So heres the tl:dr


I've got a 00 Civic with a B18C5 i built some 6 years ago. At the time we got it all happy and running with a P73-A04 and IMMO emulator board to start it right up with no CEL.
Since then i've used a Hondata P28+S300 for all my daily driving.

Well I'm going back for a ref check and I need to run the A04 again. I got myself another good ECU with a Katman installed IMMO emulator. It worked 1 time when we installed it in my civic, then it started to behave as if there was no IMMO installed at all (i.e. crank for 5 seconds, then it starts with an internal malfunction error).






We tried the ecu on another ITR OBD2B civic to see if the issue was with the ECU itself. Once installed in our 2nd civic, there were no problems, start-stop, drive, IMMO emulator and ECU are good.

So my question is, what would prevent an IMMO emulator board from either powering up or grounding properly due to my engine harness. 4 years ago we did some testing with a Hondata Traction Control unit where we had to tap into the ELD, power and ground wires... but with out knowing which Connecter pins (A, B or C) are required for the IMMO pins, its been rather hard to figure out.

Other things we found out, Both my P73-A04 (and a older P2T Si ecu I have) would not trigger the Rad-fan to turn on. Car actually began to overheat as it was idling with the A04 since it turned out the thermostat temp switch had a broken wire. That being said, the ECU should also be able to over-ride and ground the relay for the Rad fans... so again, im thinking something might be a bit off with my harness. Strange enough, the Hondata P28+S300 which has a built-in OBD1-OBD2B jumper in the case, had no problem starting the rad-fans. I had no idea my thermostat switch was actually broken lol.

 

Ecu doesn't trigger the rad fan

 

Well not directly, the ECU will ground a pin through the ECU harness which will activate the Radiator fan relay.

My Prepared Hondata P28 with its internal OBD2B jumper worked the fan relay for who knows how-many months since the thermostat switch was literally dis-connected on the green wire side, in effect my Hondata was keeping my C5 from over-heating with out the physical thermostat temp switch working.

The way the two are wired together, either the ECU or Switch can activate the relay independently, so long as one is grounded. Im just stumped why the Hondata P28 could ground the relay, but not my P2T or P73.

 

Brah.....wheres the ecu in the diagram

 

ECM green wire tied in just before fan switch.

However, what is unknown is what specifically is the ECM (ECU) doing, just monitoring or providing an alternate ground? I suspect monitoring for ground being applied from the thermo switch.

 

Exactly, its tied in only to be detect. It then tells the iacv to idle up because of three additional draw of current. Does not control the rad fan

 

Yeah the fact the OBD2B ECUs (P2T and P73-A04) wont ground the relay to activate the fan is a bit weird. Yet the Internal-jumper ed Hondata P28 does. Reading temps from the scan tool shows temps rose up to 230 and still no fan activation.


This wont be too-much of an issue as a proper functioning fan switch should solve the issue. The main issue is why my IMMO behaves as if its not installed at all when used on my harness.
When an IMMO is broken off a P73, the ecu will actualy start a car, it just takes 5 seconds of cranking before an internal malfunction CEL turns on. This is obviously an automatic fail for emission visual reasons.

 

Hondata adds significant functionality over the OEM ecu.

It's why it can run a Turbo on a 3 stage motor as it has several additional controllers that can be programmed.

 

@tony_2018 I think your confusing the thermostat fan switch with the ECT sensors on the head (the one wire for the temp gauge, and the other for the ECU). Where as the thermostat switch on the housing itself, is nothing more then a thermistor which completes a circuit to a chassis ground and activates an electromagnetic fan relay.
The OBD2B ECUs do see coolant temp through the scan tool, they just arnt grounding the circuit to activate the fans. Very odd.

@TomCat39, well clearly Hondatas can use any of the extra OBD2 pins as data point for various things, whether it be for traction control feedback, or when i used one of the P/S pins to activate a 2nd fuel map when I had Water-Meth injection with a JRSC.
I'll have to double check, but I'm pretty damn sure we didn't set up an alternate grounding solution to run the Radiator fans on w/ the Prepared P28+S300, as there should be no reason to. I'll have to look at pin A20 to verify. Though im quite sure its still there.

 

It sees coolant temps because of the sensor on the head..

What your diagram shows is the radiator circuit. Has nothing to do with the ect.

 

 

Corrected,

 

I am with tony on this one... if the HSM description stated above were true, no "fan switch" would be necessary on the thermostat housing... not to mention that fewer cars would overheat when the fan switch fails since you would essentially have a back up system to activate the fan. Really, how many Civics do you think actually have bad ECU's AND bad fan switches before they start overheating ??? Understand that your Hondata s300 system has the ability to FORCE the cooling fan on at a set ECT value... now how that actually happens inside the ECU... I can't tell you. My suspicion is it has to do with the A/C supporting systems, but that would be a guess. The bottom line is if you can turn the key on and use a paperclip to loop the two wires together at the fan switch plug and your cooling fan comes on... your fan switch is bad... replace it and move on.

As for your ECU immobilizer issue... I have an unorthodox suggestion: What about removing your P2T ECU board from it's chassis and installing it in the USDM P73 chassis ??? This way, no immobilizer is needed and the visible check would pass by the Ref. Just a thought.