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Stats, 08 GMC 5.3 4l65e 4x4. Transmission has been converted to 2wd by way of output shaft change. OS system is a e38 ECM t42 TCM. Wiring harness was modified by me using lt1.com. This is in my 65 Nova restomod.
I’m having an issue with the TCC circuit 9 on the ECM. With KO ignition I’m getting 9.5 volts. With the engine running its around 10.5 to 11. I’ve hooked it directly to the brake switch white wire NO trying to use the brake light filaments as resistors to ground it. I’ve tried a 1k 1/4 watt resistor to ground it. In both situations with the brakes applied the voltage will drop to -3.5 volts. I’ve read multiple posts on this but no one has ever concluded a solution. Has anyone here been able to fix this?
Thx for the reply’s.
I guess what I’m asking is this circuit needs to see ground until the brakes are applied. Should I be looking for zero volts? When it’s grounded or does it just need a ground seems like the volts feeding from the circuit isn’t a good thing.
It appears that I have this incorrect below for Gen IV. I will leave it, as it's correct for Gen III, if someone is researching.
Usually, its the opposite. A factory ECM wants to see 12v on the TCC line (NC switch) and then when you press the brakes, the switch opens and the line drops to zero. (Note that an aftermarket ECU like the Holley's have opposite logic of this).
Most 80s GM cars with a 700R4 should have the correct brake switch. It will have 2 sets of contacts. 1 set is for your normal brake lights wiring. The other set is NC, you will feed a switched battery (hot in start and run) to one terminal, and then the other is the TCC line to the trans. In my 1972 GMC, I used a brake switch from a mid 80s Monte Carlo (I think I had to spec with cruise control or something).
For those wondering why, the OEMs wire it this way for safety. The torque converter can only lock up when it sees 12 volts. So if there is a blown fuse, or a broken wire, the 12V goes away and lock up is prevented. In a Holley system, it *unlocks* when it sees 12 volts (and yeah, also based on gear and other factors). So if the wire isn't connected, it won't unlock when they hit the brakes.
GenIV stuff is the opposite of the older GenIII , so they only need 12v when the brakes are applied. So the OP has it wired right.
To the OP, you should get a ground through the brake lights unless you are using LED brake lights. Are you having an issue? Or just checking for the sake of checking it?
GenIV stuff is the opposite of the older GenIII , so they only need 12v when the brakes are applied. So the OP has it wired right.
To the OP, you should get a ground through the brake lights unless you are using LED brake lights. Are you having an issue? Or just checking for the sake of checking it?
Thanks for the heads up. I edited the post above to reduce any confusion...
Yes I’m experiencing problems with my transmission. It will not shift normally as though it’s not communicating with the ECM. It acts like some have described as being in limp mode. It will drive but it’s 3rd gear only it will down shift manually to 2nd. It will not run up through the gears or down on its own. I have been through the wiring several times to check continuity, grounding and power, it has it all! I’m trying to eliminate possible causes one at a time. I had read somewhere that a non functional TCC switch could cause this. So I started there but I’m not understanding the whole grounding thing, so that’s the reason for the voltage tests. My electrical schematics call for the 6311 circuit to tie together from the ECM ,TCM it then splits and one goes to the BCM were there is a 750ohm resistor inside, the other to the brake switch. Now at the point of KO with it plugged into the brake switch with non led bulbs my volt meter registers 9.5 volts. When I press the pedal it registers minus 3.4ish. Is this normal? Now I’ve thought that maybe it could be serial wake up data circuits. Some say it’s needed some say not?
If you're only getting 2nd and 3rd gears, then it's in limp mode. Have you checked to see what codes it has? Maybe it's lost communication with the TCM? Also, it did originally have a 4l60 and not a 6l80, right?
I'm not sure your current wiring configuration, but, with everything connected, when the brake pedal is not pressed, what is the resistance measurement from TCC wire to ground?
