Lock-Up Torque Converter Question

[1425956]

Alright guys and gals...I'm gonna keep this short and sweet. I have a tendency to be long winded so here goes.....

1984 GMC Sierra Classic 305/TH350C SWB 2.73 gears (Not 3.73)

So the other day, my truck started acting up at idle and I found a fuel leak from the carb to the manifold. I removed the stock Q-Jet, got a rebuild kit and float and took the carb to a reputable Q-Jet mechanic to have it rebuilt. I got it back the next day and my truck ran great.

Well yesterday, all of the sudden, my truck started acting up again and I noticed another fuel leak in the same spot. So I removed the carb and took it back to the mechanic. He said there was a little trash in it and he needed lower the float level. I replaced the fuel filter in the carb and added a 3/8" inline as well. The sending unit and strainer are new as well.

I put the carb back on my truck and it is running great again. Here is the problem, my lock-up torque converter is no longer locking up. I did notice that I broke off one of the small nipples that runs from the temp switch that screws into the thermostat housing.

So I very carefully put some epoxy on the nipple so I could properly hook back up the vacuum line that goes to it. It is the top nipple and is one of five that go to it. I held the nipple in place as I put epoxy around it it so as to not get epoxy inside the nipple and prevent it from working properly.

I double checked all vacuum lines to the carb and inspected all of the "t's" and check valves. Everything appears to be hooked up correctly and I see no problems or other breaks in the lines.

Anyone have any ideas of what I could have missed or messed up to keep the lock-up from working? I have done a search and come across a few threads with some info. Nothing real definitive though. I am 100% sure it is a lock-up TH350. Before the last carb R&R, and when driving down the road at 55 or so, I could lightly tap the breaks and the lock-up would disengage, so I know it works. It is only after the carb R&R that it has stopped working.

The vacuum line that ran to the nipple, that I had to reattach, "t's" off in 2 places and one of the vacuum lines runs to a switch mounted to the firewall directly behind the passenger side valve cover. This vacuum line attaches to the switch and the switch has two wires running from it to somewhere. I know the converter locks up via an electrical signal, but what sends that signal to the trans?

If it were RPM signal (which I don't see how it could be), I would think there would be a wire attached to the tach side of the HEI distributor that runs or connects to the trans. Maybe it is a vacuum signal to the above mentioned switch that gets signal from the thermostat housing vacuum port and that switch sends an electrical signal to the trans that in turns activates the lock-up switch on the trans?

I'll keep doing some searching but would appreciate any info or input you all might have for me.

Looks like my attempts not to be long winded failed. Thanks!

[Maxzillian]

This may help you out as far as wiring goes:



The basic components are the brake switch, a vacuum switch that is plumbed into a thermovalve and used to prevent the lock up from occurring when the engine is cold, and a vacuum switch used to disengage lock up if engine vacuum is higher than 5 inches.

I'd personally start by jumpering one and then the other vacuum switch to determine which side of the circuit is causing problems. The transmission prevents the torque converter from locking unless you're in third gear so no damage should result from this test.

[1425956]

Awesome..Thanks...I ran across this a few minutes ago as well. I think between this diagram, and the original post that came along with it that is very descriptive, I'll get it figured out. I will check it in the a.m. and post my findings. Thanks again.

[1425956]

I think I may have this lock up issue figured out, or at least part of it. I noticed my lock up works perfectly in the afternoons only. Rarely does it work in the morning. So, considering there is a thermo-valve (ported vacuum switch) in the lock up system, and considering my truck sat for 9 years without being started, I am betting my thermostat is bad which is not allowing my engine temp to reach the required temp to open the thermo-valve and send vacuum to switch that kicks in the charge of fluid to lock up the converter.

It is warm enough in the afternoon right now to where my temp will read just a little above the first mark on the gauge. It is still just a little too cool in the mornings for my engine to get very warm...Which leads me back to the thermostat. I am going to install a new 195 degree thermostat tomorrow and I will post my findings.

What do you think? Way off base? Or am I on to something??? It seems logical to me....

[Maxzillian]

Seems about right. If you want to completely verify that it has something to do with the temperature side of the circuit, jumper the connector that goes to the passenger side vacuum switch on the firewall. If the lock up begins to work after that point, you're on the right track.

[1425956]

Is this the switch that is mounted almost directly behind and above the passenger side valve cover and has one vacuum line and two wires going to/from it?

[Maxzillian]

I believe so. It should have one green and one yellow wire.

[1425956]

Well, turns out it WAS a defective thermostat. And there is good reason why.....I have all of the paperwork from when the truck was new, in addition to every receipt for parts and repair from the stealer-ship.

As I was combing through all of the receipts and the maintenance schedule book, the one thing I noticed was the thermostat had never been changed! After removing the defective thermostat, I saw there there was "AC Delco" stamped on the bottom of it along with the OEM part number.

I installed a new thermostat and gasket, cleaned off all of the original gasket on the water outlet and intake surfaces, and bolted everything back up.

Took it for a test drive and as soon as the engine temp got right around 195 degrees, the lock up kicked in and worked perfectly!