Fireindc

iTrader: (13)

Gonna make this quick and to the point.

Car setup: 1990 chassis with bp4w swap. MS1PNP. Running using all stock 1.6 wiring and CAS with DIY toyota COP ignition setup.

Issues: Bouncing tach. between 3-4k the tach likes to bounce around at random. The RPMs are not actually changing, but the tach goes wild. Sometimes at idle the tach will drop to almost 0 then hop back up to ~900 rpm. Happens at random, sometimes I'll drive it for days or weeks without the issue, sometimes it happens every-time I drive the car. Occasionally the airbag light will flash while driving. Very occasionally the car will actually cut out for a split second, particularly with the lights up at night, but I've been unable to capture this in a MS log.

What I'm looking for guidance on is my ground setup. With the chassis/motor combo I'm not sure if my grounds are correct and suspect this is the issue. I've done quite a bit of research on this but due to my setup with a nb motor in an na I'm just not sure what should be grounded where.

Here you can see my ECU, wideband,ignition, and other OEM grounds in that cluster. They are attached to the back of the intake manifold in one spot (as to share a common ground for ECU and wideband/sensors).


Then I have this ground off the VC going to the drivers side engine bay/chassis:



Then I have this ground, attached on the HEAD, going to the passenger side engine bay/chassis (thinking maybe I should relocate this one, i know it's not in direct contact with the chassis but in my mind the bolt should complete the ground as it's threaded into the chassis).



And I have this guy, also on the passenger side inner fender/chassis, slightly relocated from the factory position:


All grounds are snug and clean. Battery grounds, PPF ground, under dash grounds are all solid as well. I tried adding another ground from the intake manifold junction with the ECU ground directly to the firewall, but it seemed to actually make things worse.

What do I need to do to fix my ground setup for good?

L337TurboZ

iTrader: (1)

No offense but those grounds at the Intake Manifold look like crap. I would try to avoid having so many eyelets grounded at one location like that and if you really want to have them there, try getting a large eyelet and run all the wires into one. Are they bolted to where the EGR valve goes with a delete plate? Solder all connections too because terminal fretting is a common issue with a lot of vehicles.

If there is a plate there it could be preventing a proper ground. Another option is to ground it lower on the block itself or adding a ground from the block to the chassis. You could also buy a grounding block and use one large gauge to the engine/chassis then have a series of studs to connect each individual eyelet to. Kind of like how Volvo cars have a grounding center on their strut towers integrated to the frame.

With your tachometer issue are you running a resistor in the diagnostic box? What about the 1.6 ignitor? Got a spare to swap?

Fireindc

iTrader: (13)

I take no offense. Thanks for the suggestions.

To answer your questions:

I'm not using a 1.6 ignitor. It was removed and replaced with the COP's, which were wired up as per this thread: https://www.miataturbo.net/useful-sa...writeup-12704/

I went the method of cutting off the connector and wiring directly in. All wires are soldered and should not be related to the issue.
As for the resistor, I soldered one into my ECU IIRC when i went with the COP's. Also, as another data point, I never had this issue on my old 1.6 setup with the same COP setup, so i fully believe it to be related to my engine grounds at this point.

I'll look into fixing up those grounds this weekend. Do they all need to be grounded to the intake manifold like that? I believe I went that route because that's where the bp4w grounded them from the factory.

L337TurboZ

iTrader: (1)

I don't think they all need to be grounded there. I would just keep the factory grounds at that location and find different locations to ground for your aftermarket accessories. Also make the the lower half of the engine has a ground running to the chassis.

It is important to clean the paint off of threads on the body. If you changed the ground splice pack to a different location that was empty, and there is paint spray in those threads it can prevent a good ground. Maybe take a tap to the threads to clean them out. Or change it back to its factory location and see if it makes a difference.

If the only things that changed with the swap was your grounding, then I would try to mess with each one at a time to ensure you figure out which was the culprit.

Fireindc

iTrader: (13)

The reason they were all grounded in the same spot is because I've always read that you should ground your ECU and all sensors, especially the wideband at the same location to avoid a voltage offset.

What is strange is that I added an extra ground from the firewall to the intake manifold thinking it would help and it definitely made the symptoms worse. This just made me think I had things out of whack. I'm definitely going to be moving some grounds around and cleaning them up this weekend.

miatawelder

perhaps capacitors in the tach itself starting to go bad. This happened to a 93 Toyota that I have. Similar symptoms. Also happened to my speedo and my main ECU. everything worked OK after replacing capacitors

IanIsInTheGarage

Can you pull a data log during time when RPM is going crazy? I'm thinking either Tech, MS or crank angle sensor. But crank angle sensor you should be able to diagnose with MS. My NA was having a sudden loss of tach and car would die. The crank angle sensor wire was melted near exhaust manifold.

Fireindc

iTrader: (13)

I just redid all the grounds today. So far so good, but I only drove the car a few hours, and like i said the issue is very intermittent.

I'll snap some pics of the new ground location and update the thread.. for science! I'll post back in a few weeks if it's fixed or not.