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How To: Build an offboard PCM interface

Pure2sin

Pure2sin

New member
This is a work in progress thread but here is a list of materials (prices may have changed):

Offboard:

-Project box: $3.99 + tax

Project Enclosure (5x2.5x2) : Project Boxes | RadioShack.com

-LED switch: $3.99 + tax

SPST 12VDC/20A Illuminated Toggle Switch with Red LED : Toggle Switches | RadioShack.com

-Power supply: $7.99 + free shipping

12V 5V AC Adapter Power Supply HDD Hard Disk Drive IDE | eBay

-Female Spade connectors: $1.99 + tax

Female Crimp-On Quick-Disconnects (10-Pack) : Disconnects | RadioShack.com

-Wire Loom: $1.99 + tax

Buchanan 3/8 in. Split Flexible Tubing Connector-772240 at The Home Depot

-3ft 14AWG Power Cord Cable w/ 3 Conductor PC Power Connector Socket (C13/5-15P) - Black: $4.04 Shipped

For only $1.64 each when QTY 50+ purchased - 3ft 14AWG Power Cord Cable w/ 3 Conductor PC Power Connector Socket (C13/5-15P) - Black

-PCM blue connector: $10.00 + tax

-Bought at the junk yard

OBD II Connector: $5.00 + tax

-Bought at the junk yard

2 bolts, 4 washers, 2 nuts: $1.40+ tax
offboard_harness.jpg
 
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Monte Carlos have been known to have trouble with having to pull the radio fuse to flash the PCM. I just had an issue where I almost bricked my PCM because I didn't notice the battery was low. I have two PCMs so one will always be in the car and the other will be for new files then swapped in. Just will make it easier for me personally I guess.

It could always be used to write to other PCMs if anyone wanted to tune PCMs for other peoples cars as well.
 
Let's not forget that while you are building an offboard for a 97-03 pcm, you can also wire up the newer 04+ connector as well. Switch isn't required, since you can plug and unplug it. It's nice to have thought.

I used an old pc power supply, use the switch on the back of it and poof.. works all the same and I robbed it from something for free.

Even funnier is that WOTGTP contacted me last night looking to sell one. If you need one, toss him a pm.
 
Bill can you get in touch with WOTGTP? I messaged him about the offboard unit and he never replied I don't know if he sold it or whatever :/ I want one.. Thanks
 
KoldAsIce said:
Bill can you get in touch with WOTGTP? I messaged him about the offboard unit and he never replied I don't know if he sold it or whatever :/ I want one.. Thanks
Click to expand...
You realize that was over a year ago eh...
Build one or put a post up in the wanted classified ads, there are a couple people here that have recently or are going to build one so maybe you can see if one of them can build you one if you don't feel you can take on the project yourself.
 


its super easy to make. i just made one. also just pulled a old pc power supply out of the case. google / you tube says the yellow wire is 12 volts. any black is ground.


worked like a charm sat morning.
 
just get a 4 diode rectifier and your golden. made this for 04+ but ive added a 96-03

i love ****ing with people and connecting it to the battery backwards

 
looks like this when done. now that i have the power supply im gonna buy a two side plug to plug the off board into the p/s. so its not rigged up twisting them together. my switch has no on / off labeled on it, so i marked it.

offboardtunersetup_zpsace11944.jpg



being that it looks like your tuning now. dhp can and will brick your pcm someday. its a risk. so what i would do if i was you is to go to a junk yard, find a year car your car can use the pcm from, buy the whole air box, cut the wires to the pcm plug, rip the obd2 port out the car, and drop it all in the air box. now go pay for that air box lol i bought 2 air boxes for 12 each. one had the pcm plugs and all in it, the other just the pcm.

this way you can flash a stock tune to it, and have a back up, or flash your last known good tune to it, as a back up. you can also change the vin# in the pcm you your # in vin maintenance.
 
Okay. I'm looking to make one of these for myself, and I think I understand what's going on here. I have my PCM plug, and OBD2 port in hand. The rest I will get next time I'm by a radio shack.

So here's my question;
All my wires and numbers for the PCM plug jive except #56 (ground). There's no lead from #56. Safe to assume I'm okay without it, because #56 and #60 run together to ground? FWIW, I did get the PCM plug from a GP.
 


you pull all the wires form the pcm plug, then pick the fat wires and re pin only where you need a wire. does that makes sense?

the pcm plug has to be taken apart.
 
if the diagram says to put a wire there do it, you may have to poke a hole in that rubber part the wires go through.

you have the plug apart yet? its a bit tricky, i can direct you how to do that with out breaking it if need be. i can take some pics of how to do it easy enough.
 
I have not had the plug apart. I tried to take another one apart but couldn't because of that screw. I hoped to just jab that seal out of #56 and poke another lead through. Assuming the end stays on at least one of the pulled leads.
 
ok you just stop there. ill post some pics before you break it lol that plug is like 3 parts that all need to come apart. you need to push a clip inside with a awl or pick to release the wires from the plug once its apart.

ill take pics and post em tomorrow, im off to bed.
 


I pulled all the wires out except grounds, power and the two data lines. Connected data to the obd2, put power and ground to the power supply.. poof done.
 
you don't actually have to remove the rest of the pins in many cases there are several connection points for power and ground depending on the model of car even with the same 7440 hardware number PCM.
all you have to do is get a pic and open up the pins so that they no longer grab onto the PCM pins you don't actually have to remove all of them
 
why have 40 wires all over when all you need is 5? and why use them thin wires when you can pull them all out, and use only the fat wires. its a lot easier in my eyes, to strip the plug, then install the ones needed, vs taking the unused ones by one in the birds nest.

you try to help yet you make it sound harder. the diagram shows us where to put the ground, if you want to trust your own ground, "you" do so, dont tell some one to find a new ground cause your lazy or something. ( you dont even say what pin, just others????) < thats not help.
 
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