TH400 speedo/pcm question
TH400 speedo/pcm question https://ls1tech.com/forums/showthrea...65#post6592665
Its LT1 specific, so I probably should've posted it here in the first place... Thanks in advance guys
Its LT1 specific, so I probably should've posted it here in the first place... Thanks in advance guys
Row, the part numbers are listed in the TH400 FAQ which is stickied in the Transmission section... Alot of it is LS1 specific, but the part numbers for the 3L80 trans parts are the same no matter what F-body you put it in.
dhdenney- Turn off VSS output for A4? Why would you want to do that? My understanding of the wiring says that the pcm will see the VSS no matter what, as its directly wired to the pcm. The only VSS output I see is for the speedo. So I dont see why you'd want to turn that output off.
I was told by someone that the M6 cars needed the VSS to tell the pcm to switch from idle tables to load tables... Do A4 cars use the same thing or is there something with the wiring that tells the pcm you've switched from park/neutral to a drive gear?
I have two extra wires after swapping in the Pro Ratchet shifter and hooking up the park/neutral safety switch and the reverse light wiring. Im thinking these wires are the key here, but my Haynes wiring diagram is less than specific as to what these wires do.
These are the wires the M6 swap guys connect together for the hatch release to work. Im wondering if those are what tells the pcm that you've taken the shifter out of park or neutral. And would twisting them together make the pcm think it was still in park/neutral?
dhdenney- Turn off VSS output for A4? Why would you want to do that? My understanding of the wiring says that the pcm will see the VSS no matter what, as its directly wired to the pcm. The only VSS output I see is for the speedo. So I dont see why you'd want to turn that output off.
I was told by someone that the M6 cars needed the VSS to tell the pcm to switch from idle tables to load tables... Do A4 cars use the same thing or is there something with the wiring that tells the pcm you've switched from park/neutral to a drive gear?
I have two extra wires after swapping in the Pro Ratchet shifter and hooking up the park/neutral safety switch and the reverse light wiring. Im thinking these wires are the key here, but my Haynes wiring diagram is less than specific as to what these wires do.
These are the wires the M6 swap guys connect together for the hatch release to work. Im wondering if those are what tells the pcm that you've taken the shifter out of park or neutral. And would twisting them together make the pcm think it was still in park/neutral?
Guys, Im done with the thread in the link above... Im going ahead with installing my VSS parts and un-doing the WSS wiring Im currently using. To get the speedo to calibrate correctly Im loading the A4 base file and copying my tables to it.
Now I just need to know how the pcm knows when to switch out of the idle tables...
Now I just need to know how the pcm knows when to switch out of the idle tables...
I really need to figure this stuff out also. I swaped in a th350 3 years ago to an M6 car, and have driven with no speedo ever since. I am running a pro ratchet shifter also.
I remember when I first swaped in the trans that the car would start up and idle fine, but after going WOT the car would not idle and die. The check engine light would come on, but after restarting the car it would go off and the car idled fine again. I just went in and figured out what codes the PCM was throwing, turned them off and the car never had any problems idling after that.
I ran the reverse light wires to the shifter, and that worked out good. I also tryed to run the netural safty switch wires to the shifter, but that didnt work. The car will start in any gear.
I remember when I first swaped in the trans that the car would start up and idle fine, but after going WOT the car would not idle and die. The check engine light would come on, but after restarting the car it would go off and the car idled fine again. I just went in and figured out what codes the PCM was throwing, turned them off and the car never had any problems idling after that.
I ran the reverse light wires to the shifter, and that worked out good. I also tryed to run the netural safty switch wires to the shifter, but that didnt work. The car will start in any gear.
The VSS parts are simple to install... Different part #'s for a th350 though. They say that with a original M6 pcm you need to multiply your tire diameter by 2 or 2.5 due to a difference in tooth count on the reluctor.
With A4 pcm's adjust for tire size and gear ratio and your done.
The switch's with the Pro Ratchet will handle the reverse lights fine, but the park/neutral safety needs to be done with a relay because the current through those wires will melt/short circuit the little switch sent by B&M. Basically make the switch on the shifter activate the relay.
I believe that the two extra wires that get twisted together ARE used by the pcm to determine whether you are in park/neutral or not... Since the hatch wont open without those two wires twisted together; I assume that they need to be connected when in park/neutral and disconnected in any other gear. All the wiring diagrams I have show the PRND23 switch thats on the factory shifter, but dont show the 'diagram' for the switch itself.
The black wire is a ground, and the orange wire comes from the pcm. Neither is physically wired to the hatch release switch, but may be connected somehow through the pcm. Again all my diagrams tell me is that that wire is an 'output' to the PRND selector switch.
Can anyone confirm my assumption? That these wires are also how an A4 pcm knows that you've shifted the transmission out of park or neutral.
With A4 pcm's adjust for tire size and gear ratio and your done.
The switch's with the Pro Ratchet will handle the reverse lights fine, but the park/neutral safety needs to be done with a relay because the current through those wires will melt/short circuit the little switch sent by B&M. Basically make the switch on the shifter activate the relay.
