DIY: AT > MT swap transmission circuit reference and basic wiring DIY

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REFERENCE TO AT/MT TRANSMISSION CIRCUITS - '93-'94 models (v1.0)

There's a lot of scattered and incomplete info available in threads both here and on .com about how to wire an MT swap, but I have not found a unified and easily understandable reference to the circuits anywhere. This DIY is primarily meant to be a reference for the circuits and connectors in the Legend's transmission systems, with the aim of outlining the wires, connectors, and locations used for each transmission type. Circuit nomenclature is the same as in the ETM, where C### is the connector number, and relevant wires are noted by the color codes used in the manual as well. When wires are indicated as "branching", it usually means they are joined somewhere along the length of the wire, but sometimes the branch starts at a junction connector or terminal (those have been noted).

IMPORTANT NOTE: The first two posts are based on the '93/'94 ETM wiring, as I have a '94 and was primarily concerned with getting the wiring figured out for my particular car. Later I will add posts that also include the info specific for '91/'92 and '95 models as well. As for differences between 5MT and 6MT, I believe the only difference aside from year-specific connector/pin changes is the lack of a Reverse Lockout Circuit in 5MT cars, so you can ignore that section if you are researching a 5MT swap.

COLOR INDEX:

RED/WHT - red with white stripe
GRN/WHT - green with white stripe
GRN/BLK - green with black stripe
BLU/GRN - blue with green stripe
BLU/YEL - blue with yellow stripe
BLK/WHT - black with white stripe
BLK/YEL - black with yellow stripe
YEL - yellow
GRN - green
LT GRN - light green
PNK - pink
BLU - blue
WHT - white
GRY - gray
BLK - black

CONNECTOR INDEX:

ENGINE HARNESS

C102: 6/10/14-pin GRAY (plugs into MAIN HARNESS on passenger's side)

NOTE: Connector C102 varies in number of pins depending on the trim level and drivetrain of the car:
'L' AT has 6-pins; 'LS' AT and 'L' MT have 10-pins; 'LS' MT has 14-pins. The wiring for all three MT transmission
circuits run through this connector from the engine harness into the main harness.

MANUAL TRANSMISSION SUB-HARNESS

C121: 2-pin GRAY (plugs into ENGINE HARNESS) - MT ONLY (NEUTRAL POS)
C122: 2-pin GRAY (plugs into ENGINE HARNESS) - MT ONLY (BACKUP LIGHTS)
C144: 3-pin GRAY (plugs into ENGINE HARNESS) - MT ONLY (REVERSE LOCKOUT)
C247: 2-pin GRAY (plugs into Reverse Lockout Solenoid harness) - MT ONLY

MAIN WIRE HARNESS - LEFT BRANCH

C257: 20-pin GREEN (plugs into LEFT SIDE WIRE HARNESS)
C315: 22-pin BLUE (plugs into Fuse Block)
C316: 12-pin BLUE (plugs into Fuse Block)
C330: 2-pin GRAY (plugs into Clutch Interlock Switch) - MT ONLY
C331: 20-pin BLACK (junction connector)
C334: 14-pin GREEN (plugs into DASHBOARD HARNESS)
C341: 22-pin GRAY (plugs into SECURITY CONTROL UNIT)
C342: 16-pin GRAY (plugs into SECURITY CONTROL UNIT)

DASHBOARD HARNESS

C381: 20-pin BLUE (plugs into MAIN HARNESS - RIGHT BRANCH)
C506: 8-pin BLUE (plugs into Fuse Block)
C514: 22-pin GREEN (plugs into Dash Cluster)
C531: 14-pin GRAY (plugs into DASHBOARD HARNESS behind radio)

NOTE: C531 is the connector for the harness connected to the "AT Gear Selector Switch", which is a multi-function switch
built into the AT gear selector. This connector (the female side) and a portion of its harness is what you would typically
use in an AT > MT swap to connect the various MT circuits to the existing AT wiring.

