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700R4 question...

TommyD11730

TommyD11730

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Hey guys..
Need a little help. Have a bone stock (OK I removed the air pump and injection system) 1984 GMC K2500. 350 with a 700r4. I just noticed when pulling onto the highway from a dead spot the truck up shifts 3 times to OD. When I lift the throttle around 60mph the RPM jumps from 1500 to 1700. I'm thinking when I lift the throttle my TCC is unlocking? It's not shifting out of OD because if I pull the shifter from OD to D the RPM jumps again.
I'm guessing this has something to do with the vacuum/electric switch on the firewall. Anyone have any idea how it works..... if it should have ported or manafold vacuum... ect.
To me it seems like it's working backwards. Wouldn't you want the converter unlocked on hard acceleration and locked on part throttle?

Thanks!

20190203_132204.jpg
 
Hmmm 15 pages. About where is the transmission part in that massive thread?
 
It’s on the page in the link I sent you. It should take you right to the correct page. Page 10 about half way down.
 
Just briefed your thread. Saw some talk about your TCC, how it's wired to a mechanical switch. Nothing about how the factory vacuum /electric one works.
 
I'll check that link now, thanks!
Looking at my setup at curb idle I have no vacuum at the switch. I did not Rev the engine to see if ported vacuum was present.
I did see a vacuum diode inline. I'm guessing in the event of a backfire the don't want pos pressure on the line? Anyway the way the valve was installed you couldn't suck through it. So I reversed it. Will know more next time I drive it, or after I read your link.
 
Good link. Looks like what I'm calling a vacuum diode is a vacuum delay. Wonder is it was in correctly to begin with. I'll have to check it in the daytime to see if it's labeld.
Still makes me wonder, why the converter unlocks as I'm coming off the throttle. Wonder is it has something to do with the thermo vacuum switch. Not wanting the TCC locked when truck is cold.
 
My thinking is once it's locked it should stay locked, need it locked to keep fluid cool, in 4th. I think the tvs would work once it switched. Maybe source vacuum is to low at cruise.
 
Did a little more digging. The inline valve is a vacuum delay, not a 1 way diode.
OK, so anyways with the throttle in any position I'm not getting vacuum at the TCC switch. A quick bit of vacuum line work restored it. Checking with a test light I can see the switch now works under load. Pretty cool how the vacuum delay keeps vacuum off the switch if you just stab the throttle. In the same regard it also keeps vacuum on the switch for a few seconds after you lift. Guess the engineers didn't want it constantly cutting in and out.
Anyways back to my truck, the only way to recreate the problem is 1st drive in the am. So it's possible the thermo switch may not even be closed. So that's odd.
Im going to guess it's just an oddity, the truck see 4th gear switch close, brake lamp open , and as I cut my throttle back we get some ported vacuum on that TCC switch to unlock the converter.
Odd it doesn't happen warm...
Who knows maybe the lockup has morning sickness
 
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