I believe that the two extra wires that get twisted together ARE used by the pcm to determine whether you are in park/neutral or not... Since the hatch wont open without those two wires twisted together; I assume that they need to be connected when in park/neutral and disconnected in any other gear. All the wiring diagrams I have show the PRND23 switch thats on the factory shifter, but dont show the 'diagram' for the switch itself.
The black wire is a ground, and the orange wire comes from the pcm. Neither is physically wired to the hatch release switch, but may be connected somehow through the pcm. Again all my diagrams tell me is that that wire is an 'output' to the PRND selector switch.
Can anyone confirm my assumption? That these wires are also how an A4 pcm knows that you've shifted the transmission out of park or neutral.
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Originally Posted by Fire67
dhdenney- Turn off VSS output for A4? Why would you want to do that? My understanding of the wiring says that the pcm will see the VSS no matter what, as its directly wired to the pcm. The only VSS output I see is for the speedo. So I dont see why you'd want to turn that output off.
Perhaps I misunderstood the question. The way I read it you didn't want to run a speedo. I didn't exactly read really close either. I was told the 4l60e reluctor and speed sensor did the trick for speedo.
Last edited by dhdenney; Mar 7, 2007 at 02:50 PM.
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Verdad Gallardo Nope, I want the speedo working for sure.
I was originally trying to figure out if I would need to install my 3L80 VSS parts to be able to dyno the car. As right now, it get a speed signal from the front wheel speed sensor. This of course would only present a problem on the dyno, where the front wheel will not be turning to give a speed signal.
Since no-one could answer that, Ive decided to go ahead with installing the VSS stuff into my TH400 to prevent any issues.
Being on this topic has brought up a whole new thought process...
How does the pcm know when you take the trans out of park or neutral? Im thinking its the two 'extra' wires that go to the PRND321 switch on the stock shifter. There are 6 wires; 2 are for the Park/neutral switch, 2 are for the reverse lights, and the 2 'extra'.
I know the hatch release button will not work untill these 'extra' wires are connected... Since the logic of the factory switch would connect these wires in park and neutral only, my best guess is that these wires are also for the pcm to know whether or not the trans is in gear. Just need confirmation or denial of my theory.
TIA guys
I was originally trying to figure out if I would need to install my 3L80 VSS parts to be able to dyno the car. As right now, it get a speed signal from the front wheel speed sensor. This of course would only present a problem on the dyno, where the front wheel will not be turning to give a speed signal.
Since no-one could answer that, Ive decided to go ahead with installing the VSS stuff into my TH400 to prevent any issues.
Being on this topic has brought up a whole new thought process...
How does the pcm know when you take the trans out of park or neutral? Im thinking its the two 'extra' wires that go to the PRND321 switch on the stock shifter. There are 6 wires; 2 are for the Park/neutral switch, 2 are for the reverse lights, and the 2 'extra'.
I know the hatch release button will not work untill these 'extra' wires are connected... Since the logic of the factory switch would connect these wires in park and neutral only, my best guess is that these wires are also for the pcm to know whether or not the trans is in gear. Just need confirmation or denial of my theory.
TIA guys
Originally Posted by Fire67
Being on this topic has brought up a whole new thought process...
How does the pcm know when you take the trans out of park or neutral? Im thinking its the two 'extra' wires that go to the PRND321 switch on the stock shifter. There are 6 wires; 2 are for the Park/neutral switch, 2 are for the reverse lights, and the 2 'extra'.
How does the pcm know when you take the trans out of park or neutral? Im thinking its the two 'extra' wires that go to the PRND321 switch on the stock shifter. There are 6 wires; 2 are for the Park/neutral switch, 2 are for the reverse lights, and the 2 'extra'.
Originally Posted by dhdenney
Yeah I think that mess of wires is where you would wire your backup lights, neutral safety sitch, etc onto your new shifter.
The new shifter has 2 switches. One for reverse lights, one for park/neutral safety. I have two more wires... Do they need to be switched so the PCM knows whether its in park/neutral or a drive gear.
Only question I need answered is how the pcm 'knows' the shifters position?
Yeah I only read half your post. You're really taking me to school here. 
The way I read it in my Chilton manual, the car will only start in P and N once the respective switches are wired in properly. I assume that those switches also "tell the PCM" which gear you are in and anything not engaging the switches is a drive gear. I looked for more about the wiring but it didn't have anything too advanced.
The way I read it in my Chilton manual, the car will only start in P and N once the respective switches are wired in properly. I assume that those switches also "tell the PCM" which gear you are in and anything not engaging the switches is a drive gear. I looked for more about the wiring but it didn't have anything too advanced. Its cool. I hope someone with more knowledge of this than us will chime in here. I spent a good deal of hours last night with my face buried in wiring diagrams... The park/neutral safety wiring does not pass through the pcm, its actually the power supply for the starter solenoid. (hence why those two wires are huge)
The reverse light wiring does not seem to go through the pcm either (but I really didnt look at that wiring as in depth as I did the others).
