700R4 Lockup adjustment?

[shortboxchevy77]

Hey all, new to the forum and would love to get some input on an issue I'm having with my 77 chevy k1500. I'll give you the rundown on what has been done to the truck before my initial question

77 K1500 with 31 inch tires and 3:73 gears.
TBI 350 (low mile GM new replacement) 700R4, 208 Tcase all taken from an 87 suburban. Truck has been converted to operate all the factory 87 GM fuel injection equipment using a Painless wiring harness.

Engine has shorty headers, rebuilt and modified TBI to flow more air (no TBI spacer) and is also running higher fuel pressure (15psi). I am also using the original 87 speedo for vss reference along with the 87 brake switch for TCC control. Timing on the engine has been jacked up 4 degrees advanced. I do not have a park/neutral switch, the ECM is simply setup to always think it is in drive and seems to work fine that way.

Here is my issue. The thing is a monster around town, lots of get up and go and will spin 31 inch mud tires at 5mph with a flatbed BUT when I get it on the highway and it goes into overdrive I cannot maintain 75 without the converter constantly being unlocked. If I keep my foot off of it enough to lock the converter I can only maintain about 70.

It almost feels as if it struggles in overdrive yet it will go as fast as I want in third. I feel it would be better if I can keep the converter locked when I am in OD. This truck in my mind should pack itself 100mph no problem and its not doing it. I have adjusted the TV cable multiple times and I am almost out of ideas. Anyone else ever experience this? Sorry for the huge post, figured the more info I put out the less questions I will get back. Thanks in advance!

 

[gpmorgan]

This is just a guess. It may be the 373 gear is fooling the ecu making it think it going faster than it is and keeping it from staying in OD. Im not sure and it may be controlled by the TPS.

Welcome to the forum Mark.

 

[shortboxchevy77]

Thank you, I do know the speedo is on at 60mph so that makes me believe that the gears in the suburban were also 3:73's

 

[Boone83K10]

With a 373, you should not be having problems maintaining speed.

Are you using a vacuum switch, is it a factory version or an adjustable? (look at the 10 o clock position next to the brake booster)

They can cause cycling if your vacuum reading at speed is too close to the switch cutoff. Factory is 7.5 Hg I believe, if you have a adjustable then you need to widen the engage/cutoff ratio.

 

[austinado16]

Not sure how you've adjusted the TV cable, but it won't be set correctly unless you've used a pressure gauge in the port on the driver's side of the trans. www.bowtieoverdrives.com has an extensive article on how to correctly set the TV cable using a gauge that you can make yourself.

 

[shortboxchevy77]

With a 373, you should not be having problems maintaining speed.

Are you using a vacuum switch, is it a factory version or an adjustable? (look at the 10 o clock position next to the brake booster)

They can cause cycling if your vacuum reading at speed is too close to the switch cutoff. Factory is 7.5 Hg I believe, if you have a adjustable then you need to widen the engage/cutoff ratio.

No vac switch. I'm using all 1987 TBI components so the ECM would be commanding the lockup function.

 

[shortboxchevy77]

Not sure how you've adjusted the TV cable, but it won't be set correctly unless you've used a pressure gauge in the port on the driver's side of the trans. www.bowtieoverdrives.com has an extensive article on how to correctly set the TV cable using a gauge that you can make yourself.

While I believe that using a pressure gauge to set the TV cable would be most accurate I find it tough to believe that doing the adjustment the old way (the way every book tells you to) is going to cause things to be that far out of whack, thank you for the suggestion though. It is something to mess with in the future.

 

[Boone83K10]

No vac switch. I'm using all 1987 TBI components so the ECM would be commanding the lockup function.

well then I agree with gpmorgan that it was programmed for let say a 3.42 or 3.08 and it is confused.

 

[gpmorgan]

http://www.sonnax.com/parts/2513

Check this link out. This could be causing your problem. I have one of these valves in my 700r4 and it will stay in od at wide open throttle.

 

[shortboxchevy77]

http://www.sonnax.com/parts/2513

Check this link out. This could be causing your problem. I have one of these valves in my 700r4 and it will stay in od at wide open throttle.

Thanks for the link but the problem isn't keeping it in 4th gear. The problem is keeping the converter locked up, it kicks out past 1/4-1/3 throttle in 4th and if I hold the throttle at a point where it stays locked up it does 65mph. Anything past that speed and its unlocked the whole time.

 

[shortboxchevy77]

Finally just jumped the terminals on my trans. plug-in so it locks up in 4th as soon as it engages. Seems to work better now, power still seems a little weak to me in the 2300-3000rpm range.

 

[85 oil burner]

Try a 3.42 gear instead if ya can and it you will save a bit of fuel and go a little faster considering then you get into highway gear range, I have a little trouble maintaining speed myself at 70 mph with my burb with my 6.2 with a 700 R4 and a 3.73 rear and 2wd and am thinking of converting it to a 3.42 to go a little quicker.But if ya do that get a detroit locker not a open diff. or you will be sorry when ya go muddin or snow boggin.

 

[87ChevyR10]

Finally just jumped the terminals on my trans. plug-in so it locks up in 4th as soon as it engages. Seems to work better now, power still seems a little weak to me in the 2300-3000rpm range.

Before you did this, did your torq conv. take a long time to lock up or would only lock up with your foot barely touching the throttle?

Just curious