700R4 downshifts from OD to D on highway

[Trucknut25]

Hi Everyone, Looking for a little help/info. My 89 K1500 has a 700R4 / 5.7 3.42 gears. I've been driving it to work lately due to picking up building supplies and have noticed that on the highway it is downshifting out of OD frequently. Seems to happen any time I go over about 2300 rpms. I could be cruising at 2250, soon as it pass's 2300 it downshifts, even when I have no real load on it going down a hill. Any ideals what this could be? It kills the fuel mileage, not that I have much to begin with. On back roads running 45 or so it will stay in OD unless I really mash on it, but the interstate it wont. I've tried tried adjusting the TV cable by opening the throttle wide open when its off like they say is the way to adjust, no difference.

Any help???

 

[studigggs]

Sure it's just not the torque converter unlocking? Mine stays in OD over 40 mph unless powering up a hill. The torque converter locks and unlocks quite often depending on acceleration or headwind. You can test by manually shifting to 3rd during one of the downshifts and see if it stays at the same rpm.

 

[PlayingWithTBI]

Sure it's just not the torque converter unlocking? Mine stays in OD over 40 mph unless powering up a hill. The torque converter locks and unlocks quite often depending on acceleration or headwind. You can test by manually shifting to 3rd during one of the downshifts and see if it stays at the same rpm.

^^^What he said.

The torque converter locks and unlocks based on your TPS (Throtle Position Sensor) and MPH.

The left table shows where my 88 5.7 will unlock based on acceleration MPH/TPS. The upper right is minimum MPH to lock. The lower right shows TPS max when coasting it will unlock. Or you may have something wrong with your TPS? Hope it's your TCC locking/unlocking and not something major in your trans.

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[Madison Mehlhose]

Playingwithtbi

Can you go into more detail on how to view these settings and change them if possible. I have the necessary cable and software that you have screen shots of above. My truck TCC goes in and out depending on the weather at about 75 on the highway and I’d like to adjust the settings to not do that if I can.

Thanks!

 

[PlayingWithTBI]

Can you go into more detail on how to view these settings and change them if possible. I have the necessary cable and software that you have screen shots of above. My truck TCC goes in and out depending on the weather at about 75 on the highway and I’d like to adjust the settings to not do that if I can.

This is a screen shot from TunerPro RT which is a free download (or a small donation).

http://www.tunerpro.net/

You need to upload your bin file to it and get the proper XDF (mask) file for your truck. In this case this one is from EBL Flash-II so, I'm using EBL_F_2011.bin and EBL_V40.XDF My original files for my 88 C1500 5.7L (OBD1) AMUS.bin and $42-1227747-V5.9.3.xdf

Then you look in Scalars, and Tables.

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Can you data log to see what parameter is causing your truck to go in/out of TCC? You can with TunerPro RT. HTH

 

[Ken K]

Do you have slightly over sized tires? With the TPS input, the VSS is the other input the PCM uses to calculate engine load. The late 87 - 89 came with TCC valve body cavities but where plugged. late 89 - 92, that part was gone. The older version can have a TCC kit installed for hydraulic only.
The PCM controls TCC on and off as show above, but decides based upon "Calculated Engine Load" to kill the TCC or leave it on.
With TV cable correctly adjusted and the TPS is correct, this leaves the tire size as the only factory the PCM would choose to kick the TCC out and drop into OD only. Is that the factory final drive ratio? This should be less than 200 RPM most likely closer to 110 RPM change. Anymore than that, it's in 3rd and not overdrive.

You did not mention after the load was gone, and you where headed back for more, did it act normal? If so, it is not bad and working like it should.

(Note: Sorry for description as ECM had PROM, VCM was gas, PCM was Diesel...they are all the same for me. GM made it and gave it names, I didn't and care. They do what they do)

 

[Trucknut25]

Thanks for the replys, I down shifted after it had done this yesterday and sure enough it is the torque converter locking and unlocking. it just seems to do it so much I thought It was downshifting. I was hoping that was part of the reason for my poor fuel economy (was much better on gas when i got it a few years ago). Guess its not so Ill have to dig into the engine and see whats going on that might cause the poor fuel economy.

 

[Lanny]

Thanks for the replys, I down shifted after it had done this yesterday and sure enough it is the torque converter locking and unlocking. it just seems to do it so much I thought It was downshifting. I was hoping that was part of the reason for my poor fuel economy (was much better on gas when i got it a few years ago). Guess its not so Ill have to dig into the engine and see whats going on that might cause the poor fuel economy.

Driving around on the highway a lot with an unlocked converter will definitely hurt the mileage.

 

[Ken K]

GM info indicates 100 RPM reduction of engine speed when the TCC is engaged. Road test with TECH2 in hand shows as much as 175 RPM's. Some line of codes in the VCM/PCM in certain years and models show the ability to achieve lockup in 2nd, 3rd much less OD.
For fuel mileage loss, all kinds of things can eat away it. Write down mileage and gallons to the tenths and repeat to make sure.
Without any check engine light, try using Techron by Chevron. The trick is this is safe in the gas tank, not true with GM / ACDelco X66P Upper Engine & Fuel injection Cleaner."
Shop around to find the Techron, then with a low tank, empty 1 bottle to increase the ratio fo cleaner to fuel. Then drive and it will clean injectors that improve the spray pattern. It can't hurt.
It used by fuel supply companies as the only approved fuel injection cleaner (5 or so Car Manufactures signed on) added prior to tanker delivery. It is measure at an exact ration to qualify as "Top Tier" fuel.
Now, online shopping may bring up the GM/ACDelco X66P. Be warned, it is for injector rail only and not to be poured into the tank. Fuel pump damage will occur. I have the actual kit that allows for a pressurized tank, that fuel with 20% X66P is added to fuel. With the return line is plugged, the engine is run until it stalls. EGR plates are provided for direct cleaning. X66P will remove the carbon from the intake valves, but if applied thru the EGR, it is dispersed to all cylinders equally therefore it will clean intake runners as well. GM's TSB / Warranty indicates when a customer has a properly diagnosed restricted injector, it is to be cleaned and varied as repaired, the pay the claim. But, the owner is instructed to use "Top Tier" fuel from now on, otherwise there will be a charge.

So try a few normal things, cleaning the air filter, MAF is tricky but can be done, throttle valve, IAC passage, tire pressure, tire wear, etc.

I will never forget the days decades ago at the dealer, when customers asked about why the transmission felt like it was shifting "TOO" much. The explanation of TCC satisfied some...others not so much. Then came the crowd of men who where convinced they had a short, as certain electrical things stayed on when the key was off, open door, went off. "Sir, its retained accessory power." Then came "Gas Cap" CEL. Got the service advisors to tighten the cap or check it before writing a repair order that dos not pay.

Well, hope this helps. And perhaps a little laugh.

 

[whitewheels]

Had that exact problem when I bought my truck many years ago,that said I can't remember how I solved it but I do know that I ended up puting a new TC in. Worked fine after that. ... [emoji848]

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