MAIN WIRE HARNESS - RIGHT BRANCH

C351: 26-pin BLUE (plugs into PCM) - AT only
C386: 26-pin GRAY (plugs into ECU/PCM)
C388: 16-pin GRAY (plugs into ECU/PCM)
C389: 12-pin GRAY (plugs into ECU/PCM)
C424: 16-pin GRAY (plugs into TCS Control Unit) - TCS ONLY
C436: 20-pin BLUE (plugs into MAIN HARNESS - RIGHT SIDE BRANCH)

REAR WIRE HARNESS

C278: 20-pin GRAY (plugs into LEFT SIDE WIRE HARNESS)

[devolution]

All but one of manual transmission (MT) circuit functions are also present in an automatic transmission (AT) model. These circuits consist of backup lights, cruise control, ignition interlock, and neutral position. The reverse lockout circuit only exists in 6MT models.

CIRCUITS PRESENT IN BOTH MT AND AT:

BACKUP LIGHTS

The backup lights circuit is a simple one, compared to most of the other ones.

In an MT, the circuit runs from fuse 13 (C316, pin 2) on the fuse block, on a YEL wire to C102 (pin 8; pin 9 if TCS) on the engine harness*, then to C122 (pin 2), and then to the backup lights switch on the transmission. Coming out of the switch, the circuit runs on a GRN/BLK wire back to C122 (pin 1), then back to C102 (pin 9; pin 10 if TCS), then to C257 (pin 3), then to C278 (pin 8), then to the backup lights, grounding on the other side of the lights at G451.

In an AT, the circuit runs from fuse 13 (C506, pin 2) on the fuse block, on a YEL wire to the AT selector switch (C531, pin 11). Coming out of the switch, the circuit runs on a GRN/BLK wire (C531, pin 10) through C334 (pin 2; pin 1 if '93), then to C257 (pin 3), then to C278 (pin 8), then to the backup lights, grounding on the other side of the lights at G451 in the trunk.

* if the car is a 6MT, this section of the circuit actually runs through the reverse lockout relay first, then continues to C102; otherwise it runs directly to C102 from the fuse block.

NEUTRAL POSITION SWITCH

The neutral position circuit indicates to the starting, security, ECU/PCM, and TCS (if present) systems when the transmission is in neutral (or park, in an AT). Like the Backup Lights circuit, the actual switch is on the transmission in an MT and in the gear selector in an AT. The routing of this circuit is considerably different between MT and AT, other than the TCS branch, which is the same in both (though the wire color is different).

In an MT, the circuit runs from the ECU (C388, pin 4) on a WHT wire to C102 (pin 10; with TCS, pin 11), then to C121 (pin 2). From C121 the circuit runs on a YEL wire to the neutral position switch located on the transmission. Coming out of the switch, the circuit runs on a GRN/BLK wire back to C121, and from there to ground location G101 on the engine harness. IF YOU HAVE TCS, between the ECU and C102 (WHT wire), the circuit also branches to the TCS control unit, running through C436 and continuing on WHT wire back to C424 (pin 9), at the TCS control unit.

In an AT, the primary function of the neutral position switch is to replicate the Clutch Interlock Switch function in an MT system (refer to the CIS description below for more info), preventing engine ignition if the AT gear selector is not in park (or neutral). This branch of the circuit runs from the starter cut relay on the fuse block (C315, pin 19) on a BLK/WHT wire through C331 to C334 (pin 14; in '93, pin 12), through the dashboard harness to C531 (pin 6) and then to the AT gear selector switch. The other leg of this circuit comes out of the AT selector switch on a BLK/YEL wire back to C531 (pin 14), back to C334 (pin 5) and then to the security control unit at C341 (pin 10).

A second branch of the neutral position circuit in an AT runs through the gauge assembly and to the PCM: from the AT selector switch (C531, pin 7) it runs on a GRN wire into the gauge cluster at C514 (pin 19) where it lights up the "N" indicator when active, then out of the cluster at C514 (pin 1) on LT GRN wire to C381 (pin 16). After C381 it runs to the PCM (C388, pin 4). IF YOU HAVE TCS, the LT GRN wire branches after C381 (before the PCM), runs through C436 and continues to C424 (pin 9) at the TCS control unit.

(CLUTCH) INTERLOCK CIRCUIT

In an MT, the clutch interlock circuit prevents the starter from engaging unless the clutch pedal is depressed, and the Clutch Interlock Switch, located on the back of the clutch pedal bracket, indicates this to the system. Similarly, in an AT the interlock circuit prevents the car from being started unless the AT selector is in park (also neutral, I think), and the switch for this is located in the AT selector switch. There's also a control unit under the dash for the AT interlock system.

