89-91 rx7 ecu limp home possible fix
 

Hi all,

I am wondering if anybody out there could try/has tried this.

I am just in the process of removing the S5 (89-91) engine I fitted to my 81 rx7 about 1.5 years ago, so can't try this myself.

One of the problems I had during the install using the stock N374 ecu was the limp home mode some minutes after the car was running with the oil metering pump removed.

While I am pulling the car apart, I thought I'd pull the ecu and have a poke around inside. I discovered the ecu pin 2A connected to the OMP position sensor and ecu pin 2F connected to the narrow range TPS both run to an IC HC289 on the analog board. HC289 seems to have dropped off the face of the earth, can't find any mention of it on the web. The full range TPS (ecu 2G) does not connect to HC289 interestingly. So my theory is this: is the ecu simply comparing the signal on the narrow range tps with the omp position sensor to decide when there is a problem and hence activating limp home?

So if someone was kind enough to:
A) disconnect their omp position sensor
B) without stuffing wiring up tap into the narrow range tps signal (green and red on the TPS plug according to my wiring diagram) and connect to where the OMP position sensor input goes (green and black - runs to ecu2A)
C) go for a drive, say 20 minutes
I could get some info on my theory.

The voltage range expected from the omp is the same as the voltage range of the narrow range TPS so ecu damage should not be possible.

If the car does still go into limp home mode, you can either turn off the car and restart it, or plug the OMP position sensor back in.

Regards,

Arran

PS my website is http://home.acenet.net.au/cootea

Try contacting a guy who goes by the handle "Henrik" on the TeamFC3S forum. He has pulled apart the ECU and mapped the whole thing. If anybody would know the answer I bet it would be him. I am sure there are others out there too.

NO.
The e-omp has a feedback circuit that is supposed to verify the position of the OMP (4 position) or else it barfs.
It really doesn't have anything to do with the TPS, except for input from the full-range side.

Either replace it or figure some circuit that can mimic the 4-pole stepper + position.


-Ted

I had a similar problem. I've got the OMP plugged in for now, although it's not being used, until I go with a standalone this summer.

Hi Guys

Quote from RETed
"NO.
The e-omp has a feedback circuit that is supposed to verify the position of the OMP (4 position) or else it barfs.
It really doesn't have anything to do with the TPS, except for input from the full-range side."

What I am say here is that the OMP position sensor and the narrow TPS
both run to an IC (HC289) inside the ECU. The stepper drive is on plug 3 and the driver IC lives some physical distance from where the position sensor signal comes in on the PCB.

The "feedback circuit" you are referring to is the OMP position sensor I am referring to. Because the narrow TPS and OMP position sensor both run to the same IC I am thinking that this IC does the comparison between where the OMP should be and where it actually is. I am inferring that the ECU uses the narrow TPS to tell the OMP stepper where to move to, so comparing the OMP feedback signal and the narrow TPS would make sense in this case. This is why I am thinking that connecting the narrow TPS and the OMP position signal together might stop limp mode.

As I mentioned in the first post, the OMP position signal and the TPS signals are electrically the same. They have the same ground and the same supply rail, looking at the wiring diagram.

rmaier :

This is how I have been running my car for 18 months, OMP plugged in but cable tied in the engine bay not doing anything except keeping the ecu happy

Cosmo Donk:

I think Henrik has been concentrating on S4 ECUs, which don't have electronic OMP. Correct me if I'm wrong.

Anybody want to give it a go????

Arran

/

Considering the small range of throttle movement the narrow-range TPS measures, I can't see how it's much use in the operation of the OMP system. We do know that the full-range TPS (along with other inputs) is used to determine how much oil the OMP flows, much like the mechanical connection on the S4 OMP but with better control.

The narrow-range TPS is used by the ECU to control closed throttle injector shut-off and some emissions functions, and the full-range TPS is used for things like A/C cutout control and WOT injector shut-off for unflooding while starting. Given that these functions are totally unrelated to the OMP, it doesn't seem to me that the fact that the narrow-range TPS and OMP position sensor happen to be connected to the same chip is proof that the two are used as you've suggested.

I agree that it does seem strange that the narrow range TPS goes to the same IC (HC289). I guess the proof is in the pudding so to speak. It is not too difficult to try out if somebody wants to give it a go and prove me wrong.