The two 'extra' wires for the hatch release are the only ones going through the pcm... Unless someone else can chime in, Im going to try and borrow a buddy's MATCO scan tool and do some 'experimenting' with those two wires.
The reverse light wiring does not seem to go through the pcm either (but I really didnt look at that wiring as in depth as I did the others).
The two 'extra' wires for the hatch release are the only ones going through the pcm... Unless someone else can chime in, Im going to try and borrow a buddy's MATCO scan tool and do some 'experimenting' with those two wires.
Ahh I figured the safety switches went through the PCM. So anyway, I've not been much help here but take the following for what you will. I used a 700r4 in a factory M6 car with no codes and proper speedo. I didn't wire up anything other than the VSS. I had no backup lights, no park/neutral safety switch, but I had a speedo and the car ran fine.
Yeah that worked because M6 cars use the VSS to tell the pcm when to switch out of idle tables and into load tables. I bet you noticed weird idle characteristics (probably a stumble) when you would put the shifter into gear from neutral or park as well. (At least I did, which was my reason to go back to an A4 file from the M6 file I had.)
On the same token, when I originally swapped from the stock A4 to a M6, I simply deleted the A4 codes and doctored the speedo calibration... It worked well, but the pcm never went into any idle tables. Staying in load tables caused a hanging idle when slowing down from whatever speed to a stop. Basically the pcm would correct with the IAC but it was a SLOOOW correction. And changing from an A4 base file to an M6 base file solved that problem.
I guess I'm gonna have to wait untill my new Fuel pump gets here so I can start the car and connect/disconnect those two wires via a toggle switch to try and determine what I need to do.
BTW, dhdenney - I like the avatar! Ive got a pic of my fiance in a similar pose, but she wont let me put it on the net
On the same token, when I originally swapped from the stock A4 to a M6, I simply deleted the A4 codes and doctored the speedo calibration... It worked well, but the pcm never went into any idle tables. Staying in load tables caused a hanging idle when slowing down from whatever speed to a stop. Basically the pcm would correct with the IAC but it was a SLOOOW correction. And changing from an A4 base file to an M6 base file solved that problem.
I guess I'm gonna have to wait untill my new Fuel pump gets here so I can start the car and connect/disconnect those two wires via a toggle switch to try and determine what I need to do.
BTW, dhdenney - I like the avatar! Ive got a pic of my fiance in a similar pose, but she wont let me put it on the net
Haha. Actually there was never any sort of stumble. Of course every car is different and each person can tolerate different things. I had 0 issues with using the M6 file with the 700r4.
Originally Posted by dhdenney
Of course every car is different and each person can tolerate different things...
These OBD2 pcm's seem to get involved in alot more things than the older ones did. I'll experiment some, and report back when I figure it out.
Ok, on your car I dont know how it would work still. But I got my question answered over in the PCM section.
My 2 'extra' wires from the factory shifter postion switch ARE the way the PCM knows that wheather shifter is in Park/Neutral or any other position. They need to connect when the shifter is in park or neutral, disconnect when in any drive gear. On both 96-97 cars the wires are orange and black. I have wiring diagrams for the OBD1 cars as well, so I'll see if I can find anything similar in the diagrams for your car
The response I got in the other section also said that the pcm uses these wires along with VSS and other inputs to determine the correct load on the engine. So this confirms all my suspicions about the weird issues Ive dealt with swapping tranny's in and out of this thing.
Obviously an original M6 car swapped to an A4-A3would not have these wires, therefore 'should' have problems with engaging a drive gear due to the pcm not being 'told' that there is a load being applied via the TQ converter. This could be solved by installing an A4 specific harness and PCM file.
On the flipside, someone swapping from an A4 to an M6 can just permanently connect these wires so that the hatch release still works. Then install a M6 PCM file for the correct load tables and hardware usage.
Hope this helps someone besides me!
My 2 'extra' wires from the factory shifter postion switch ARE the way the PCM knows that wheather shifter is in Park/Neutral or any other position. They need to connect when the shifter is in park or neutral, disconnect when in any drive gear. On both 96-97 cars the wires are orange and black. I have wiring diagrams for the OBD1 cars as well, so I'll see if I can find anything similar in the diagrams for your car
The response I got in the other section also said that the pcm uses these wires along with VSS and other inputs to determine the correct load on the engine. So this confirms all my suspicions about the weird issues Ive dealt with swapping tranny's in and out of this thing.
Obviously an original M6 car swapped to an A4-A3would not have these wires, therefore 'should' have problems with engaging a drive gear due to the pcm not being 'told' that there is a load being applied via the TQ converter. This could be solved by installing an A4 specific harness and PCM file.
On the flipside, someone swapping from an A4 to an M6 can just permanently connect these wires so that the hatch release still works. Then install a M6 PCM file for the correct load tables and hardware usage.
Hope this helps someone besides me!
Interesting. I would like to know myself why nothing was affected in my swap. My current project involves an LS1 and a TH400 so I might in fact encounter this problem. I plan on doing a custom Speartech harness so it's possible that we can work around this.
