifold bolts snap commonly...
 

Envoy_04

Well-Known Member
kenzoil said:
I did check for exhaust leaks, and found none. I will change the filter and tranny fluid, since it is about due also. What I was thinking , and didn't check is maybe the exhaust manifold gasket has a small crack/ hole, between the header pipe and head. Sometimes hard to see, but that kind of a noise maker. How common do you see the exhaust manifold gasket going, in these engines?
Not sure about the gasket, but the manifold cracking is SUPER common.
 
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kenzoil

Active Member
so you are talking about the actual exhaust header pipe off the block, cracking?? any place to look in particular? I would try some JB weld first, if that was the case, before replacing the actual pipe.
 

HARDTRAILZ

Moderator
Yes. Headers replace manifolds...
 

strat81

Well-Known Member
kenzoil said:
I can follow most of what you guys are saying. So I do the brake test and I'll see what happens. Any good in doing another fluid change? done about 40,000 Miles ago. Can anyone explain the loud engine rev noise? It is not the fan clutch, since I already had that problem and replaced everything.. Even going down a hill, with my foot off the gas pedal, coasting, it seems like the engine is loud reving, even though the rpms are low??
If you notice it at low speed, it may be the fan clutch (again). A bad fan clutch is pretty noticeable under 30mph.

If the noise gradually gets louder as your vehicle speed increases, regardless of RPM, it may be bad tires or wheel bearings (hubs). An unsafe way to test this would be to get up to 65mph, put it in neutral, and turn the engine off.

Do the vehicle's tires show any abnormal wear? Is there plenty of tread left? Are they winter tires, mud tires, or all-terrain tires?

Have you ever changed front wheel bearings? If so, what brand did you use?
 
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kenzoil

Active Member
The right front has been replaced. It really sounds like it's combustion related. Is it a big deal to take off the exhaust manifold heat shield to get a good look at the exhaust manifold ? I think this is where I am going with this.



strat81 said:
If you notice it at low speed, it may be the fan clutch (again). A bad fan clutch is pretty noticeable under 30mph.

If the noise gradually gets louder as your vehicle speed increases, regardless of RPM, it may be bad tires or wheel bearings (hubs). An unsafe way to test this would be to get up to 65mph, put it in neutral, and turn the engine off.

Do the vehicle's tires show any abnormal wear? Is there plenty of tread left? Are they winter tires, mud tires, or all-terrain tires?

Have you ever changed front wheel bearings? If so, what brand did you use?
 

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