Mazda Rotary Club

I experienced a P1688 error code earlier this year and my RX-8 went into a (very unpleasant) "limp mode". The error code pointed to the ECU detecting the OMP not working correctly on a start-up check. I checked all the electrical connections were OK, then checked the 4 stepper motor coils, and position sensor resistances were all OK, which they were.

To get my car back on the road quickly I decided to purchase a replacement OMP supplied by Boston Rotary, which fixed the problem. I recorded my DIY test and replacement of the OMP in a video, which needs some editing before publishing.

However, there seem to be an increasing number of reported failures of OMP's, so I was curious to see if they could be tested and maybe fixed. Some research led to several useful documents on the operation of the OMP, the most useful of which was this one, which describes the check sequence the ECU goes through to determine if the OMP is working correctly.

I needed a bit of a technical challenge at the moment, so set about putting together some hardware (based on the cheap Arduino controller) and some software to simulate the ECU test sequence. The worst bit was the small bolts attaching the OMP top cover were seized solid, so I had to use a Dremel to cut/grind them off to enabled me to check the mechanical arm inside was moving as it should. Here's what it looks like inside (not my pic in this case):

Here is the result of a few hours work (note I have not edited this, so please excuse DIY nature of the footage!):


https://www.youtube.com/watch?v=y9TyjN8NBdE&t=5s

The next stage was to convert this prototype into something that is portable and self-contained, so it can be plugged into an OMP in situ.

First, a breadboard version with display added for extra info:

And here we are, as of today.

Hardware now soldered onto small board and tested with the software. A Schottky diode added to provide simple protection against reverse polarity connection. A few bits to tidy up and a small "press to test" switch needed for the final version.

It looks like it will juuuust fit into the small black box pictured. A bigger one would make it easier, but I have a few of these lying around.

The slightly puzzling thing is that the unit I'm using for testing PASSES all the published Mazda tests; resistances all in spec and the stepper motor works and moves to correct positions. BUT, it's the unit taken from my car that the ECU kept reporting as faulty.

So, are Mazda doing any tests that are not published? Weird, as the unit appears fully functional (so why would Mazda need to do any further checks?), but it threw P1688 codes, despite resetting a couple of times........... :dontknow:

Shame I binned my broken one
You could have had that to test
Good work Malcolm


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Just 1 thing Casey...it has no oil init...could be stalling the stepper motor.

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Thanks Jon - unlikely I think, but I'll leave it on the "possibles" list.

If anything, the presence of oil would help to lubricate the moving parts in the oil volume control mechanism and ease any load on the stepper motor. On the bench, those parts have to moved "dry", with more friction, although, as this is a used unit, there will be an oil film still in there I reckon.

How about warming it up to engine temp..see if that effects it.
I lost 1.. then moved away from heat and no more probs. Clives has been under his dash for 60 races...

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Good thinking Jon. Again, worth adding to the list. The first failure was when the engine stalled a junction when hot.

Now, blowtorch or rig up a hot oil bath?  ;)

Old metal saucepan...bolt it to it and heat up water on a camping stove or such like.

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Quote from: boosted on August 27, 2021, 12:27:08 AM
Old metal saucepan...bolt it to it and heat up water on a camping stove or such like.

That's the sort of thinking that made this country great!  ;D

Great video & impressive work!  :sign0098:

The prototype is now complete and I hope to test a few more OMPs to see what results I get, trying to solve the mystery of why mine failed in service, yet passes all the published tests. I have an old saucepan ready to adapt for some high temperature tests too  ;)

I'm quite pleased with the final size and ease of operation - Sharpie pen for scale in pics ;)

The loom is long enough to allow for connecting to the car battery (or any other 10-30v DC source that can deliver 600mA), and to connect to the OMP in situ. Reverse polarity protection is built in too  :icon_thumleft:

Absolutely amazing, good job sir

Lähetetty minun G8141 laitteesta Tapatalkilla

Very nice bit of kit I think you will be asked for that for testing a lot  Amazing well done

Fantastic work Malcolm.

Ive got my mate Richard ( did all my speedo work) on the same table as you on Friday...................

.................... you can talk to each other in binary ! lol

Quote from: Jamie R on October 11, 2021, 06:14:17 PM
.................... you can talk to each other in binary ! lol

Darn, I only know hexadecimal  ;) :))

Quote from: Casey on October 11, 2021, 07:18:35 PM

Quote from: Jamie R link=topic=85237.msg922010#msg922010 date=1633972457
.................... you can talk to each other in binary ! lol
/quote]

Darn, I only know hexadecimal  ;) :))

Are you up there with laplase transforms? I know he hates them


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You mean Laplace Transforms  ;)

Haven't touched then since A level maths  ;D

I've not touched them since college. Hated them !


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