If you're only getting 2nd and 3rd gears, then it's in limp mode. Have you checked to see what codes it has? Maybe it's lost communication with the TCM? Also, it did originally have a 4l60 and not a 6l80, right?
So long story! But originally the ECM and TCM were sent out and bench tuned which turned into a nightmare. The tuner basically did nothing he left the vats on and some other things that when you pay good money for should have been taken care of. Anyway a friend came over with his hptuners and we got it fired up but he could not load a trans file. Like it’s non existent. I then sent the ECM and TCM to another reputable tuner and it came back and it ran much better. I then put the rest of the car together and this spring took it for a ride finally after 10 years of building. This is when I noticed it wouldn’t shift. It is the matching 4l60 the only thing is I had it converted to 2wd by changing the out put shaft when I had it rebuilt. I have not checked trouble codes for transmission but I got a crank sensor code after driving it and changed the crank trigger switch.
I'm not sure your current wiring configuration, but, with everything connected, when the brake pedal is not pressed, what is the resistance measurement from TCC wire to ground?
The ohms are about 25 ish is that what you’re meaning by resistance? That’s no key on.
So I repined the accessory wake up serial data. Pin 18 on the X1 connector ECM and pin 11on the TCM connector tied them both into 12 volt keyed. Started it up and almost instantly got a check engine light. Checked the code and it is U0101 lost communication with TCM. This is the first time I’ve been able to pull any info on the tranny and it’s just on my hand held scanner. After reading possible causes I’m not sure if I have the can bus wired right. Should it loop together before it goes to the data link connector?
Any help would be much appreciated!
Is my assumption of the abrupt silence to this thread an indication that I may be on the right track?
Please I’m desperate! I know there’s some super intelligent people on here that could help me sort this out and in the process help others.
That's a very clean install! I can imagine your frustration not being able to drive it and wish I could be more help, but trying to diagnose things over the internet can be hard. That said, you may have to go back to the basics and look over everything you have done. I would start with the wiring harness and make sure you don't have a broken wire somewhere, which will require you to start doing continuity tests between the ECM, TCM and the transmission connector. Also check the wires for continuity to ground. I once had a wire rub through and shorted the 3-2 shift solenoid wire intermittently and it would throw the transmission into limp mode, start in 2nd and only shift to 3rd. Took a while to figure out what was happening but finally found it was shorting to ground. Ran a new wire from the PCM to the trans and it was fixed.
Have you double checked that you actually have 12v ignition power to the transmission and the TCM?
Thanks for the reply anything at this point is helpful. I’ve checked and rechecked wiring and power and grounds ,continuity. I’ve started with the TCC wire to make sure this is working correctly.
Now I’ve been at this build for some time. So long that my first wiring schematic came from Brenden at lt1.com for the identical setup year make and model by chance it worked out that way. It’s been so long that the paperwork I have for this gen IV TCC switch wire he described as working like a gen iii 12volt all time until the brake pedal is depressed.
Since then he has changed his description on the web site as to how it works. Grounded till pedal is depressed than 12volt.
Ive read that if the TCC switch is not working properly it will cause a limp mode situation? True? False? Dunno?
It seems there is a lot of confusion as to how this works. Someone here has to know?
Now in my quest for an answer to this I came across some discussion on that the gen IV TCC switch wire looks for a ground. Which most people use the tail light bulbs or a 1k resistor to help pull it to 0 volts?
I’ve tried this, neither approach works for me.
It starts at 9.5 volts and when the pedal is pressed it’s -3.4 volts it never pulls it to 0 volts however it’s configured? Someone here has had to deal with this. All of the threads and countless post I’ve read on the net on this never comes to a conclusion and I’m hoping someone here can help end this for me and others by solving this for whatever why mystery!
I feel as though I can’t move beyond the TCC signal until it works properly and move on to the next possibility if it doesn’t solve the problem.
Not sure it works that way? The way I comprehend it is that it needs to see a resisted ground like it would get if the BCM were there but then there are pull up resistors and pull down resistors this is we’re my knowledge stops!
Anyone?
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TCC brake switch 
