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1G Replacing 1G MPI Relay with Bosch-style Relays

In this guide I will be describing how I replaced the MPI relay for a 1990 Eclipse GSX with 3 Bosch-style relays. This approach is useful if you had a MPI relay go out and you don't want to pay over $100 for a new one or if you are worried about sourcing one of these units in time for a track-side repair. Keep in mind that at the end of the day it's probably still quicker/easier to just buy a new MPI relay but as these get harder to find I like the idea of having an alternative strategy.

Disclaimers:
  • This has not been tested very extensively. I put this in my car and it started/idled as it should which makes me feel pretty confident that this works. However, in testing this I determined that the rest of my wiring harness was too far gone to continue without redoing a lot of it first (it's a hot mess of shorts and random wires). I will continue to use these relays after I get the rest of my harness more sorted and will update this post then.
  • I believe this would work for any 1990-1994 4G63 cars as the MPI relays I’ve seen for sale are advertised as working for all these cars but I have only tested this on my 1990 GSX so proceed with caution if you have a newer 1G. If you know a reason why this shouldn't work for other 1Gs it would be much appreciated if you left a comment here!
Guide:
First step is to remove the old MPI relay from your car. It looks like a small golden box and is located in front of the ECU under the radio. Just remove the two bolts holding it on in this location. Then unplug the relay from the connector so you can remove the relay from the car. For me it was easiest to use needle-nosed pliers to get the connector off.

Once you have removed the relay from the car I would highly recommend maneuvering the connector back behind where the ECU is so that it is poking out in the location shown in the image below. This will give you a lot more slack to work with the wires as well as making it much easier to see what you are doing.

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The MPI relay is essentially three relays in one and we are just separating them out as seen in the diagram.

For more info on how the MPI relay works check out this very helpful thread (https://www.dsmtuners.com/threads/1g-basic-ecu-mpi-circuit-function.435961/).

Below is the electrical diagram I made to show how the wiring should be done. The circles indicate wires that will need to be spliced together. The two letters on the MPI pins are labels indicating wire colors (I also tried to show this on the connections themselves, dashed line represents a white wire).

I also included a diagram from Project Zero G's website which will help locate the MPI pins in the connector.

How exactly you wire and mount your relays is up to you. I would recommend wiring up the splices that go between relays on the bench and then cutting off the old MPI connector and attaching the relays last. I found keeping good notes and labeling as much as possible to be very helpful, just like with most larger wiring projects.
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Feel free to let me know if you have questions. If there's demand for it, I could make a video going through this wiring diagram and showing in more detail how I wired the relays once I have tested this setup a bit more.

Hope this is helpful.
 
Last edited:

Laika

Probationary Member
17
12
Jun 6, 2019
Dundee, Oregon
Update:

I thought about it a bit and decided to switch to using actual Bosch relays with flyback diodes. It's only necessary on the two relays that connect to ECU pins and it may even be overkill there but I'd rather be safe than sorry, especially since it looked like the MPI relay had a diode in it. In case anyone's wondering I got Bosch part # 0332019155 relays off Amazon and they were $9 each. I had to make some changes to my electrical diagram for using relays with a diode so I went ahead and replaced the diagram in the original post with the new updated one.

I'm going to wire this up with the new relays and hopefully try it out this weekend to double check it. Then on to making a new wire harness.
 
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