Spyder New DSM P0300

[itty Bitty “Spyder”]

What’s up! I finally found another DSM.
What are some of the things I should check out?
What maintenance and repairs do I need for daily driving?

Thanks

 

[itty Bitty “Spyder”]

Ok I made it back with a cheap 35$ meter from O’Reillys. Im kinda bummed out because when I got home i noticed it only goes up to 12v dc. I know you guys been saying to stop relying on ai but I was able to confirm with google that my volt meter might get damaged because the highest setting is 12v dc.

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At idle-

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I reved it up a little bit but forgot to turn on ac and lights.

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I also had charging voltage at the b+ terminal on the alternator I wasn’t expecting to have charging voltage.

B+ terminal on the alternator

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Engine off

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Lights on,Blower on max,Ac button on, reved it up a little.

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[dwb]

Voltage looks good at the battery. I have to wonder why your scan tool is reading so much lower. Was the engine running when you used the scan tool?

 

[itty Bitty “Spyder”]

I went out and checked the battery and alternator with my scan tool again..Yes engine was running when I did the alternator test.

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[itty Bitty “Spyder”]

I honestly thought I was on to something when my scan tool told me the alternator was bad and my charging voltage was low but after seeing charging voltage at the battery I feel like I'm getting voltage from the alternator to the battery so I feel a little better about the wiring…. My scan tool is still saying the battery is bad and charging voltage is below the acceptable limit. Is my alternator going out? Thanks

Ok guys I’m sorry for relying on AI and Google but sometimes it's the only answer I can get. - A reading of 12.98 V during an alternator test is too low. When the engine is running, a healthy alternator should output between 13.5 V and 14.5 V to properly power your electrical system and recharge the battery. [1, 2]
The 12.98 V reading is effectively just your battery's resting voltage, meaning the alternator is likely failing to provide adequate charge and your battery is slowly draining.

Google - Yes, your alternator is failing or unable to supply enough power. A healthy, running charging system on your '99 Eclipse should output between 13.5 and 14.5 volts. A reading of 12.98 volts means the alternator is barely maintaining the battery and will cause electrical glitches and intermittent misfires (Code P0300). [1, 2, 3]

When the charging system voltage drops below 13.0 volts, the electrical current supplied to your ignition coils drops with it. This results in a weak, intermittent spark that fails to fully ignite the air-fuel mixture, directly triggering your P0300 (Random/Multiple Cylinder Misfire) code

 

[itty Bitty “Spyder”]

99Dsmer was saying to check the g control circuit first this is what I found online.

It is best to check the alternator B+ circuit first.
The B+ wire carries the actual charging current to the battery. If this heavy-gauge wire is loose, corroded, or broken, the alternator cannot charge the battery, regardless of what the "Generator G" control circuit is doing.[1, 2, 3]
On a 2G Eclipse GST/GSX, the Generator G circuit (pin 33 on the ECU) controls whether the alternator is in high-voltage mode (around 14.4V) or low-voltage mode (around 12.3V). When the ECU grounds the G terminal, the alternator drops output to low-voltage mode—typically to reduce engine load during heavy acceleration. If the G circuit is shorted to ground, the alternator will remain stuck in low-voltage mode and undercharge the battery. [1, 2, 3]

How to Test the Circuits
1. Check the Alternator B+ Circuit [1]

• • Test: Set your multimeter to DC Volts. Put the red lead on the thick white wire at the alternator B+ stud and the black lead on the negative battery terminal. [1, 2, 3]
  • Spec: With the car running, you should read standard battery voltage output (around 13.5V – 14.5V). [1, 2]
  • Voltage Drop Test: If you read low voltage at the battery but good voltage at the B+ stud, put the red probe on the alternator B+ stud and the red probe on the battery positive terminal with the car running. You should see less than a 0.5V drop. If it is higher, you have a bad or corroded charge wire. [1, 2, 3, 4, 5]

