siragan

iTrader: (1)

Hey guys,

I've been trying to get my 94 touring FD to run for about a year now on and off, dealing with an issue few have which is a CEL/MIL that returns no codes, but the vehicle is in limp home mode. This has been experienced on 2 different ECUs also. It's been a lot of fun.

I basically went through the whole engine wiring harness, literally unwrapping the whole thing. It seems there's a lot of replacement on this forum, and in general, without very much understanding involved. Contrary to what I believed initially, the engine wiring harness while brittle on the outside, the interior wires remained pliable. I was surprised and I encourage those in the market for a new harness to actually unwrap their existing harness and assess it at greater than face value.

Anyway, I was basically going down the list of every engine peripheral and FSM troubleshooting procedure and got to the MOP / OMP. As far as I can determine, the OMP used on the FD and S5 and up engines is nothing more than a 6-wire, four phase stepper with sister coils wired together and positional feedback via some type of linear or rotary potentiometer. I saw that one of the four stator coil resistances was low. FSM spec is 16-31Ohm (centered @ 23.5) across each respective coil. 3 of 4 coils measured 22.3Ohm on the fluke meter. The final coil measured 3.3Ohm. Feedback potentiometer on the stepper was fine.

I compared this to a spare OMP I had from a S5 turbo keg, which measured the same 22.3Ohm on coupled poles and 44.2Ohm on the other coupled set of poles. As far as I know, this OMP was working when it was pulled from the engine, which was an osakajdmmotors engine up Quebec (I know, it's filthy). This is technically 13.2Ohm above FSM specification, however it does NOT demonstrate a difference in resistance between coupled poles as my presumably defective FD OMP does.

I'm curious about two things. First, what have other people measured on their 'alive' or dead OMP pole resistances? Second, why wouldn't this throw an error code? I am thinking that if one and ONLY ONE coil of a twinned set fails, the stepper will continue to operate in half-stepped mode (I'm assuming the FD ECU uses half-stepping to increase resolution). However you will only get an OMP code 26 (iirc) if the twinned set fails completely.

I doubt anybody has gone this in depth into the OMP and its relation to the stock ECU, but I figured I'd document something up here to serve as help to others in the future if for nothing else.

If anybody has pictures of a disassembled stepper for the OMP and failure analysis type pictures of broken stator windings, etc, I'd love to see them. It'd be kinda nice to definitively say what the hell goes wrong with these things rather than just replace it and call it good.

I was also curious if anybody had written OMP defeat hardware/code. If I didn't have this extra OMP, I was considering writing code for the arduino to receive and interpret stepper pulses and relay corresponding feedback potentiometer voltages to the ECU. To the ECU it would look like and respond to controls just as a real stepper would. This would allow a person to run alternative oiling systems, such as premix, and would circumvent the cost of a metering oil pump and/or PFC/aftermarket ECU. The stock ECU is actually quite good, I was impressed, especially by the independent front/rear rotor injector driving on overrun and other really nice integral failsafes. Of course the whole limp mode/no CEL codes was a nice feature too. Maybe muythai/ken has something to say. Thanks for getting this far.

JM1FD

iTrader: (1)

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/omp-declassified-part-iii-mikuni-897507/

The OP seems to know quite a lot about the technical details of these pumps...

siragan

iTrader: (1)

Wow that is some awesome information JM1FD, thanks for the link. I suppose there are people out there who are crazier and more obsessive than I am. I'll have to buy you a beer next time I'm in Houston.

I'll try to remember to update this post to inform if a ~43Ohm coil resistance will keep the ECU from pissin and moaning. I hope so, because driving my honda around is not conducive to my sex life...

siragan

iTrader: (1)

Alright, the spare stepper motor I had was actually in spec, I just measured the resistances incorrectly. If you measure across two poles it'll read double the resistance of course, which instead of 22.3ohm is 44.6ohm. The old stepper was indeed faulty, with one of the poles reading 2.3ohm instead of 22.3ohm.

I installed the spare I had (which passes FSM spec procedures) and I'm currently in the process of re-wrapping the engine harness and providing thermal insulation. Will post an update to see if any of this garbage works...

siragan

iTrader: (1)

okay... put everything back together with the in-spec (as per FSM) OMP. car wouldn't start. no spark. changed out the two ECUs I had. twice. replaced plugs. received intermittent spark. finally, in mid-crank, as if going from a broken state to a fixed state, the ECU decided to deliver spark (as witnessed on the leading coils via a timing light). car kicked over and got it to start. no idea why the ecu was withholding spark. throttle was not at flood-clear state.

upon start, the engine hunted like crazy, cyclic hunting between 1k and 2.2k rpm, even while warmed up. exhaust EXTREMELY hot - burned hand on exhaust gas while waving it by the exhaust tips.

funny thing was, before I took everything apart, the engine started and idled just fine - only problem is that it was in limp home mode. now it starts 'reliably', hunts like mad, and is not safe to drive. my friend reported that sparks were seen out the exhaust tips during 'idle'.

now don't get me wrong - I'm not new to rotaries... I've put over 20 grand into projects over the past 7 years and have had several successful megasquirt installs. I've been working on and off on this FD for two years with no resolution. Both ECUs STILL report no CEL code when in TEN mode - the MIL/CEL light flashing on, then off, then staying on indefinitely. I am completely stumped and seriously considering taking this piece of garbage triangle clusterf*** and putting in a decent KA24DE/T with megasquirt. I have absolutely zero ECU visibility, and even if I put a PFC in I'd wonder if I would run the risk of burning it out and effectively killing 1,000 bucks of my hard earned money. Any suggestions would be welcomed. I have never been as stumped with a vehicle issue as this in my 25 years of living... (if you can classify an existence in the presence of rotary engines 'living').

signed,
isosceles in seattle

siragan

iTrader: (1)

After significant work, it turns out it was the pole on the OMP stepper that was the problem. I replaced the OMP and tried a new (known good) ECU with success. I had 3 ECUs:

1. The original ECU (car stuck in limp home, rarely intermittent EGR CEL, normally no CEL (stuck on, but not a short)
2. A replaced ECU which ran on the original defective OMP (same symptoms)
3. A 'new' ECU, run with the replaced OMP.

I measured the resistances of all the 24 terminals of the ECU WRT the ground plane of the ECU and recorded them. On one of the 4 pins of the OMP stepper circuit, the resistance was very high. This was only seen on ECU #1. ECU #2, while it exhibited the same symptoms, did not show this out-of-spec resistance on the OMP pins.

However, I did observe on the printed face of the 12 pin stepper IC (I forget the name of the IC, but I do know that I was unable to find a replacement for it online) that the casing was cracked/burned on ECU #1. It was not very noticeable. On ECU #2, one could *barely* see cracking on the printed face of the IC, it was a hareline crack at best, just above pin 4.

Replacing the low resistance stepper OMP with the new one, in conjunction with replacing the ECU (which had a defective 12-pin stepper IC - located near the header on the base board of the ECU), resolved the issue.

RE.Amemiya.FD3S

I got the 26 CEL error code, along with 23(Fuel Thermosensor), 25(Pressure Regulator Control Solenoid) and 44(Turbo Control solenoid)

Can you please tell me how and where did you replace the stepper OMP?
I may sound a rookie, as i don't know much about Rotaries

Your reply is highly appreciated.

Thanks