The "right" way to tap into ECU...?

[NachtRitter]

Here is a question for those of you that are a lot smarter about electronics and/or electrical stuff than I am (which is probably the vast majority of you... ).

My '94 Honda VX is not OBDII, so no handy-dandy port to plug into so that I can access the plethora of wonderful engine stats coming and going...

Of course, I do have an ECU, so there is still a lot of that good info coming into the passenger compartment from the engine compartment... and a lot heading back out again too.

I'd like to tap into that information for a variety of things, not the least of which is to get the VSS and injector pulse width for the MPGuino... there are other things that (over time) I'd like to read or possibly even over-ride ... for instance, I might like to be able to stop the crank position sensor signal so that I have a "quick & easy" engine kill mechanism without having to switch off the ignition.

I am not really a fan of stripping the insulation off wires so that I can tap into the signal that way. Nor am I a fan of those 3M "guillotine" style connectors that provide a "brute force" approach to tapping into wiring... I've had too many of those cause the original wire to break at the tap point over time.

So my preference is to go to the wrecking yard, get myself an old ECU along with the section of the wiring that plugs into the ECU, pull the wiring harness socket out of the ECU, and build a "breakout box" that goes between the car's current wiring harness and the ECU.

I know that Honda has these in order to do diagnostics at the dealership... I also know that they would charge me ~$800 if I wanted to buy one from them.

So my question for you experts would be: What is the right way to build the circuits inside this box so that I can tap into the various signals (both analog and digital) without screwing up the signal that the ECU sends and receives? For instance, I recall a vague reference in college to the fact that introducing an alternative path to ground via some resistance could drop the voltage sensed by another portion of the circuit. I don't want to try to detect that voltage, and by doing so, screw up the voltage the ECU receives. Nor do I want to accidentally feed in a "digital" signal that confuses or overrides the ECU. And I certainly don't want to toast the ECU accidentally either. For some reason, I have a feeling that simply doing a "T" connection for each signal (digital and analog), I won't get that protection I'm looking for.

For the basic power and ground connections, I'd likely just pass those through the box without trying to break them out. Similarly, there are probably several signals I'm not even going to bother with and so will send straight through.

As an additional advantage, this approach would allow me to keep the original wiring "unmolested", which is another preference of mine.

Thank you in advance for your comments, suggestions, and thoughts.

[Daox]

I did this a while back when I wanted to use an Greddy E-manage on my Tercel. I wrote up an articl here:

E-manage Plug & Play Harness

[TomO]

I'm facing this same conundrum for my VX. The only company that makes an OBD1 to OBD1 jumper harness for Hondas is Rywire.com and they want $110 for one. I've asked the Ebay seller that sells the $30 OBD1 to OBD2 jumper harness to see if they would make a straight OBD1 to OBD1 jumper and they told me they don't have the resources to do so (go figure!?). About the most cost effective way to get this accomplished is to make one yourself if you can find a junk ECU for cheap/free and the ECU wiring/connectors.

AFAIK, the HONDA ecu jumper has nothing special in it aside from having "ports" for you to easily insert the testing points of a DMM into it. No special circuits in it or other components.

[NachtRitter]

Hey Doax,

Great writeup! Thank you for sharing! Looks like you did a basic "T" connection for those signals you wanted. No concerns about affecting / disturbing the signal or toasting the ECU (accidentally, I mean of course)?

Thanks TomO,

The $110 is waaay more reasonable than the $800 that Honda wants!! Still pricey, I agree, but now it's within a reasonable time vs $$ tradeoff area. It'd prolly take me ~ 6 tedious hours to get all those connections soldered...
Also a bit more "comforting" to know that the Honda ECU diagnostics harness has nothing special inside to "buffer" the signal from the external test points.

Maybe I'll look at it as an opportunity to enhance my soldering skillz...

[Daox]

Well, I'm sure what you all connect to it to monitor things is going to effect voltage drop and what not. The e-manage is designed for what it does and thus there are no problems.

[TomO]

Originally Posted by NachtRitter

Thanks TomO,

The $110 is waaay more reasonable than the $800 that Honda wants!! Still pricey, I agree, but now it's within a reasonable time vs $$ tradeoff area. It'd prolly take me ~ 6 tedious hours to get all those connections soldered...
Also a bit more "comforting" to know that the Honda ECU diagnostics harness has nothing special inside to "buffer" the signal from the external test points.

Maybe I'll look at it as an opportunity to enhance my soldering skillz...

Best way to make your own would be to get the actual Sumitomo connectors and virgin pins and thus create your own harness without having to solder. the problem with that logic though is the fact that Sumitomo connectors need to be bulk purchased in about 1000 or more units at a time. That makes it a bit cost-prohibited unless you were to make some OBD2 to OBD1 jumper harnesses and sold them on EBay to recoup costs. but that would require more time and effort and research.

I'm thinking that if I could convince the $30 Ebay jumper harness seller to sel me a "kit" that would work out better. Or if I could just purchase a prebuilt jumper and just re-pin it.

I'm much more inclined to pay $30 and some time rather than $110. but you're correct in stating that the $110 price tag is much better than the $800 Honda one.


And there is the possibility of the MPGuino and other electronics being mounted in the circuit to change reading, but that would be negligible unless the circuit faults for some reason.

[NachtRitter]

Originally Posted by TomO

Best way to make your own would be to get the actual Sumitomo connectors and virgin pins and thus create your own harness without having to solder. <clip>

I'm thinking that if I could convince the $30 Ebay jumper harness seller to sel me a "kit" that would work out better. Or if I could just purchase a prebuilt jumper and just re-pin it.

Aaaaah... brilliant idea!! Let me know if you'd like assistance encouraging the Ebay feller to sell a "kit" ... If it's basically a "crimp & clip" operation, I would definitely prefer that over soldering!

Originally Posted by TomO

And there is the possibility of the MPGuino and other electronics being mounted in the circuit to change reading, but that would be negligible unless the circuit faults for some reason.

OK, thanks for the inputs... I'm a bit more at ease about tapping into the signals based on both your and Daox's feedback