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-   -   th350c lock up, does not function? (

stevetankl 05-26-2012 02:29 PM

th350c lock up, does not function?
2 questions...Q#1, besides my selonoid, what else can be the cause of my lockup not engaging? It's a brand new converter...brand new clutches, seals, everythings new. I there's a wire running from the selonoid to a pressure switch. Could the switch be bad? Is so can I bypass it?
Q#2, there's a pressure switch on the passenger side under the transmission lines. What's it for??? :confused:

pricesgarages 05-26-2012 04:07 PM

the psi switch under the lines is for a advance lock out on the distributor it was some of the mid 70s smog equipment
as for the lock up have u checked to see if u have 12v going to the soleniod
also if the axuliry vave body could have the tcc valve stuck

spinn 05-27-2012 07:09 AM

How many pins are on the HEi module?

Connect a 12 volt acc voltage switched to the tcc. Drive it around and see if it is on. Could be a bad harness.

LATECH 05-27-2012 10:28 AM

The switch on the side is the third gear switch. It closes when third gear is achieved.Also the power side has a brake light switch circuit, when you apply the brakes, it cuts the power to the switch and the converter unlocks.
If you mash the accelerator and the trans downshifts to seconfd , the 3rd gear switch opens and unlocks the converter also, untill it upshifts.
The ground is controlled by the ECM on most of them,turning on when a required speed is achieved, along with 3rd gear, and minimal throttle inclination.
The vacuum switch is for the distributor vacuum , cuts it off, during deceleration IIRC.

spinn 05-27-2012 11:23 AM

You saying the trans vacuum switch dumps the distributor vacuum podadvance signal?

That I did not know.

Why not leaveit on the carb,or manifold source.

LATECH 05-27-2012 06:11 PM


Originally Posted by spinn
You saying the trans vacuum switch dumps the distributor vacuum podadvance signal?

I did some checking on that, and found i was wrong. The vac switch was manifold source fed, and low vacuum opened the switch circuit, disallowing lockup,so when in high gear and lockup, if applying throttle, and engine vacuum dropped, it would unlock to prevent chuggle, or driveline lashing (like when lugging the motor).
That would be on the 78 up models, when the 80 models came out with computer controlled carbs and dizzys, it then became an ECM function ( rpm, tps, and map sensor algorithm ) that controlled that part of the circuit.

So the vacuum switch setup is actually the best way to go, it is simple and needs no computer.
Good lookin out spinn. :thumbup:

cobalt327 05-28-2012 03:33 AM

Good info; '81 began ECM TCC control (unless CA possibly). Before then (like on my '80 Malibu wagon) it was analog, used a vacuum switch mounted on the fenderwell or firewall.

My engine doesn't use an EGR valve so I run manifold vacuum directly to the TCC vacuum switch and to the distributor vacuum advance, w/o the intake water crossover-mounted temperature switch. Using a temp switch wouldn't hurt, it would only prevent TCC lock up (as well as vacuum advance if that was also hooked through the switch) until the engine was up to temp.

stevetankl 05-28-2012 08:10 PM

Can the governor oil pressure switch be over ride?

cobalt327 05-29-2012 04:51 AM


Originally Posted by stevetankl
Can the governor oil pressure switch be over ride?

You could, but if there was no governor pressure switch to prevent lock up under a certain speed, the vehicle could be lugged when coming to a stop until it downshifted.

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