In an MT, the circuit runs from the starter cut relay on the fuse block (C315, pin 19) on a BLU wire to the security control unit at C342 (pin 9). This wire also branches between the fuse block and the security control unit, running to C330 (pin 1) and then to the Clutch Interlock Switch (C422). This leg of the circuit continues from the Clutch Interlock Switch (C422) on a BLK/YEL wire back through C330 (pin 2) to the other connector of the security control unit, C341 (pin 10). (There are TWO connectors plugged into the security control unit, C341 and C342.)

In an AT, the circuit runs from the starter cut relay on the fuse block (C315, pin 19) on a BLK/WHT wire, through the junction connector at C331, to the security control unit at C342 (pin 9). The BLK/WHT wire also branches at the junction connector (C331), runs through C334 (pin 14; pin 12 if '93), through the dashboard harness to C531 (pin 6) and then to the AT selector switch. From the AT selector switch, the circuit continues on a BLK/YEL wire back to C531 (pin 14), back to C334 (pin 5; pin 4 if '93) and continues to the security control unit at C341 (pin 10).

CRUISE CONTROL

The cruise control circuit is also relatively simple. Like the neutral position circuit, the cruise circuit relies on a switch to indicate whether the car is in gear or not, and like the NP circuit, the switches are in different locations depending on the transmission type. In an MT, the switch is located on the top of the clutch pedal bracket; in an AT, it's part of the AT gear selector switch.

In an MT, one leg of the circuit runs from the clutch switch (C307, pin 1) on a PNK wire to the ECU at C389 (pin 4; there will be no wire in this position of C389 if the car is an AT). This wire also branches to the cruise control unit at C373 (pin 14). The other leg of the circuit runs from the clutch switch (C307, pin 2) on a BLK wire to ground at G301.

In an AT, one leg of the circuit runs from the AT gear selector switch (C531, pin 5) on a PNK wire to the cruise control unit at C373 (pin 14). The other leg runs from the selector switch (C531, pin 9) on a BLK wire to ground at G501.

MT-ONLY CIRCUIT:

REVERSE LOCKOUT CIRCUIT

This is the one circuit that has no matching function in an AT or 5MT setup. It's purpose is to prevent accidentally shifting (grinding is probably more accurate) into reverse instead of 6th when going faster than ~12 MPH. The circuit consists of the reverse lockout (RLO) solenoid on the transmission, a relay, and the wiring. The solenoid and its transmission harness should come installed on a 6MT transmission, and you should make sure you have it if you are buying the parts for a swap from someone (or from a salvage yard). The relay is in Relay Box C, which is located under the hood on the passenger's side of the car. Relay Box C has a divider in it, with space for two relays on the side closest to the fender (TCS and RLO) and six relays on the other side (cigarette lighter, ABS fail-safes {x2}, ceiling light, seat heater, A/C compressor clutch). All of these circuits use identical relays, so if you pull any one from this box on a parts car, it will be the correct one (assuming it's functional). The spot for the RLO relay in the relay box is the one closest to the fender (note that the diagram in the 93/94 ETM shows TCS and RLO relay positions reversed from the way they are actually installed), and while it's not strictly necessary to install it in Relay Box C when doing a swap, it's a good location to consider.

The RLO circuit has three legs. The first leg provides power for the relay and the solenoid and comes from fuse 13 on the fuse block (C316, pin 2) on a YEL wire and runs to the RLO relay (pin 1). Pin 1 and pin 2 of the RLO relay are jumpered together by another segment of YEL wire. A third segment of YEL wire runs from pin 2 to C102 (pin 8; pin 9 if TCS) on the engine harness. From C102 onward, the YEL wire becomes the backup lights circuit, providing the power to the backup lights switch on the transmission.

The second leg runs from pin 4 of the RLO relay on a RED/WHT wire to the ECU (C386, pin 17). There will be no wire in this position of C386 if the car is an AT or 5MT.

The third leg runs from pin 3 of the RLO relay on a GRN/WHT wire to C102 (pin 3; pin 8 if TCS) on the engine harness, then to C144 (pin 3). At C144, the wire color changes to YEL and runs to C247 (pin 3) and then to the RLO solenoid. From the solenoid, the circuit runs on another YEL wire back to C247 (pin 1), then changes color to GRY and runs to C144 (pin 1), then through the engine harness to ground at G101. This leg also branches from pin 3 of the relay and runs on a GRN/WHT wire to a diode located behind the right kick panel (from the factory, it's taped down to the thick harness bundle, next to a diagnostic connector). This part of the circuit continues out of the diode on a BLK wire to ground onto another BLK wire that's part of the main harness section that runs into the passenger's side door.

