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#1 |
Registered User
![]() Join Date: Apr 2004
Location: Sacramento
Posts: 3,737
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lock up on th350
I think the wiring is bad to my tranny on my '81. does anybody know how the lock up gets its signal. Does my truck have a computer that runs it? Is it turned on a specific RPM? A diagram would be wonderful if anyone has it. I suspect a problem because at freeway speeds my RPMs a way to high. 3000+. I have 3.08s out back.
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#2 |
Registered User
![]() Join Date: Apr 2004
Location: Sacramento
Posts: 3,737
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31 views and no one can help? does the question make sense?
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#3 |
Senior Member
Join Date: Jul 2003
Location: Indianapolis, IN
Posts: 1,277
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I am pretty sure the lockup is controlled by a vacuum switch, I know your truck does not have a computer. I think it is as simple as 12 volts on one side of the vacuum switch that is routed to the trans when the vacuum signal closes the switch. Seems like ithe power was also run through the brake light switch to unlock the converter when you hit the brakes.
I think the switch is mounted on the intake near the carb. My 84 had one that I pitched when the 305 was replaced with a 350. The Th350 lockup should be similar if not identical to the lockup on the same era 700r4 in a truck. Sorry I can't be of more help, this is all from memory. |
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#4 |
Registered User
Join Date: Mar 2001
Location: Fayetteville, Arkansas
Posts: 160
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I have an 84 with a 350-C.
The lock-up is vacuum operated by a switch mounted on the firewall just to the inboard side of the brake booster on mine. A vacuum line runs from the carb to the switch and a 12v wire runs from the switch to the tranny. Another switch runs from the brake pedal to the tranny to un-lock it when the brake is applied. Either switch bad (or no vacuum to the lock-up switch) can cause it to malfunction. |
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#5 | |
Registered User
![]() Join Date: Apr 2004
Location: Sacramento
Posts: 3,737
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Quote:
HRH |
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#6 |
Slots go on anything!
Join Date: May 2003
Location: Richardson, TX
Posts: 5,957
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As you are driving down the road at a steady speed, tap the brakes as you would to disengage the cruise. You should hear the truck pick up RPM a bit. If it does, the TCC is working. If it does not change at all, I would suspect the vacuum switch first.
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1974 Jimmy- 5.3/4L80e/NP241 |
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#7 |
Oh,you can't buy that new
![]() Join Date: Apr 2000
Location: Pennsville,N.J. 08070, USA
Posts: 2,039
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professor is on the money with local and signal but I'm pretty sure ?? all those other POS switches that the MAIN vacuum lines run into at the front of the engine are thermally activated and only work right for about a year or two after the trucks were new. My big problem with YOUR problem is that there is NO WAY you should be running 3000 even without the lockup. with 3.08 and any normal size tire you would have to be missing third gear to run that high of RPM and I'm serious. DAVE
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"been there, done that, ruined the T-shirt". 2007 LBZ GMC Sierra, ECSB 2006 LM7 RCSB Silverado 98 Vortec project. 94,3500,6.5L 4L80 81 SWB GMC POS finally gone ![]() 73 Pinto, stock, w/CragarSS's-eww ![]() LOTS of Cummins trucks. |
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#8 |
Registered User
![]() Join Date: Apr 2004
Location: Sacramento
Posts: 3,737
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I just had the tranny rebuilt a few months ago and it has been that way sense. Im not sure how it was before because I only drove it a few time before I swapped out the motor and tranny. Thanks for the tips.
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#9 |
State of Confusion!
![]() ![]() Join Date: Jul 2002
Location: Gulfport, MS USA
Posts: 46,844
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This may sound silly, but it sounds like the tranny is only in second with those RPMs, since it acted up since you did a rebuild/swap, maybe the linkage point is off a bit and your not really in drive when on the D. Maybe it is off enough that when you put it in D, it's in 2 instead.
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Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God! |
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