Automatic to manual conversion?

This is more of a curiosity than a need! THEORYS ARE WELCOME! Don't bother to ask why or mention that "this is impossible", this is just a plea for greater information. ("Impossible" and "hard to do" are not synonyms in my vocabulary. "Waste of time" is merely an opinion. Feel free to spout negative opinions, I'm not here looking for affirmation, just information. I believe these machines were built by humans and quite obviously can be modified by humans!)
I have built many old cars. With the old stuff, Auto to manual conversion was usually just a matter of abandoning some electrical circuits, IE: Tq converter lock up switch {no Tq converter on a manual}, moving neutral start switch from the shifter to the clutch pedal.
Lonny has probably already discovered how this might be done? If that investigation is available somewhere I would love to see that!
I have a(cadaver) engine wiring harness and fuse block out of an 07 auto and noticed that an AUTOMATIC used a TCM and a ECM, a MANUAL has only a ECM. But that on both systems, only the ECM communicated into the greater wiring harness.(Holy Hot Wires, Batman!, A. B. Normal may be on to something!??)
I have not delved into the body\dash harness but feel fairly sure the only switching differences are a clutch in\out switch and outputs for auto shift display and reverse lights. There may be other systems involved such as cruise control, display and diagnostic?
ANY input is appreciated! Sometimes naysayers are unknowingly holding the keys to success!
I believe every Goblin built so far has abandoned the Cobalt's ventilation system, wipers, interior lights, Electric windows/door locks/trunk release, Audio/On Star, SRS and ABS systems. Maybe my actual question is, "what other systems I can abandon?"


Goblin Guru
Anything is possible with enough money!

It could be done simple by adding a clutch pedal and MC and running a standalone ECU and fuel management system.

Would be worth a shot just to remove the TCM and moving the park/neutral sensor to the clutch pedal like you mentioned. I don’t see why it wouldn’t work. But you’d want to pull the wiring diagram for the donor and verify the GM high speed LAN circuit remains complete. If it goes from the ECM to the TCM and then back to the ECM, just complete the circuit at the ECM. If it goes out of the TCM back to the BCM, then complete the circuit from the BCM to the TCM directly.


Goblin Guru
On some of the newer GM stuff, it's not as easy as saying the ECM is handling the engine and the TCM is handling the transmission, the two have to be programmed to work together. You also can't count on the pinouts for ECM with a manual is the same as ECM/auto. I don't know how much of this is true with the Cobalts, and there are possible differences for the various ECM used in different models. There are aftermarket auto controllers that can be made to work to make the transmissions stand alone.

The HP Tuners LS V8 Gen3 & Gen 4 forums are covered up with people that thought the switch is as easy stick them in, rewire a few pins and you are good to go. Most of the time it's much harder than that. The 4T45 transmission may be old enough that it isn't as bad, but you need to investigate before you start. There may be some people on here that have the info you need, but I would search the Cobalt and HP Tuners forums also.
To clarify: Auto to manual. (I imagine the reverse is a REAL nightmare)
To quote Poppa Robertson on Duck Dynasty, "Happy, Happy, Happy!" Thank you both for the quick and thoughtful responses! Thank You for your time! This forum is a miracle! Being interested and curious is exactly how mysteries get solved! The more INDIVIDUAL brains and their experiences we/I can bring to bear toward this goal, the more likely it will get solved!
Cobalt Forums is an excellent suggestion.(Play Of The Day!)
On a side note, I have installed a resistor in anti theft key systems(following the directions of Real computer people!) in order to make a "security system" cooperate. Avoiding spending $$$ on keys and stuff to make a 20 year old truck functional again. Scrapping a one ton truck with 30,000 miles on it because the "theft computer" wouldn't let it start and no one at the dealership had the ability to fix it, is a travesty and a crime in my book. (My long way of saying I hate to lose.)
As I mentioned in my post, "HEADS WILL ROLL!", err, sorry!, Modules will be bypassed!