Here is a good explanation of the relay and also the purpose of the diode:

Originally Posted by The Crowing

Now, you 6-speeders that want your reverse lockout to work; IMO it's a good idea to do this, as it is *very* easy to accidentally go into reverse instead of 6th gear. You can train yourself into the feel, but I promise you'll still have "accidents" every now and then, and it is not a pleasant experience. Before I explain how to wire it, I have to explain how a "Normally off" relay works. You have four pins, A, B, C, and D. Pins A and B will receive constant on 12V+ power. Pin C will run to the "control" device, in this case, pin A8 on the ECU. Pin D will go to the electrical component you're trying to control, in this case, the reverse lockout solenoid. Here's a good diagram:



Now, there's a few more components to the reverse lockout solenoid, that I recommend adding in. Take a look:



See that diode coming off the pin that goes to the solenoid? That is there to control voltage spikes created when powering and un-powering a coil, as found inside the relay. Every time voltage spikes, it arcs across terminals and slowly wears them out. Think spark plug gap. The diode, however, prevents voltage from flowing in a given direction, and when wired as shown, will eliminate this problem. Is it really needed? No. Will it prolong the life of the relay? Yes.

Anyway, you'll want to mount that relay somewhere safe. A typical favorite spot is up near the ABS module. You will run two wires, through fuses to a 12V+ source (Or run one larger gauge wire and split it into two at the relay). Connect these two to the proper terminals on the relay, the switch power and the coil power. Run another wire down to the positive wire of the solenoid on the transmission. You can also try to wire in a plug, if you wish. Run the second wire of the solenoid to ground. The last wire of the relay, the ground side of the coil, goes to terminal A8 on the 93-95 ECU found here:



Yes, it has to be 93+, as the 91-92's work differently, and do not have the same wires running through the same terminals. Now your lockout should work! Above 12mph, you should be unable to shift into reverse!

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Reserved for future update ('95 version)

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Reserved for future update ('91-'92 version)

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Reserved for future expansion #1

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AN OUTLINE OF A SIMPLE APPROACH TO SWAP WIRING

This section outlines what I consider to be the simplest approach to wiring all of the MT functionality into a factory AT car, as I understand it. For those who have already done this and want to suggest corrections, please feel free, as I want this to be as accurate as possible. I can't attest to whether this is actually correct as outlined here, partly because it's not how I did it and partly because I have yet to start my car to find out what does and does not work . The steps outlined are solely based on my understanding of how the circuits work from researching the ETM, how others have done it based on what info I could find on the forums, and to some extent on what I did in my car. I chose a "custom" approach to wiring my 6MT swap because I had a 6MT donor car to pull the wiring from, and because I wanted to get as close as I could to factory MT wiring (not really that close in the end, so mine is a weird hybrid of the two). At some point, I'll post how I did mine; however, I think for most people the simplest and wisest approach is to either make your own harness using the existing AT gear selector harness or to just buy one of Alperovich's pre-made harnesses. I have one of those that I chose not to use, but it appears to be well-constructed, and I think it's well worth the money in terms of time and hassle saved.

1. I'm not going to go into any detail about how to do the mechanical stuff, how to route the wiring, or anything like that. Crowing's DIY is very good about outlining the mechanical steps to a swap; as for the wiring, I encourage you to figure out something that works for you, your skill level, and the amount of work you want to put into it. To get ideas about good approaches for routing wires and that sort of thing, ask people who have already completed a swap about what they did. I will say that I think it's well worth the time to wrap your custom wiring in the black plastic tubing stuff and electrical tape everywhere you can. You don't want "bare" wires laying around that can end up worn through to the copper or being cut or damaged where you can't see it.
   
   
2. You will need the harness and connector that is used to connect the AT gear selector to the dashboard wire harness. It's a 14-pin gray connector (C531). Cut the wires, leaving yourself as much length as you can because you can always shorten it if needed. You will also need something thin, flat and strong to use to remove the terminals from connector housing. I took a paper clip, straightened one end of it, placed it on a flat piece of metal and used a hammer to flatten the very end of it, no wider than about 1/8" but as flat as I could get it. This works really well to release the plastic catch that locks the wire terminals inside their slots in the plastic housing.
   
