OK, I will go pin by pin on the 1227747 harness. FIRST- You will NEED the set of diagrams available from here Or attached below. There is also a very nice 1 page pdf file in this thread! There are 2 connectors. If you look really close you can see they are labeled. The small one is A & B. The large one is C & D. There are also numbers on the plug for each pin. Some basics: I run 2 relays. 1 for 12v to the ignition, ECM power and injectors, and 1 for the fuel pump. I use the basic 30 amp aux lighting relays from the parts store. If the harness came with the factory fuel pump relay than I retain that relay. If not I use the 2nd 30 amp relay for the fuel pump. Both relays are energized from the 12v ignition wire that used to go to the + side of the coil. Since it is only used to turn the relays on there is less draw on the stock 30 year old scout wiring. I don't run the ign from the switch. Due to voltage drop from the old IH wiring. I run all switched(from the key "on" position) 12vdc thru the relay. If you use a bosch style relay here are the pins: There are several different versions of the relay out there so look at the pin numbers before you buy one. 30-12v fused input(should also hook up your org ECM constant 12v to this wire) 85- Gnd 86- 12vdc from ign sw(black wire that was hooked up to the coil + or grn/wht from ECM for fuel pump ) 87- pink/blk, pink(ignition, ecm power) 87- red, white(injectors) there are 2- 87 output terminals on the SPST 5 terminal relay so you can use both. For a SPDT there is 1-87 and 1-87a. See the diagrams. They are opposite in operation and can not be interchanged. If you wire to 87a your circuit will be on all the time with the key off and then be off with the key on. Using factory fuel pump relays.... To start with go to this link and get the diagrams. You want to get sheet 2 and 3. At the bottom of sheet 2 is the FP relay circuit for the S/T M series of trucks. I will use it for the explaination. Here's the diagrams...they are also attached below. diagrams Note: Pin A comes from the ECM pin A1 and is used to apply 12V to the relay to turn it on. Pin C is the ground wire. Can be connected to one of the black/wht wires. Pin D is your 12V from a fuse. This is normally tied to the org wires B1 and C16. It is the same that provides the bat 12V to the ECM. Pin B is the output of the relay going to the Fuel Pump or any other load you have connected to it. This also ties to pin B2 of the ECM to tell it when the FP is actually running. Now the pin letters may not be exactly the same as this diagram. That's not important. It you follow the diagram here and the description and just match it up to the diagram on your relay!!! Here is a modified diagram (sheet 2 attached at bottom of post) showing one of the alternate pin designations on the relay and also shows the removal of the oil pressure switch. _____________________________________________________________________________________ Now Let's go through the harness wire by wire A1- grn/wht- this wire is used to energize the fuel pump relay. A2- not used A3- not used A4- gry- to EGR relay.(optional) This is a ground from the ECM to control the EGR relay. A5- brn/wht- SES(service eng soon) light. This is a ground to turn on the light. A6- pnk/blk- switched 12v from the ignition relay. A7- not used A8- orn- ALDL Serial data. This is your ALDL pin E data for your lap top. A9- wht/blk- ALDL pin B diag mode. When jumpered to ground will set the ECM to ALDL diagnostic mode. A10- brn- VSS speed sensor signal to the ECM (optional) A11- ppl or blk- MAP sensor ground. A12- blk/wht- System ground. Can be tied with D1. Goes to engine ground. Just to clear the confusion on which pins are grounds I have taken a C3 ECM apart and labeled the pin locations on the plugs. If you look you can see that D1 and D2 are solder connected and A11 and A12 are also connected. So if either one of the two, example: D1 and A11 are grounded the other one will be. D2 and A12. B1- orn- 12v fused from battery. I use an inline fuse on this. It also ties into C16. B2- tan/wht- Fuel pump signal from the relay. This one ties into several places, the fuel pump, the relay, and the ecm. When the relay is on it sends 12v thru this wire to turn on the fuel pump, at the same time it tells the ECM the fuel pump is on. B3- blk/red- EST reference. B4- not used B5- ppl/wht- EST high reference B6- not used B7- blk- ESC signal This is the knock sensor signal to the ECM. The knock sensor circuit is optional. You will need to program the chip to prevent the SES light from coming on if not used. B8- dk grn- AC signal. This tells the ECM that the AC is turned on. This can be used to set the idle speed higher when you turn the AC on. I use it to increase the idle when using the compressor for on board air. Same thing!! Just tie it to the wire going to the AC clutch. B9- not used B10-orn/blk- Park/Neutral sw. This is optional but I have hooked it into the park sw on the tranny. It tells the ECM when you are in Pk or N. It can be used to give a slight adjustment to idle speed when switching from park to drive and back. Manuals do not use it and I no longer use it for auto or manual. Just my pref. B11- not used B12- not used C1- not used C2- not used C3- IAC- gn/blk to IAC D / 2.5l flat plug A C4- IAC- gn/wht to IAC C / 2.5l flat plug B C5- IAC- bl/wht to IAC A / 454 flat plug B / 2.5l flat plug D C6- IAC- bl/blk to IAC B / 454 flat plug A / 2.5l flat plug C The ECM controls the IAC motor. It adjust the amount of air that is bypassed around the butterflies to adjust the idle speed. Note: For the IAC pins C5 to A and C6 to B are reversed on the 454 vs the SBC so swap those 2 pins if using a 454 TB with the bolt on IAC. To connect the 1BBL 2.5L IAC, all 4 wires are different. See above. C7- not used C8- not used C9- ppl/wht- Starter crank signal. Goes to the small terminal of the starter. The stock scout wire is white that goes there. Tells the ECM you are trying to start the engine. C10- yel- Temp sender signal C11- lt grn- Map sensor signal C12- (1228746 only) IAT/MAT sensor C13- dk blu- TPS sensor signal C14- gry- 5v reference signal to map and TPS. C15- grn- inj B ground signal. Most systems do not use this. It was used on some of the bigger engines to share the load of the injector signal. C16- orn- fused 12v tied to B1 D1- blk/wht- System ground, ties to A12 and engine. D2- blk- Sensor ground. To eng block or head. D3- not used D4- wht- EST control D5- tan/blk- EST bypass. This is the wire that has a plug near the distr that you disconnect to set your timing. D6- Tan- O2 ground to engine. D7- ppl- O2 sensor signal D8- D13 not used D14- lt grn- Injector B ground signal D15- blu- optional injector A ground signal D16- blu- Injector A ground signal And that's it. If you sit down on your garage floor and have each sensor plugged into the harness, then trace each wire back to the ECM connector, you will be able to separate a lot of uneeded wires. Once you have each wire identified and layed out, then you can set it up on the engine and start tailoring it to fit your engine. Placement of the sensors will determine wire length. Most stock harnesses are plenty long and only need to be shortened or wrapped and bundled. When I'm all done and it's running then I go back and reinstall the plastic loom over all the harness.