{{{{{Warning, babble ahead, feel free to skip!... Every Goblin builder bypassed their SRS and ABS modules. As I learn more about the LAN(LocalAreaNetwork), the modules are wired "in series" meaning they are a long chain. From what I have read, any link(control module) in the "chain" can be removed so long as the "chain" is reconnected. This is obviously an over-simplification but the basics of LAN function. I suspect LAN is at the root of halting many modifications. I'd bet broken/shorted LAN wiring is more often the culprit than bad computers. I've seen many cars for sale claiming it needs a new BCM! I accidentally fixed a bunch of problems in a wrecked 200? Regal, I saw and fixed a bunch of pinched/ rubbed through wires. I soldered them back together and insulated them, then I found other problems like power windows were fixed as well?! I never bothered to discover what I had done, just happy this car ran and functioned! As I learn more about LAN and BCM's controlling accessories, many things are becoming simpler or more logical. My wife claims I am a stone ax caveman in this 21st century silicon chip world.}}}}}} Now, back to our Show!

But I'm getting ahead of myself. I am going to have to dig deeper into my wiring harness to discover where the individual wires in that connector from the ECM terminate. I am in an odd(and possibly enviable) position in that I have months of free afternoons to delve into this and other seemingly worthless problems(What ever happened to the beige M&M?, flat beer?, soggy nachos?, general layer cake dis-satisfaction?, Does Bigfoot pay any taxes?(I think he does!, if not the IRS would have found him!), What exactly is a tomatillo? a Fruit? a Vegetable? None of the above?) without suffering adverse effects. I won't even be near my victim, errr uhh I mean, my car until February!
I feel sure I've only just seen "the idea of the concept of the shadow of the iceberg". FULL SPEED AHEAD!!
MODERATOR: IF this "rabbit hole" needs to be somewhere else, feel free to relocate it.


Goblin Guru
Sorry, all of your talk about the auto trans having a separate control module threw me off, Auto to manual might be easier, but a lot of what I said will still be true since there are probably portions of an ECM tune that are looking for information from the TCM. Depending on whether the auto/manual shared the same basic ECM, you might could have the operating system swapped.
All of this will still vary with the exact details of the ECM and what information it is needing.


Goblin Guru
Standalone automatic transmission controllers have been used by Goblin builders already, so I'm reasonably sure that we can treat the ECM and TCM as separate entities and not concern ourselves with any interplay.
GT, What you write is spot on! Lots more research for me in the offing. I'm just hoping to start where others have left off, if I can. Tuners and makers of stand alone ECM and TCM systems must make their product interface with the car's standard systems. I don't want to re-do their laborious trek to discovery.
I'm not saying this is simple, but as problems go, it does have a fixed number of variables. If the easiest way is to swap all wiring harnesses and computers from a manual donor, then that is one way.
I have assembled stock automobiles and a few race cars from multiple wrecks, reconstructed wiring harnesses from the remains.(A 1980 DeTomaso, Pantera, several Chevys and Mopars) In my High School years I helped a Frat at Georgia Tech build their wreck for Homecoming, 79,80. My sister was a student there, Textiles. The Frat guys didn't know I was a high school kid, they thought I was a grad student?, I did not disabuse them of that notion.
Your solution is to have the ECM aftermarket(no TCM in Man Trans) "stand alone" and interfacing with the existing car. This is likely not as cheap a solution as I would like, but it IS a good option!, you've narrowed my aim. MORE RESEARCH!
I do not have emissions to worry about, so that's a plus.
My Son says I'm still Fishin', just not Catchin' Yet!
K! An excellent bit of info! Multo multo Gratzi! I've read your build, I think.
I am prone to wordy-ness, and it is easy to lose my trail. So, let me clarify: Auto TO Manual. But your input IS useful and fills in a few blanks. Also glance at my sig, I'm not exactly on the Goblin page with y'all. But DF is filling in many mystery parts of my build! FREE THE SCIENCE! Their "Toe End Link" that replaces the steering rack in the rear was what brought me here. My finished Cobalt will have the engine and trans on a subframe mounted in the rear with a "dummy" subframe in the front. A'La Porsche, Corvair, Fiat X1/9, Fiero, Renault R5 Turbo, Toyota MR2, Roadkill Garage's Twinpala and Maserati's Bora. To me this is old ground, now it's my turn! Trying to keep it under $4K. Let me get it driving first, Then I'll TURBO IT!
I'm in Yellowstone just now, headed east in a week. The Gulf of Mexico by Christmas, 2 months in GA with my victim and I'll see the Atlantic by fall. Then??
Thank You very much for your interest, time and sharing your experience/knowledge. (I feel sure some will claim this is impossible but I'll build it anyway.)
You're up towards Mt. Vernon. I'll be in touch if we get up that way again! Prolly '24.
GT, I'll try to be in touch in the Spring! I would like to see your goblin. I saw a couple driving somewhere? in Red Oak, TX; 2 years ago. No! I didn't get a chance to drop in, Darn it!


