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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8612
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Mitsubishi Eclipse Spyder GT: 2001 Mitsubishi eclipse spyder

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2001 Mitsubishi eclipse spyder gt 3.0 v6. 4speed auto tranny model f4a51. Have two issues - at idle battery and brake light in dash illuminate but go out when rpm is raised. Replaced alt & battery still does it . At idle alt not charging only shows 12.6v increase rpm shows 13.8 . Second and biggest issue and the one that's costed me the most is code p0715 turbine input speed sensor! Started off by changing sensor .worked about a week then same thing .Decided to service transmission - drained fluid found metal shavings. So I pulled trany had to pull any ways beings you have to split cases to change filter any ways !!! Dumb design but way to make money !!! Took tranny apart and found that output planetary spyder gear had a sheared dowl pin causing spyder gear pin to rub on overdrive planetary !!! So was like no prob I got this ordered overhaul kit & planets from wit transmission parts. Replaced all o-rings clutches cleaned valve body installed trany didn't even get two blocks p0715 holy crap all that for what ? Although I did find something wrong but not the issue . Reyes replacing neutral safety switch no affect to back got money back . Put original sensor back in for turbine speed sensor . Worked fine for two weeks shifted perfect was like thank god finally but bam same thing again !!! Please help

As I'm sure you've figured out the Battery/Brake light combination is a charging output problem. Any time the system voltage drops below roughly 12.5-12.8V the computer interprets this as a no-charge condition as it assumes it is seeing battery voltage only.
When certain conditions occur in the charging system you will result in just enough power output to keep the vehicle running without depleting the battery, but not enough to put it into "normal" range and have the lights go out.

There are two causes for the charging output to drop like this (apart from wiring/cabling problems of course), the alternator and the PCM.
Even though this vehicle uses an alternator with an internal regulator, the regulator is commanded by the PCM based on system load. If the PCM does not output correctly to the regulator, charging suffers.
For what it is worth, stock alternators rarely fail in this capacity (where revving brings the charge up), they are usually all or nothing.
Aftermarket alternators for this car are quite common to perform this way brand new (avoid them like the plague), and PCM internal failures are somewhat common to cause this.

With regard to your P0715.... I'd be willing to wager this is an aftermarket input shaft sensor? Not to be overly hard on aftermarket parts, but electronics on Mitsubishis are extremely sensitive and hall type sensors pretty much never work on these cars... more than a few of these have been towed in for no starts because of aftermarket hall sensors (crank etc) that were replaced but didn't fix anything due to the quality of the new part.
Beyond that, you really only have wiring or PCM to consider here since you've been through the transmission. Theoretically we could see the reluctor alignment be off, but it would have been corrected during reassembly.

Are you familiar with how to test the alternator regulator input on this car?
Customer: replied 4 years ago.
So more then likely the PCM is causing both issues !!! Not sure what wires to check on alt.
Customer: replied 4 years ago.
Going to get oem sensor and try it . Can you tell me what wires at PCM are the T I S wires? So I can ohm them out as a double check . Thanks
I apologize for the delay, our notifications have been a bit haywire lately.

Yes get the factory sensor in there from a dealer only and compare notes on the P0715... odds are it will resolve then, especially considering putting the original back in improved the condition for a short while.

With regard to the alternator, you need a multi meter, some T pins or similar to help in probing the wires.

On the back of the alternator is a four pin connector. We are interested in pins 1 and 4, the outer two.
Turn the key on and probe one pin for voltage (other lead to ground) and note the voltage. Turn the AC fan on and note the voltage. Turn the high beams on and note the voltage. Turn the defogger on and note the voltage.
Perform the same test incrementally adding loads while measuring the other pin.

You should see inverse voltage changes as loads are added... each time a load is added the voltage will increase one one wire and the voltage on the other wire will decrease.

What we often see is a driver burn out on one of these wire in the PCM and you will get the voltage (example) increasing on one pin but the other pin does nothing. This is when you normally get stuck with 12.5V etc.

If both pins invert voltage as loads are added, the PCM and wiring are good. If you see a problem, inspect your wiring and connections for damage. If nothing is found, the PCM will need to be replaced (and keys reprogrammed).
If the PCM and wiring are good as displayed by the pin tests, you will need an alternator... and this is another case of OE or bust. The track record on the aftermarket alternators is abysmal, mainly due to the partial internal regulation that is not used on other models.... the aftermarket suppliers just cross over based on amperage and mounting ears with no regard for the regulation differences on these Mitsubishis. Luckily Mitsubishi alternators are pretty reasonably priced compared to most manufacturers.

If you have any questions just let me know.
Customer: replied 4 years ago.
What plug and wires are for turbine speed sensor at pcu
Give me just a moment and I will pull the schematic for you.
Here is the pin out for the PCM. Note that it is looking at the wire side of the connector, buttons on top.
Pin 103 is the 5V reference voltage for the input shaft sensor.
Pin 57 is the return ground from the input shaft sensor.

The easiest way to check these is to probe for 5V on pin 2 of the sensor harness with it unplugged and key on. If you have 5V, you know you are good there. If you don't, then proceed to check voltage at the PCM and if you have it, check your wiring (if you don't, the PCM is kaput).
Pin 53 should have ground, checked at pin 1 of the sensor harness unplugged. If you do, then all is fine. If there is no ground, check at the PCM pin 53. If ground is fine, then check your wiring. If there is no ground, the PCM is kaput.

If you have an oscilloscope, the preferred test would be to probe between 1 and 2 of the sensor with it plugged in or 53 and 103 of the PCM with the sensor plugged in. With the transmission in 3rd gear and the engine at 2000 rpm you should see a clean 5V squarewave. If it is less than ~4.2V or so, there is a problem. If the squarewave is jagged or inconsistent, there is a problem.
Doug and other Mitsubishi Specialists are ready to help you
Customer: replied 4 years ago.
Your rite about aftermarket sensors don't use them got got new one from dealer. Car drives like new. Thanks for your help
Haha, fantastic!

I had another customer just a few days ago that I suspect is going the same way (I'm waiting to hear back on the test results). It's pretty much a once a week to once every two weeks thing on here, and about once a month at the dealership in person. No one wants to hear that aftermarket sensors aren't any good, like it is just trash talking.... but they really are that bad as you've seen.

If there is anything else I can do for you just let me know. If you want to be sure to deal with me in the future just put "For Doug Only" on your question to alert others you are waiting for me (I am the only Mitsubishi employee here).

Have a great day!