2. Check the Generator (G) Control Circuit

• • Test: Unplug the 4-wire connector from the alternator. Turn the ignition to the "ON" position (engine off). Check for continuity to ground on the G terminal wire. [1, 2, 3]
  • Spec: If you measure near zero voltage or a dead short to ground with the key ON, your ECU might be incorrectly commanding low-output mode, or the wire is shorted to the chassis. [1]
  • Troubleshooting: As a test, you can unplug the generator (G) wire from the alternator and drive the car. If the alternator's internal regulator is working normally, the alternator will default to high-output mode and charge the battery. [1, 2, 3]

Before doing these tests, review the ECMTuning Alternator Control Guide to understand the ECU's role. If the charging system is still struggling, check the related Generator Relay and the 100A Alternator Fuse under the hood, as these commonly fail or cause phantom battery drain. [1, 2, 3]

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In the DSMtuners community, "Itty Bitty Spyder" is the notable username of a forum veteran and trusted DSM Wiseman known for contributing heavily to 2G Mitsubishi Eclipse Spyder platforms and resolving engine issues like the P0300 random misfire. [1, 2, 3, 4]

I came across this when I was looking for my thread on google guess there is an Og on the forums with a similar user name cool! I don’t think I can change my username anyway it is what it is not intentional. Sry

Itty Bitty Spyder" is a recognized expert on the DSMtuners forum, known for technical expertise in 4G63/4G64 engines and ECMlink tuning [DSMtuners]. Detailed insights and specific build information from this user can be found in the DSMtuners forum archives, including a featured 1997 Mitsubishi Eclipse Spyder GST build [1997 Mitsubishi Eclipse Spyder GST Vehicle Profile & Build Thread Showcar Build https://www.dsmtuners.com/dsm-profi...ilt-tucked-engine-bay-street-show-vert.11452/]. For more information, visit DSMtuners.

 

[itty Bitty “Spyder”]

I feel like the alternator is going out do you guys approve of me taking the alternator out to bench test it or should I do some pinpoint test first thank you.

 

[itty Bitty “Spyder”]

Guys I also wanted to mention I used to have a 98 GST with this same code - P0300.
It had a 1g CAS for some reason. It had no balance shaft I'm guess it went went out and took out the crank sensor they switched it or it had a 1g head I’m not sure but when I would unplug the TPS the code would go away but I would have a TPS code. I’m sure you 1g in a 2g guys know about this I didn’t have DSMLink!

A failing generator field control circuit (often triggering code P0622) on your '99 Eclipse GST Spyder means the ECU cannot properly regulate the alternator's output. This usually results in an undercharging or overcharging state, presenting as dim/flickering lights, a dead or boiling battery, and intermittent electrical glitches like cutting-out radios. [1, 2, 3, 4]
Common symptoms to watch for include:

• • Charging System Warning Lights: The red battery icon and/or the brake light on your dash may flicker or stay illuminated. [1, 2]
  • Dead Battery / No-Start: Because the ECU is commanding zero field current, the alternator won't charge the battery, leaving you stranded after short drives. [1]
  • Overcharging (Boiling Battery): If the circuit shorts, the alternator may continuously run at full field, pumping 15V to 18V into the system, which can boil the battery electrolyte or cause a burning electrical smell. [1, 2, 3]
  • Engine Stalling & Surging: At idle or low RPMs, erratic voltage can cause your 4G63 engine to surge or even stall if the voltage drops too low for the ignition and fuel systems to operate. [1, 2]
  • Dim or Flickering Lights: Interior and exterior lights will dim or brighten abruptly as the engine RPMs fluctuate. [1]

Experts on DSM Forums and the Club 3G Forums note that nine out of ten times, this issue stems from a faulty internal alternator voltage regulator, poor ground connections, or corroded wiring in the harness, rather than the ECU itself.

Should the light come on when disconnecting the alternator-

Yes, the charging system (battery) light should come on. The warning light circuit detects voltage drops or a lack of power generation. When the alternator is disconnected, the car runs solely on battery power, and the system senses an immediate charging failure. [1, 2]

On a 1999 Mitsubishi Eclipse GST, the warning light is controlled by the alternator's internal voltage regulator, specifically through the "L" terminal wire. When you unplug the alternator, the light grounds out the circuit, triggering the dashboard warning. [1, 2]
Important Warnings:

• • Do not disconnect the alternator while the engine is running. This can cause a severe voltage spike that will permanently damage your ECU or other sensitive electronic components. [1, 2]
  • If you disconnect the alternator and the light does not come on, it indicates a burned-out bulb in the dashboard, a blown fuse, or a damaged wire in the alternator's wiring harness. [1, 2, 3, 4]

I wonder if disconnecting the alternator plug will test that that g control circuit I have to do same research.