Goblin Guru
Standalone automatic transmission controllers have been used by Goblin builders already, so I'm reasonably sure that we can treat the ECM and TCM as separate entities and not concern ourselves with any interplay.
I’m not sure that is true. The aftermarket TCM can possibly replicate the data the ECM needs to run.


Goblin Guru
I’m not sure that is true. The aftermarket TCM can possibly replicate the data the ECM needs to run.
I have my doubts, but I'm no electrical engineer; I studied vehicle design lol

If the ECM doesn't rely on the TCM, then we can simply remove the TCM (my auto had the wiring/connector for the clutch switch already, and I know some manuals have the automatic transmission shifter plug in their harnesses, so there may not be need to add/repurpose circuits), add a clutch pedal and master cylinder, swap brake fluid reservoirs (or use an aftermarket one), run lines, and the transmission itself.

Lively discussion guys! Keep it up! But I think you're actually headed 180 in the other direction, away from my goals. Auto car converted to manual.
I do KNOW this though, the deeper you dig into this general question the greater your/my/our combined understanding of these systems will become, and that is a GOOD thing.
WARNING: More of my history, skip if desired! I spent the month of May helping a friend put 1991 Chevy 5.7 w/ GM TPI in a 1980 car. {We had swapped an entire TPI system from an 87 C10 into a 77 Nova, but in that application we used the C10 Donor's entire fuse box.} This latest time we started whittling down the system wiring and made it virtually stand alone, using the original computer, sensors and distributor. We learned a lot about the care and feeding of those older computers and the fragility of the TBI it self!
So, a 2008 Cobalt system is just my next research project. The engine/ trans harness in the automatic Cobalt is built/wrapped together. BUT just because they are wrapped together does not mean they intermingle!(they probably do, sigh. To build lots of cars quickly, sometimes things that are near by are built into sub assemblies for manufacturing convenience and ease of installation.) The harness for a manual Cobalt is missing the entire pigtail that would go to a TCM(and of course has no TCM box at all). I guess I'm going to have to get a cadaver harness for a manual Cobalt and rip into them both to compare and evaluate their differences.
But first I'm going to open up the cadaver harness I have and see if I can easily physically separate the Big white auto trans control plug and the Shifter quadrant plug and the bulk connector(to the TCM) and any other transmission related plugs(except the speedo) from "the engine harness". I strongly suspect that the ECM and the TCM are sharing some sensor data, I bet I'll find LAN wires jumping between those two! it is time for me to test and quantify that theory. I'm off to find out what data the TCM is "listening in on" inside the engine wiring harness bundle.(the big black connector that installs under the fuse panel)
Actually this is a job for Monday. Tomorrow and Sunday I have to go hiking in Grand Tetons Natl' Park. Bummer, right?
"Auto car converted to manual."
After I re-read my sentence it dawned on me that DF could benefit from this info. If other models(Chevy Cruise?) are to be used as future donors, I feel sure they will need to address this situation.
K! You found an excess plug, a clutch safety start switch plug, buried in your dash harness?!! That is an interesting twist! I need to get myself more cadaver wiring to experiment on. Dr. Frankenstein would be so proud of us!

