Engine/Transmission (1994 - 1998) Do I misunderstand or is there something wrong with my truck???

[blacksheepdiesel]

Took a 1000 mi round trip this weekend to bring Christmas trees from Wisconson. Was pulling a 30ft g/n and total weight truck/trailer/trees was approx 22,000 lbs. Just put gauges on the stock 1996 with auto. I was under the impression that as the engine speed was reduced by, for example, going up a hill that the egt's would go up which would make more boost? Is my assumption incorrect? Or, is there something wrong with my truck? When the transmission would shift from 3rd to od and the tc would lock up the egt's would rise and the boost go down from 18 to 10. Mind you the load was about 10 feet tall, therefore I was pushing lots of air.



TIA



Trent

 

[Joe G.]

There may be a bit of confusion here. EGT is exhaust gas temp. The boost is controlled by how much exhaust you are makeing not how hot it is. When you shift from 3rd to OD and the TCC locked up the RPM dropped. All things being equal you will make more exhaust if the RPM is higher, thus more boost. Of course, at high EGT you are burning a lot of fuel which makes a lot of exhaust, but it's not firing as often so the total "blow" out the exhaust is less. Basically, if you run low RPM with a lot of fuel (lugging) the EGT will get high and there will be less boost. I get best power and lower EGT at about 2300 RPM or so. Running uphill with a load at about 1800 RPM mine smokes like a forest fire and the EGT is high.



If I have a heavy load I lock out the OD and force the TCC to stay unlocked.

 

[blacksheepdiesel]

Got it!

It's the higher CFM of exhaust, not the egt's that make boost! Duh!!! Learn somethin' every day!



Thanks Joe!



Trent

 

[akline]

TCC

I am guessing by TCC you mean torque converter clutch. This clutch when applied forces the speed between the two halves of the torque conveter clutch to be equal, the main benefits being increased fuel mileage and less damaging and fuel milage robbing heat produced in the transmission. I was reading your discussion with some interest and would just like to point out that even when locked out of overdrive, the TCC will still engage in third gear as low as 25 MPH depending on how hot the trans is and as long as the throttle position is not to high (it will unlock to keep from lugging the engine under heavier throttle) This has been my experience with my Dad's 1996 with auto trans. (I am somewhat partial to manual which is in my 1995)

The reason I point this out is because it is important for the TCC to be locked up as much as possible to protect the trans from heat buildup. If your load is so heavy as to require too much throttle too allow the TCC to engage in O/D over an extended time, you should definitely hit the button and drop to third.

Just some food for thought

Also how do you like the TST plate. If you can make smoke at 1800 RPM it must really make a difference!

 

[Joe G.]

I have a manual control option for my TCC. I can force it to stay locked if I wish or force it to stay unlocked. If I see that my EGT is getting too high I will shift down and/or unlock the TCC. I have enuff torque to slip the TCC in 3rd or OD so I have to watch what I am doing. I have added a set of 370 injectors recently so with the #5 plate I can really get some fuel to it. With this setup I have to pay attention to my gauges when towing my 32' TT in the mountains. So far, I have had no transmission temp problems.

 

[akline]

All I can say is: SWEET!