Yes, disconnecting the alternator should trigger the charging system (battery) light, but doing so is highly dangerous for your car’s electrical system and is not a recommended test method. [1]

Why You Should NOT Disconnect the Alternator

• • Severe Voltage Spikes: Unplugging the alternator while the engine is running causes massive voltage spikes (often jumping to 16-18 volts). This can instantly fry your 2G ECU, sensors, and sensitive electronics. [1, 2]
  • Inaccurate Results: Modern electrical systems do not test well this way; it only tests if the alternator produces some power, not if it is functioning optimally. [1, 2, 3]

How to Safely Test the Charging Circuit
Instead of disconnecting components while the engine runs, use a simple multimeter (available at auto parts stores for about $15): [1, 2]

1. • Battery Test (Engine Off): Measure the battery voltage directly at the terminals. It should read around 12.2 to 12.6 volts. If it is below 12.0 volts, the battery needs to be charged or replaced. [1]
   • Alternator Output Test (Engine On): Start the car and measure the voltage at the battery again. A healthy alternator should charge between 13.5 and 14.5 volts. [1, 2, 3]
   • The Light Bulb Circuit: On your '99 Eclipse GST, the charging light on the dash is wired in series to provide the initial excitation voltage to the alternator's field coil. If the bulb burns out or the 4-wire harness plug is corroded, the alternator will not charge. [1]

Ok that doesn’t answer my question about this alternator g control circuit we speak of.

 

[pauleyman]

Ok I made it back with a cheap 35$ meter from O’Reillys. Im kinda bummed out because when I got home i noticed it only goes up to 12v dc. I know you guys been saying to stop relying on ai but I was able to confirm with google that my volt meter might get damaged because the highest setting is 12v dc.

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The DC voltage settings are on the other side of the meter. You're on batt test. Go select DC 20 volts

 

[itty Bitty “Spyder”]

The alternator has two “modes” - low voltage/low load and high voltage/high load. The ECU controls which mode the alternator should be in using the “Generator G” pin on the ECU harness (pin 33). When G is grounded, the alternator is put into low-voltage mode (around 12.3v). When G is left floating, the alternator is in high-voltage mode (Around 14.4v).

Think that answers the question about how to test the g controle circuit but do I need to check this at this point think the voltage regulator is going out and that is why the light is not coming on.can I just replace the thing or am I forgetting something thanks

 

[itty Bitty “Spyder”]

Pauleyman?

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[pauleyman]

Pauleyman?

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Yes that's correct. You can google why there are two different settings and what it does.

 

[dwb]

It never hurts to get the alternator tested. For piece of mind.
*I* believe your alternator is ok but you have voltage drop somewhere. I'd start by making sure your grounds are tight, clean, and secure. Battery to starter bolt and firewall are the two main grounds. Then make sure you have a ground strap from the firewall to the intake manifold. I know there's a diagram around that shows all the additional grounds that might be useful for you. It's in the Factory Service Manual, which you can find in this thread.

2G - 1998 Mitsubishi Eclipse and Spyder Factory Service Manual Volume 2 - including Electrical

1998 Mitsubishi Eclipse and Spyder Factory Service Manual Volume 2The binding on my copy was falling apart so I decided to end the fight and scan in all the pages. The big difference here - I scanned in the complete second book by section so you no longer need to download 2000+ pages of a...

www.dsmtuners.com

 

[pauleyman]

Please stop posting all your google and AI results. Your posts are extremely difficult to read. Ask your question and we will answer. Before I even did any research I would've tested battery, cleaned and checked grounds, checked ALT fuse and THEN I would've started testing alt etc if necessary.

 

[itty Bitty “Spyder”]

Ok I went out and tried testing everything on the 20v DC scale!

