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Discussion Starter · #1 ·
I have just finished putting my car back together.
Turbo 1.6 with megasquirt.

Got it running and everything was working. Started the running in procedure and it was all working fine.Started on the nose every time and idled and ran perfectly.

Went back to it yesterday (after about 24 hours of sittng) and had a wierd problem.

1. Fuel pump primes, but kept running.
2. Starter would not turn over.

There is fuel, and it is getting to the rail and back again
I can turn over the engine by hand, so it is not siezed


I found this thread: www.miataturbo.net/general-miata-chat-9/fuel-pump-wont-stop-priming-98026/

This says that it is the light green wire that triggers the fuel pump, and it should be grounded to make the fuel pump run. i.e it should not be grounded after the fuel pump has primed.
On my car it is grounded when the key is in pos 1 and 2 and doesn't 'un-ground' itself.

Anybody got any ideas?
Sounds like it could be a megasquirt issue?
The only changes i made between it working and not working were to plug in a hazrad/pop up switch (whic I have no unplugged just to amke sure!)
 

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Have you done the tests in the link? can’t see it being megasquirt related.
More likely linked to your recent loom change.
Double check all earths to the pump circuit.
 

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M52TU Turbo Swapped MX5 (No that's not a joke or a typo)
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That seems a really odd combination of issues.

So not entirely sure what year your car is but looking through the wiring diagrams for a 97, 96, 95 and 92 MK1 and the starter is independent of the ECU. The starter links to the Ignition switch, Starter interlock and Main fuse block (on the 95/96 I think there is a starter signal fuse so maybe check that if yours is 95/96)

as for the fuel pump try the troubleshooting in the article you posted if you haven't already.
 

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Ok did some digging and based on another thread I'm guessing you have a 93 (correct me if I'm wrong)

but here is the starter diagram:


No link to the ECU so your megasquirt shouldn't be causing the starter to not run. According to the diagram the 93 also has the "ST SIGN" fuse so check that and see if its blown
 

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Discussion Starter · #6 ·
thanks all for the input.

it would be nice if it wasn't the megasquirt.
I had some earthing issues previously, but thought I had sorted that out.

It is a 93 engine with a different loom (i broke the previosu one) from an unknown year... but (and I will keep saying this to myslef in a whiney voice) it was working the day before...

Will go earth/fuse/relay hunting this evening.

I really can't afford for this to be a big problem. It's gettign shipped to the US imminently and it needs to be running or it will cost me a fortune.
 

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Discussion Starter · #7 ·
Just spent a bit more time prodding around.


Fuse
I don't remember having to remove one, but it was running for a few years before this rebuild, and it was running 2 days ago...

Tests in my linked miataturbo post
Yes - i looked at that, and the post said that the light green wire should be grounded (by the ECU) when the pump should run.
I found that it is grounded when the ignition is in pos 1 and 2. I think the pump should only run for 2 seconds (?) when ignition is moved to pos 2.
What i did notice is that the multimeter normally goes to zero when testing a ground, however this time it showed some resistance.
I think this points to grounding issues?

I also tried going into the test mode in the megasquirt and trying to run the fuel pump form there, hoping that i might be able to turn it on and off, but that didn’t work.
The miataturbo post states the ecu switched the light green wire to ground to run the pump, but I can’t trace that wire back to the ECU. Does anybody know which wire that is out of the ECU?




Testing the voltages from igntion switch
My next test was to look at the voltages out of the ingition switch, and i got the following - which I also think is wierd.
The down arrow indicated that the voltage was slowly dropping.


Given I have had earthing problems in the run up to getting the engine working, it looks like it is more of the same.
What I don't understand is that I have done nothing to it between it working, and not working.

If the starter is only getting 11.8, would that prevent it from running?


I guess my only option is just to go hunting round the loom checking earths.
 

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11.8v doesn’t sound healthy, if your battery is putting out more than that then it looks like you have another earthing problem.
 

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M52TU Turbo Swapped MX5 (No that's not a joke or a typo)
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So a TIP125 is just a PNP power transistor so assuming that nothing else is toast yeah you can replace it. You can pick up a new one for pennies so if you're competent enough with a soldering iron give it a go!
 

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On the original part in the photo

Mospec - is the manufacturer
TIP125 - is the actual Transistor type (5A 60v PNP Darlington array power transistor in a TO-220 package)
MSO 4N - Is likely the manufacturer part number
 

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Discussion Starter · #18 ·

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SMD markings are slightly different, in this case the 2 letters are a code that Identifies the type, the numbers identify the year and month of manufacture. So specifically that is a GF1B diode manufactured by Vishay in a DO-214BA package (The 49 would likely be September 2014). Here is the datasheet for it: https://www.vishay.com/docs/88617/gf1.pdf

with this one, because the code on the part is reference to the specific manufacturers datasheet it would be best to stick with the exact same part. Thankfully if you google GF1B you get results from Farnell, RS and digikey etc so it shouldn't be too hard to find and order the right part

Hope that helps
 

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Discussion Starter · #20 ·
How can you weld and know about electronics?...
I should understand electronics - comuter science at university, and maths/physics/chem for alevels. And i don't think i am stupid... but it is utter witchcraft.

Thank very much for helping with this btw,,,
 
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