So About 2 months ago I completed my engine swap from another car, The other car had a 1688 code aswell before the swap was completed. After the swap was done the car drove for about 80 miles with no issue then started to throw the code and went into limp mode.

So I swapped the omp that was on the new motor for the old good omp. This fixed the problem for about 400 miles. Then she threw the same code again.

Its been on and off since the swap. I have re zeroed the mop countless times and the car seems to do it randomly. This last time the thing that seemed to fix it was cleaning the ecu pins and plugging them back in. That lasted for 2000 miles!

But about a week ago I had to disconnect the battery to diagnose a coolant leak and ever since then an inescapable p1688 and limp mode. I have re-zeroed the position sensor. Tried a different position sensor. Cleaned the contacts and put dielectric grease on them.

Nothing.

I have looked at many p1688 forums and cant find anything quite like it so I made this post. I'm pretty desperate at this point, please help.

 

So if replacing the OMP hasn't fixed it, and the issue shows up intermittently, you're probably looking at problems in the OMP wiring. Especially if moving the wires around, and nothing else, appears to fix it. Possible the connector to the OMP is damaged? Any signs of damage in the insulation? It's pretty low to the ground, water/salt/etc can kick up in there, especially if driven without the undertray.

 

There is no loom or heat insulation on the wires on the 3 pin position sensor connector right where the wires meet the base of the connector, I didn't think it would cause issues as the solder looks perfectly intact. Though, Now that you mention it it could very well be the issue. Should I maybe check voltage with a multi meter? Perhaps snip wires and reloom then solder again?




The 6 pin stepper motor control connector is perfectly intact, with no visual issues.

 

Wait, solder? You shouldn't be seeing solder anywhere? Possible someone already tried a fix?
And can you confirm no undertray? You do need it on there for stuff like this as well as to avoid overheating.

If the wires are soldered, maybe resolder and heat shrink them properly. If they're all original, you can add extra sidesplit loom without undoing them.

 

So, there was no undertray for a long time Until very recently when I made A chassis mounter splitter/undertray combo. The factory one was lost from previous owner (Car is a salvage title).

This undertray goes all the way back to the radiator.

Maybe What I saw was not solder, I just remember exposed metal. I'm just waking up for the day so I'm going to go check it. and get back to you. Thanks for your help so far btw.

 

2 pin on close inspection is actually in good condition. No exposed metal at all.

Same with the 6 pin stepper motor controller. Its in good shape too.


Steps from here?

 

Hmmm. My thinking would be to try to reproduce the problem reliably. Once we know what causes it to trip we can figure out a fix. Can you try to spray water on the connector, or jiggle it while the engine is running, while someone watches for the CEL?

 

For the past week I have started it the problem has been there. After disconnecting battery and resetting ecu it will usually come on within 15 seconds.

On 3 occasions in the past week it lasted up to 5 mins running without the light, but on a restart of the car it will come back 100 percent of the time.

The issue used to be intermittent but now seems to just be permanent. But its very ******* strange because It will run for a few mins without the CEL on rare occasion.

Is there a way to inspect the MOP for proper function?

 

UPDATE:
I have seen on other p1688 forms that you can actually plug an mop into the car without bolting it in and the car knows no difference. So Just to test to see if its a bad mop I plugged in the old "bad" omp which used to throw a 1687 which is a mop code not a position sensor code and It actually only threw p1688 still.

I'm thinking that there is a short in the wiring harness between terminal a and c of the 2 pin connector.


The resistance of the position sensor when tested was 1132 ohms which is well above the failure specification of 400 ohms. But I think it isn't sensor side I think its harness side. Could I maybe check voltage of the harness side of the connector to see if there is a short? Or how would I go about detecting a short. I'm really not knowledgeable when it comes to wiring.

 

Problem fixed
Drove 50 mins away for a used mop from a buddy of mine who has a non running rx8. Gave it to me in exchange for helping him with a motor swap.

Fixed for 5 miles so far.