Alternatively I could make a better guess if I can find a data sheet on HC289 to work out what it does. Been looking, hard to find.

Arran

Try it and report back to us of your findings!


-Ted

My guess:
Maybe the ECU is using the OMP position signal to verify the sanity of the short range TPS signal, or vice-versa.
Some kind of cross check?

bump

Has anyone tried it yet? I have many customers with bad omps. Brand new s5 omp runs for almost $1500 from mazdatrix..

solution is the rtek chip..
and if you wish to retain a working EOMP ( that has a faulty feedback circuit ) you can use the standalone EOMP controller and use it in rx8 mode
.. where it only needs to see start and full stroke signals from the feedback and it will count the subsequent winding position

prototype EOMP controllers are currently underway for total PNP on the rx8 ECU
... no current plans to try and retro a PNP for s5
( which appears to be in fact more complex than rx8 software self checks )

8 year old threads getting resurrected?..what the Hell?

$1500 for a new eomp is highway robbery, better off finding another solution for sure

or rebuilding them.

i officially have stopped doing research into little projects like that but someone should step in and take a look at it. i doubt it is a sophisticated piece of electronics... same goes for the ABS units that no one rebuilds and are astronomically priced.

maybe if i wind up with nothing better to do i will rip one apart and look into it, 'til then don't hold your breath.

also surprised no one has worked out an emulator for it yet.

maybe we have... for the rx8 ecu .. s5 ecu not

for s5 eomps that pass the motor pole resistance tests , issue is usually in the feedback sender, full of oil .. its just a simple wiper at 2.5 K ohm full stroke

yes but the 8s aren't at a critical phase where the units are dropping dead left and right with no alternatives but a platinum plated wall trophy that weights about a pound.

even the few good units i thought i had seem to have gone faulty from just sitting on the shelf. frustruating for those who need a functioning OMP.

if it's just an oil impregnation issue causing resistance then a cut open of the factory sealed casing and clean up shouldn't be rocket science. i have no idea why people are so afraid of the unexplained, if it's broken then what is the risk in dissecting one and possibly figuring it out? that is exactly my approach to working on these cars.

just about any nitrile(Buna-N), viton or HNBR o-rings will work and stretch to fit for a quick reseal. just about everything on these engines is sealed with o-rings. doubt the ABS units are any exception but an assortment of o-rings that failed.

again,, the fix for poor souls stuck with a stock ecu is the rtek chip... no one else is going to reinvent this wheel
.. done well already

then for those who wish to retain the EOMP unit with faulty feedback sender
,, to make it work,, you need the aftermarket eomp controller, ,, used in rx8 mode it will ignore a wiper with faults mid stroke ,, and only rely on a count back of winds from the known max positions

-- this will make what is a faulty EOMP unit to a stock ecu.. re-usable
( only those EOMP units that pass the motor pole resistance tests )

Rtek S5 chips came along after i stopped dicking with those things, so i never realized they created an OMP deletion code in the chip. i guess that's one way of fixing the issue but not for those who do not want to premix.

You can Rtek, then convert the front cover to S4 and use the S4 OMP. I seriously considered doing this on my S5 vert when the OMP failed burning out the ECU. I ended up finding a good used OMP and a non burned ECU. When I rebuild, I may still to the Rtek/S4 front cover/S4 OMP way.

This is why I prefer the s4 motor with mechanical omp, way too many people blowing motors because of this, part is basically a lemon that costs $1500 bucks. F that.

Premix Is great a great way to solve the problem but not everyone wants to do it, and I'm personally not too keen on the idea of oil passing through the entire fuel system. (Maybe someone can.shed some light as to.if the fuel system will suffer any long term effects)

Not to.mention if you forget to premix you
can blow your engine, as well as it being
hard to.precisely measure how much oil to
put in, too little Is bad, but too much probably isn't ideal either.

Id assume more protection is a good thing but overprotection will just cause carbon buildup.

#mytwocents

I wouldn't call the S5 pump a lemon when it usually last over 100K. only problem is the price Mazda charges, and when it's dead (oil filled) it created a short and rape the ECU.

I think I see some Denso word on the OMP, I assume they made it? I wonder why it's not sold thru regular channels like spark plugs and everything else, without Mazda's "OE" boxes aka PREMIUM, it should be a lot cheaper.