MPI Relay not triggering

jeffball610 · Post by **jeffball610** » Tue Aug 09, 2016 10:04 am

I'm in the final stages of getting my Datsun 510 project #4G63510 running and have hit a road block. The first issue is the MPI relay not triggering properly. I might also have some issues with not getting a signal from the CAS and that in turn sending a signal to get spark and fuel, but let's tackle one at a time.

I believe I understand the proper function of the MPI relay and Bill did a great job explaining it to us: http://projectzerog.com/forum/viewtopic.php?t=1501

I've got constant 12v on the Black/Red wire (BR) and a good ground on the solid Black wire (B). There is also some voltage running through the Black/Blue wire (BL-Bill has them listed as just Black). When I turn the key on, I get 12v in from the Black/White wire (BW), BL goes to ground, I get 12v from the two Red wires (R), and I get 12v at the White/Red wire (WR). However, I do not get a signal on the Black/Green (BG) wire to trigger the fuel pump.

My current solution is to simply trigger my remote fuel pump relay I mounted using the BW wire. The fuel pump comes on, but does the ECU need to see a signal here? Does the WR wire need to do something inside the ECU to make other things function?

If this is not a critical system, then I'm fine with bypassing it. However, if this is needed so the ECU can make other things function, then I need to have it resolved so I can troubleshoot other things.

jeffball610 · Post by **jeffball610** » Sat Aug 13, 2016 2:21 pm

Whoops. This should be a part of the "no CAS" thread.

No help huh? This is what I've done to test:

Pins #1 and #2 on the CAS test as 5v with ignition on. #3 tests as 12v and #4 is a ground. So wiring should be fine. I don't have a testing procedure for the CAS itself, but have tried the one in the car and a separate unit by spinning them and there is no reaction from the ECU or motor letting me know they are working. They could both be bad, but that seems unlikely.

Is there a way to "manually" send a signal to the ECU and see if the coil or injectors will react? Maybe ground one of the 5v wires? I'm at a loss. What else can I test?

jeffball610 · Post by **jeffball610** » Sat Sep 03, 2016 3:20 pm

Still no responses huh?

My next thought is the MPI relay isn't functioning correctly. I do get it to trigger on one half. I have constant power and a good ground. When it receives ignition power, it does trigger the ECU to go to ground and send power to the sensors.

However, it does not trigger the fuel pump circuit. In the previous post, I mentioned that I bypassed this since I was running a remote fuel pump relay. I still don't quite understand how this section functions or why it's necessary. As far as I understand it, the fuel pump should trigger as soon as it receives ignition power. I'm still not clear on what the White/Red wire does. The Black/Yellow wire is for the starter solenoid and I think when the MPI receives this signal, it also sends power to the fuel pump while cutting power from the Black/White ignition wire.

I know I'm missing something simple here and I can't see it. Literally, ANY suggestion is welcome. My only other thoughts were the CAS or ECU. The CAS seems fine and I swapped in a known good ECU and there was no change. I could really use some help. It's almost on the road after 11 years of sitting.

turbostellar · Post by **turbostellar** » Mon Sep 05, 2016 5:34 pm

I believe there is another power wire to power up the other side of the relay to supply current to the pump during cranking. It's been so long I don't quite remember. If your current setup is like what I had. As soon as you stop cranking you hear the fuel pump power up

jeffball610 · Post by **jeffball610** » Tue Sep 06, 2016 5:26 pm

There are 2 power wires from the ignition. One is for run, BW, and the other is for start, BY. I believe you're talking about the BY wire that goes to the starter solenoid. I believe it does cut power to that part of the relay, but I still can't figure out why it doesn't work.

I had another thought that maybe my ignition switch sends power in a different way than the MPI relay wants to see. I'll be testing that next, but I doubt that's the issue. This is so frustrating.

turbostellar · Post by **turbostellar** » Wed Sep 07, 2016 11:40 pm

Are you still using your Datsun ignition switch? If so you need a switch with a reference voltage for your ecu

jeffball610 · Post by **jeffball610** » Thu Sep 08, 2016 9:09 pm

This is what the stock system looks like

The WR wire (1) is fused power in.
The LB wire (4) is my fuse panel ignition on power
The BW wire (2) is my MPI relay ignition on wire and does have 12v with ignition on only
The BY wire (3) is power to the starter solenoid and the other part of the MPI relay
I have eliminated the BL wire (5) from the system as it shouldn't be needed. Maybe I'm wrong.

Does any of this sound wrong? Both the ECU and MPI have fused battery power. Maybe I'll do up a quick wiring diagram just to show what I have, but in my mind, everything is right. Obviously I've missed something or did something wrong.

turbostellar · Post by **turbostellar** » Sat Sep 10, 2016 1:19 am

Does be have 12v while you are cranking the starter? If not then your fuel pump will not run at that time.

turbostellar · Post by **turbostellar** » Sat Sep 10, 2016 1:21 am

Also make sure you are getting 12v to the ecu while cranking.

jeffball610 · Post by **jeffball610** » Sat Sep 10, 2016 10:47 am

I can check that today. However, the fuel pump portion shouldn't matter and I don't get why it's not working would matter. Since that portion wasn't working, I bypassed it and ran the fuel pump off my ignition trigger wire. I have a separate relay near the fuel pump and used it as a trigger instead of running power through the MPI relay.

I know the ECU is picky and wants the MPI relay for some reason instead of regular relays and such. But what I don't get it is why this part would keep it from running since it doesn't seem to control anything other than the fuel pump.

jeffball610 · Post by **jeffball610** » Sun Sep 18, 2016 6:44 pm

I'm officially wrong about everything. The MPI relay was triggering just as it should. The fuel pump does not fire with the key on, only during the crank and while running. I know this because #4G63510 officially started today. It ran briefly before a skyrocketing idle forced me to shut it down, but it runs. Video forthcoming.

All of my frustration was due to a bad connector on the CAS. The wiring tested good, but the connection to the CAS was bad causing the ECU to not receive the signal. Good times are ahead.