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Greetings all. Please Help!
My Miata was driving wonderfully for a long time. I moved so I didnt drive it for about a year. I charged the battery to go to a track day and it started up awesome as usual. I got halfway to the track, turned off the engine to get gas and it wouldnt start again.
No power to the fuel pump.
I wired the pump directly from the battery and got to the track and back home.
I traced the issue down to the fuel pump / circuit opening relay because I couldnt get power to the fuel pump even with the fuel pump pin jumpered to ground in the diagnostic electrical box. I replaced the relay and it will now run with the fuel pump pin jumpered to ground in the diagnostic box but it will not power the fuel pump without the jumper.
I have been investigating this and have found that the LT GRN wire which used to get grounded by the OEM AFM is not getting grounded.
Has my gen1 megasquirt pnp given up the ghost at the same time as the relay?
Did one issue cause the other?
How do I check if the megasquirt is sending the right signals to the fuel pump circuit AKA grounding the LT GRN wire?
The VIO wire is sending a constant 12v signal when the key is on. The ground at the fuel pump relay works. If I jumper the fuel pump relay connector WHT-RED to BLU-RED the fuel pump is powered with key on.
The pin on the fuel pump relay connector connected to the LT GRN wire is a functional ground when the FP pin in the diagnostic box is jumpered to ground, but if the diagnostic pins are not jumpered, the LT GRN wire pin will not function as ground with the key on or during cranking. My cranking settings in tunerstudio are set to always prime with key on and turner studio is showing the fuel pump as turning on.
Beyond all this im not sure what to look for next.
Thanks so much!
Jon
Last edited by HMB; 04-15-2024 at 02:20 PM.
Reason: additional info
Here is the diagram I am working from. I have the LT GRN wire and the fuel pump switch highlighted. Im not sure how the MSPNP takes over duties of the fuel pump switch in the AFM. I have verified that the grounds in the AFM connector are functional.
Thanks for ANY input.
sounds like you need to probe around that relay and test the relay. They are known to fail, especially with aftermarket fuel pumps that draw more current.
ECU/diag box/AFM are either/or signals for closing that relay. If all 3 are not working, the relay is most likely the culprit. The car would not run with the jumpered pump if your issue was main relay (wht/red)
Ted,
Thanks for the response!
I like that your diagram shows the connections to the diagnostic box. It looks like that is missing from my diagram, but other than that they seem to be the same, am I missing what you saw?
I agree, that the main relay is OK.
I have bench tested the new circuit opening / fuel pump relay by putting +12v on the relay contact that goes to the violet wire and the negative on the relay contact that goes to the black wire. This changed the state of the relay / closed contacts that power the fuel pump circuit (WHT-RED and BLU-RED). The old relay that I replaced failed this bench test so it was definitely bad.
I understood that the relay needs both key on and the AFM switch to close to the energize the fuel pump. But the bench test did not indicate that that is true. When installed in the car, the relay does not function as it did on the bench, AKA close the fuel pump circuit when there is +12v to the violet and ground on the black. I am guessing that this is because when the relay is installed in the car there is also 12v to the WHT-RED which is affecting the relay's ability to close (maybe it needs ground on LT GRN also?) (I can try this bench test later today)
Attached is a picture of the relay schematic. Can someone explain this to me?
What I have figured to correlate the relay schematic and the miata fuel pump circuit schematic is;
1=BLU-RED - wire to fuel pump
+2=LT GRN - wire to fuel pump switch in AFM
+3=BLK - ground
4=WHT-RED - power from main relay
5=VIO - power from ECU - ST SIG fuse removed.
Yes, it was the DIAG connection that I was looking at.
I am not sure I understand your question. The Key energizes the main relay, which supplies power to WHT/RED. At this point either grounding LT GRN (via AFM or the diag box), or energizing VIO via the ignition switch will close the relay. So during cranking, where very little air is flowing through the AFM the relay is closed via the "key", and then once the car is running and drawing air through the AFM, it keeps the fuel pump turned on. This way if the car stops running, the fuel stops until the operator cranks the motor over via the key.
Stand-alone installs remove the STOP_SIGN fuse and then supply 12V to the VIO net directly from the ECU, which allows deletion of the AFM if desired. With a stock install, the VIO net is only energized during cranking.
Progress: MSPNP isolated as cause. No signal power to FP relay
the 12v I was reading on my multi meter was a ghost voltage (I made this term up?) it was not enough power to energize the relay coil / change the state of the relay, I found this by testing with a small test light.
Ted, as you stated, and helped me determine, the issue is the violet wire.
I get a strange reading from the violet wire / its connection at the MSPNP at initial key on its 12v then it drops down to 1v for a few seconds then it stays back up at 12 until key off. again, this is only a trace voltage and nothing shows on a test light.
The 12v signal also drops down to 1v during cranking.
I know now the issue is in the MSPNP. What do I do next?
I have had the MSPNP apart before and will look inside, but if any one can get me on the next path of troubleshooting its components that would be great. Please please.
What would cause the megasquirt to show 12v of ghost voltage when the fuel pump should be off and drop down to 1v when it should be sending 12v to the fuel pump relay/ when the fuel pump should be on? What ever it is, when it failed, it likely killed the coil in the old fuel pump relay too.
Have you confirmed in test mode that you don't get 12V? The prime pulse at key-on is super short. The MS won't turn the FP back on until it detects cranking.
There likely was a short 1v period at initial key on for prime pulse that I missed / didnt note.
What do you mean get 12v in test mode? With the FP jumpered in the diag box? why would this affect the signal from the MSPNP?
As I recall I did key on key off cycle excessively when the power to the fuel pump first went out. A fail to start had happened a couple times before and I couldnt trace it down, but key on key off cycles worked so I tried it a lot when it wouldnt start back up on the way to the track. Maybe I burnt something in the MSPNP due to trying to get a failed fuel pump relay to activate?
I opened the MSPNP and did smell a smell. There is some light corrosion in spots on the board but I dont visually see anything obvious....
There is an output test mode via tunerstudio that you can use to test the fuel pump output directly.
It has been almost a decade since I have supported any MS2 HW, but if something did let go inside I would think DIYautotune or maybe even google would be able to tell where to look.
Ted, Thanks again for your time and putting me on a good direction.
I have reached out to DIYautotune about the issue and will let you know how this works out.
Thanks.
After less than a week DIYAutotune responded to my email and said the issue was likely the Q5 transistor. I had it replaced and POW! the fuel pump operates as it should.
Thanks Ted! Thank you to this community and this forum for the countless hours I was able to spend on it getting this ecu dialed initially many years ago, and thank you to DIYAutotune for supporting a legacy product. Excellent product support!
Kudos All!
04-14-2024, 10:21 PM
Fuel Pump Circuit Issue
0
Fixed!