   
3. You can remove the wires from the following pins on C531: 1, 2, 3, 4, 8, 12, 13. None of those wires will be needed. Use the following for reference about what the remaining wires will be used for:
   

   5: PNK - cruise control
   6: BLK/WHT - clutch interlock
   7: GRN - neutral position circuit
   9: BLK - ground for neutral position circuit (and reverse lockout if you want)
   10: BLU/YEL - backup lights
   11: YEL - power for backup lights and reverse lockout relay
   14: BLK/YEL - clutch interlock

4. Run wires for the circuits using the following info as a guide (suggested wire colors are indicated in parentheses):
   
   
   1. BACKUP LIGHTS: From pin 2 (YEL wire) of the connector (C122) on the end of the harness for the backup lights switch, run one (YEL) wire to pin 11 of C531. This wire will also need to be joined to the wire running to pin 1 of the reverse lockout relay, when you get to that part, so you might want to plan ahead for that now. The power for both circuits will come FROM the fuse block, THROUGH the dashboard harness on a YEL wire to pin 11 of C531, then TO the relay and the backup lights switch. From pin 1 (GRN/BLK wire) of C122 on the backup lights harness, run one (GRN/BLK or BLU/YEL) wire to pin 10 of C531 (the continuation of the circuit to the backup lights at the rear of the car).
      
      
   2. NEUTRAL POSITION SWITCH: From pin 2 (BLU/GRN wire) of the connector (C121) on the end of the harness for the neutral position switch, run one (BLU/GRN or WHT or GRN) wire to pin 7 of C531. From pin 1 (BLU/YEL wire) of C121, run one (BLU/YEL or BLK) wire to pin 9 of C531.
      
      
   3. CLUTCH INTERLOCK: From the switch on the lower back of the clutch pedal bracket (C422) run a (BLK/WHT or BLU) wire to pin 6 of C531. From the other pin of C422, run a (BLK/YEL) wire to pin 14 of C531.
      
      
   4. CRUISE CONTROL: From the switch on the upper front of the clutch pedal bracket (C307), run a (BLK) wire from pin 1 to the nearest ground, which is bolted to the left side of the car very close to the switch. From pin 2 of C307, run a (PNK) wire to pin 5 of C531.
      
      
   5. REVERSE LOCKOUT SOLENOID: From pin 3 (YEL wire) of the connector (C144) on the end of the reverse lockout solenoid harness (at the top of the transmission, not the one at the bottom next to the solenoid), run one (GRN/WHT) wire to pin 3 of the reverse lockout relay. Add another section of the same color wire, joined either at relay pin 3 or somewhere down the other wire, and run this section to one pin of a diode*. From the other pin, run a BLK wire to ground (the ground spot behind the right kick panel is a good location that's easy to access).
      
      * NOTE: I do not have a specific part number for this diode, if you don't have an existing one from an MT and need to buy one somewhere; hopefully someone can post that info in a reply to this thread and I will update this section with that number.
      
      From pin 1 (GRY wire) of the connector (C144) on the end of the reverse lockout solenoid harness, run a (BLK or GRY) wire to ground somewhere (pin 9 of C531 is an option, though you likely have closer locations depending on how you choose to route your wiring).
      
      From pin 4 of the relay, run a (RED/WHT) wire to C386 (pin 17) of the ECU. You'll likely need to pull a wire from another connector that plugs into a control box of some kind, as you'll need the small version of female terminal to fit into C386; a good source is C351, the 26-pin BLUE connector that plugs into the "transmission brain" in the PCM of AT cars. (Note: connectors that plug into control units mostly use smaller terminals than connectors that plug into other connectors). You can solder just the end of this wire onto the wire you run from the relay, or pull a long enough section and use the whole thing if you have access to a parts car.
      
      From the wire already connected to C531 (pin 11), run a second (YEL) wire (joined to the first one, for the backup lights switch, either at the connector or somewhere along the wire) to pin 2 of the RLO relay. Run another segment of (YEL) wire from pin 2 to pin 1 of the relay.

[devolution]

Reserved for outline of my version

[Alperovich]

Awesome write up!

[Canadian144]

+1 very helpful stuff man! Thanks a ton.

I was able to wire up everything for my 5 speed swap based on Crowing's DIY but I was pretty lost when it came to the 6 speed RLO. This really clears it up... thanks!!