lock up on th350 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

hotrodhomi · 12-01-2004, 11:47 AM

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.

hotrodhomi · 12-01-2004, 05:12 PM

31 views and no one can help? does the question make sense?

beaterC10 · 12-01-2004, 05:28 PM

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.

Professor · 12-01-2004, 05:35 PM

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.

Jonboy · 12-01-2004, 05:48 PM

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.

**crossy** · 12-01-2004, 06:50 PM

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

hotrodhomi · 12-01-2004, 08:06 PM

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.

**Palf70Step** · 12-01-2004, 08:45 PM

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.

hotrodhomi · 12-01-2004, 09:43 PM

Quote:

Originally Posted by Professor

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.

For the vacuum line/carb connection..does it matter where on the carb it is pugged into. I have the stock carb now but Im expecting my edelbrock 1406 any day now from Jegs. I know the 1406 has more vacuum ports. Anybody know which one I should use.
HRH

12-01-2004, 11:47 AM	#1
hotrodhomi Registered User Join Date: Apr 2004 Location: Sacramento Posts: 3,737	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.

12-01-2004, 05:48 PM	#5
Jonboy Slots go on anything! Join Date: May 2003 Location: Richardson, TX Posts: 5,957	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. __________________ 1974 Jimmy- 5.3/4L80e/NP241

12-01-2004, 06:50 PM	#6
crossy Oh,you can't buy that new Join Date: Apr 2000 Location: Pennsville,N.J. 08070, USA Posts: 2,039	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 __________________ "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.

12-01-2004, 08:45 PM	#8
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 46,768	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. __________________ 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!

12-01-2004, 05:12 PM	#2
hotrodhomi Registered User Join Date: Apr 2004 Location: Sacramento Posts: 3,737	31 views and no one can help? does the question make sense?

12-01-2004, 05:28 PM	#3
beaterC10 Senior Member Join Date: Jul 2003 Location: Indianapolis, IN Posts: 1,277	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.

12-01-2004, 05:35 PM	#4
Professor Registered User Join Date: Mar 2001 Location: Fayetteville, Arkansas Posts: 160	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.

12-01-2004, 08:06 PM	#7
hotrodhomi Registered User Join Date: Apr 2004 Location: Sacramento Posts: 3,737	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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