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V8 Miata Wiring, Electrical, and ECUs v8 conversions on a Miata can be tricky. Post your comments or questions within with regards to swap wiring and electrical.

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Old 06-22-2017, 01:44 AM   #1
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Default Question about the 2000 Miata PCM module

It may be a bit late but I am asking the question anyway.
Do I remove and not use the Miata factory PCM module when installing the LS1 ECU?

Originally I was thinking I could get rid of the original Miata brain and sold it with the old engine and engine wiring harness.
In hind sight I am thinking the PCM runs a whole bunch more than just the engine and I still need a Miata unit to run everything that the LS1 ECU is not controlling.

Do I need it or not?
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Old 06-22-2017, 09:55 AM   #2
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Go through the factory service manual wiring diagram and check out the circuits going in and out of the ECU. Then you can figure out if you need any of these anymore. You'll find that the Miata ECU is very standalone and doesn't interact much with the rest of the car. The only thing it really interacts with is the instrument cluster. The tach signal will come from the LS1 ECU, speed comes from the transmission directly into the cluster (Miata ECU gets speed from the instrument cluster, not the other way around), coolant sensor uses a separate circuit to the cluster, oil pressure switch goes straight to cluster, and fuel gauge has no interaction with the ECU.

When I decided what I needed and didn't, I just went page by page through the factory service wiring diagram and followed the circuits I knew I needed all the way from component to sensor, power, ground, etc, and figured out what was necessary to keep or not. If there was a whole page on the wiring diagram that I was deleting (HVAC for example), then I'd just place that page in a separate pile labeled "delete". Then other pages I marked up with what wires and circuits were going to be cut out. Then using this info, I populated connector pinouts of the main harness connectors, and labeled which ones I could cut out. Then I basically cut those wires at the connectors, and followed them through the harness to remove what ever was at the other end. This led to taking out the entire engine harness, which included all ECU connectors. YMMV because I deleted a lot of stuff, but the ECU specifically should be able to be taken out of all V8 swaps.
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Old 06-22-2017, 01:21 PM   #3
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I am not real electrically inclined and am having a tough time making heads or tails of the various wiring diagrams so I did what I probably should have done from the start.

I called Flyin Miata tech support and found out from the guy on the phone that the Miata PCM is not needed after doing the LS1 swap.

BIG relief.
Guess I will have a couple of large connectors loose under my dash.
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Old 09-26-2017, 12:25 AM   #4
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Hi! So on the cooling fan question just wire it like the camaro was but using the Miata fans. There is 2 fans on the Miata just like the Camaro had but there is one slight difference in that the Camaro had 3 relays to run them and the Miata only has 2. Which is logical since there is 2 fans and 2 control circuits coming out of the LS1 computer.

Hopefully the attached diagram will help you navigate this. Do you have the wiring diagram for your Miata? You will need that and it is in the builder resources section of this site.

OK on your LS1 computer there is a red and blue plug. The low speed fan is controlled by wire 42 on the blue plug and the high speed is controlled by wire 33 on the red one. These are ground side switched which means the computer doesn't send power to activate the relays it sends ground.

So on the Miata side it's a simple matter of turning on the key and seeing where you have power on the control side of the relay. One wire will have nothing and if you hit it with an ohm meter will register nothing and the other wire will show 12v power. The wire you want to hook to the ls1 computer is the one that shows nothing.

When you are done your wiring will look just like the diagram here but no fan 3 relay and it will work perfect.

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Old 09-26-2017, 09:05 AM   #5
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Thanks for the diagram but it is a few months late for me as I sold my Miata ECU with the engine.

This diagram is my best guess for the correct way to wire up the fan using the LS1 ECU.
Will this work or do I need a relay in the loop to keep from burning up the ECU?

Last edited by BGordon; 09-26-2017 at 10:13 AM.
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Old 09-29-2017, 02:15 PM   #6
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Definitely need a relay
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Old 09-29-2017, 03:51 PM   #7
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Thanks for the confirmation.
I did go ahead and put in a relay and changed the fuses to 30 amp. The wires are 16 ga. which is acceptable for that size fuse with only 4 ft. of distance according to a couple of online reference charts.
Can't seem to imbed an updated diagram because the way images get imbedded has changed since my last post.
The fans are working now, which is my goal.
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Old 09-30-2017, 10:39 AM   #8
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I'm doing a JV6 swap and keeping it in place primarily so I can still pass OBD2. On the plus side though doing so means I can use it to run my tach, speedometer and water temp gagues so I will not have to purchase new gauges or decoder boxes for them (keep in mind the JV6 conversion still uses the Miata trans and therefore the speedo sender so this makes sense) and my fuel pump, cruse, fans, etc.

At this point I'm hoping all I need to do to at least get tachometer/speedometer/H2O functionality out of it is adding a 1999/2000 Miata crank trigger wheel and sensor to the front of the J32a3 crank pulley as well as adding the Miata water temp and air temp senders. If need be I may also need to grab the cam signal from the J32a3's front cam wheel and somehow fool it into thinking the mass air meter is there (I may just cut the element out of the Miata tube and retrofit it somewhere in the new air stream). I'm also going to have to locate the Miata's O2 sensors on one side of the new exhaust. If I do all of that I may be able to convince it its still running a four cylinder and not have a check engine light.

When I built NA Ford Monster Miata conversions I completely removed the Miata ecu. In pre OBD2 NA Miatas the ECU really does not control much more than the engine so keeping it for anything other than fan control would be pointless. Even the cruse could run without it. The Miata tach reads directly from the coil, the speedometer is mechanical, the water gauge has its own sender, the alternator is internally regulated, and cruse just reads from the back of the speedometer. In an NB the gauge cluster has it's own mini computer in it that needs to talk to the ECU to do anything other than show the fuel level and run the idiot oil gauge (I'm retrofitting a pre-1995 NA sender and gauge).

-Jason
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