KOEO - engine off

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Koer - engine on

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Loads on

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Load on - rev engine

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I really don’t want to replace an alternator and have the same problem can I trust my scan tool or do I need to test electrical.

 

[itty Bitty “Spyder”]

20v DC scale alternator b+ to battery ground

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[itty Bitty “Spyder”]

Batt. And alt.

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[itty Bitty “Spyder”]

Can anyone give me step by step instructions on how to confirm that all wiring is good before removing the alternator thanks

 

[pauleyman]

Can anyone give me step by step instructions on how to confirm that all wiring is good before removing the alternator thanks

My apologies but this needs to be said. You went to UTI but don't know how to test wiring?

For the larger gauge stuff its probably enough to just make sure the ends are clean and mating surfaces are clean. You can always test continuity and resistance on grounds.
What problem are we chasing anyway? Your last voltage test looked ok to me. What is the car doing that you're trying to solve? The p0300 misfire? Your charging system wouldn't be my first suspect.

 

[itty Bitty “Spyder”]

The last picture I posted with the DMM was available voltage at the alternator B+ - forgot to do a voltage drop of that B+ to the battery negative? (Mod - Wrong, you test voltage drop from Alternator B+ to Battery Positive. You're looking for the voltage difference between two points)

Thanks yes I graduated UTI back in 09 and never worked doing electrical so I don’t have as much hands on experience as I would like.
Now I am trying to refresh as much as I can to get ready for ASE certifications I never got after graduating. At school we had software - pinpoint test it would tell us check available voltage at the battery. It’s good do this next. it is bad this is your problem. Here you dude it’s embarrassing but 2009 was a long time ago and I don’t do these things everyday.

 

[pauleyman]

Thanks yes I graduated uti back in 09 and never worked doing electrical so I don’t have as much hands on experience as I would like.Now I am trying to refresh as much as I can to get ready for ase certifications I never got after graduating.At school we had software-pinpoint test it would tell us check available voltage at the battery. it it’s good do this next.it is bad this is your problem.Here you dude it’s embarrassing but 2009 was a long time ago and I don’t do these things everyday.

Fair enough. I forgot what I did last week. Clean connections. Run continuity checks as necessary on smaller wires. On charging systems i find corrosion is the biggest issue.

 

[itty Bitty “Spyder”]

Yes I am considered a professional technician “the best of the best” I graduated from Ford, fact which is a top-notch electrical program.

 

[itty Bitty “Spyder”]

thanks terminals look clean

One day I will live up to the school moto- a professional technician the best of the best!

 

[dwb]

I really don’t want to replace an alternator and have the same problem can I trust my scantool or do I need to test electrical

I trust the volt meter over this scan tool. However, I think it's pointing to a possible wiring issue.
I'm not a fan of your negative battery terminal. Those wire clamp-type are notorious for poor connections and corrosion that still "look" ok. You also have a mess of wires that should be gone through, making sure all the splices are good and tight.

 

[itty Bitty “Spyder”]

I’m definitely a little ruff around the edges. I see room for improvement but. Have potential. Like that Supra from F and F! Lol

I agree Dwb- I think I’m in denial Of the wiring issue because I don’t fully understand it but honestly would diagnose it before uninstalling any parts. Yea, I noticed a bunch of aftermarket wiring and suspected this

Thanks what should be my first pinpoint test and I can go from there.

How to do pinpoint test on a 99 Eclipse Spyder GST charging system? I See allot of possible answers but no accurate pinpoint test?
I’ve been asked not to post AI or Google answers so I’m just going to wait for an answer. But the answer I’m looking for, is a step by step process. I think we have already completed the first step by the way.

 

[steve]

While I don't care for either of the two battery terminals I don't see a problem with the charging system based on the posted voltmeter measurements.
You might want to try measuring inside the car like at the ECU, OBD2 connector, or the cig lighter to see if that's where the scanner is getting messed up.

I'd think at this point going back and focusing on other potential causes of the P0300 random misfire would be a better use of time. This round don't put so much weight on what Google or it's AI says. Research is good but posting every 5 mins or so doesn't leave much time for thinking or getting your hands dirty.