Dale E

Well-Known Member
On my 3800 SC V6 wiring (automatic) there was a large yellow wire going from ignition switch to the transmission gear selector plug - (neutral safety). The purple wire from that same plug went to the starter, like the purple wire on the Goblin. So ignition yellow in to transmission and through neutral safety and out to purple starter. When I changed to Fiero Getrag 5 speed I hooked the purple to ignition bypassing the automatic transmission neutral safety. No clutch switch. Brake switch only for brake lights and cruise if I wanted to add that sometime.

Yours may have to be hooked up differently than this wiring.
Dale! You are a shining example of the "trouble"(ACHEVIMENT!) we all can get into, if we try!:cool: You're the Best! Is that 3.8 Buick in a goblin? With the Getrag? I've gotta see pix of this! You're the "Cadet" Kirk of Goblineers! (Does the Kobiashi Maru test ring any bells?) Keep On Dreamin' Y'all!
Well, I couldn't wait, I pulled out my box of cadaver Cobalt wiring and started pulling off the tape! I have a fuse panel with all the box connectors underneath and a 3" cut pigtail of wires hanging off each one, junk yard guys just cut the wires to get the fuse box out of their way!, and the slightly hacked up engine harness/ automatic(several other wires were hacked or broken, but I figured them out to label them). Sadly this means my engine harness had the lg black (engine harness connector)fuse panel plug cut off, LUCKILY this is all from the same unit!
There are two trans connectors, a Lg white w/ blue lock rectangle and a sm black circle type bulk electrical connectors in the "engine harness" and the one that locks to the TCM.
I found the twisted pair from the speedo pickup{purple, yellow} goes to the White w/blue plug on the transmission. Once I cut(and knotted) the speedo twisted pair at the white trans plug, that leaves only about 8-10 wires that go to the lg black fuse box plug, plus two twisted pairs(tan & tan w/blk stripe, LAN?). Three(or 4?) ground wires follow with all the other Engine Harness grounds to the block ground connector.
When I take the back cover off the Lg Black fuse panel connector I hope to find several wires obviously have only one terminal they can go to. Others may be paired on terminals and thus are only represented once in that connector, I hope. Or, sadly, that color wire may go to multiple locations making it impossible to trace without a schematic or another example harness to compare. I really hope to find the homes of many of these wires tomorrow. Which makes chasing the remaining wires that much easier!
It appears these 2 harnesses, though built together as the "Engine Harness"(for manufacturing ease) barely intermingle! (So, except for the feed from the speedo sensor, the auto trans part of the "engine harness" thus far, appears to almost stand alone.)
I'll make a list of the remaining cut wire colors, name their still connected terminations and list their possible home locations.
Well, Trouble in Paradise. Not really bad, I'm just stalled. This cut up(cadaver) Engine wiring harness w/ pieces and fuse panel I have is not all from the same year/trans car. I'm not even sure the year of any of these bits. I thought I was "dissecting", but apparently I have been doing "a forensic reconstruction" and my subject is contaminated.(I draw the line at true vivisection!)
I have currently narrowed my research to 07-08. Staying away from VVT, DI. But this was all junk, dirt cheap, prolly worth more than I paid, just for the misc plug ends I'll have at my disposal.
We move in a week to Durango. Much bigger town, likely to have a junkyard. Or a yard in the big city of Denver may sell and mail me some used pieces.(We were living in Denver working for a mining company when the pandemic sent her job to full online work.)
I wasn't too careful about requesting exact year parts as I had no idea about breadth interchange. 07 and 08 appear to have very few variances, especially the dash harness. Or maybe Ebay/Amazon? Does anyone have all the wiring from a 7/8 they'd like to sell and ship to me?