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From the test results of the MFI relay, it sounds like at least a defective MFI relay at this point. You can also check to see if the ground signal from the ECU (I think it’s a white/red wire IIRC) also drops. That ground signal is what activates the MFI relay as seen here:


Note: I think the wiring pins at the MFI relay are numbered differently on the 1G cars. But you’re still looking for the ground from pin 8, I believe.


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I agree with the reasoning provided to this point including beegeezy's. I think we can take it even 1 step further knowing that unplugging the airflow sensor (MAF) kept the pump and the car running. If crank/cam /run signal to ECU was dropping out or B+ voltage into MFI at pin 7 was lost, or MFI was bad and opening the circuit after only seconds,the car would shut down despite the MAF being unplugged...that didn't happen. So the ECU logic and/or some circuitry components failing is the branch point as to whether it gets ground to pin 8 W-R or not to keep the pump running. Confirmation done by seeing if ground suddenly lost on W-R at ECU end or MFI end. Pull ECU and post some good up close pics if that's happening.
 

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The most common problems are with the radial electrolytic capacitors failing from age causing erratic current in some circuits, which often have no visual signs. In many cases they leak electrolyte that can corrode and also damage near by components and/or circuit board traces, in these cases there’s usually visual signs of leakage as shown in link @OhioSpyderman linked about removal post #26. Sometimes the electrolyte will damage sub boards attached to main mother board because of ECU being mounted vertically on it’s side, see Important Note: Specific to repair for Stealth ECU (in red) ~ 1/3 down page of the below link.

No matter if there’s visual evidence of leakage or not, original electrolytic capacitors in a ’92 ECU needs to be replaced.



The IACV readings you got of 25-26Ω is probably OK if it is the unit with tan colored top, which should be in ~30Ω rage @ 68°. The later solid black units run ~40Ω @ 68°. Ohm readings a little off or infinity reading from a open coil will not damage ECU, just casue a malfunctioning IAC unit. Damage to ECU from IAC usually occurs when one of four coils shorts to housing (ground), causing ohm readings of near zero ohms and complete continuity to IAC metal housing can be read from pin #s for that coil. When that occurs IIRC what happens is burn driver boards that can be seen as in below picture.

View attachment 307542 click on image to enlarge
The IAC resistances seem fine to me as well since all 4 are so close to each other. Plus the IAC not working might allow the engine to stall if it was faulty but the engine would have also stalled when the maf was unplugged and when you ran pump straight from the battery which it did not. Also if IAC caused stall, the ground from the ECM to the MFI would not cease causing the MFI click and MFI voltage drop to 0V to occur until after the engine stopped ending the crank/cam signals to the ECU. That's opposite of your observations.
Finally, I said back in my post #18 and giving my reasoning why.. I believe the most likely problem is the ECU. I have tried to think of any "out there possibilities" to account for the MFI to click open before the engine stalls besides the ECU being bad and cutting the ground. Could the speed relay be filled with water or the speed relay coil be toast and directly shorted across itself, or relay pins be shorted to pin 1 causing voltage and amperage to quickly drop as soon as ECU applies ground to pin 1 to shift to slow speed circuit? If so could voltage drop low enough causing the MFI coil to allow the MFI pump contacts to open/click first followed by car stalling just after from pump stopping? Or could a shorted low speed resistor or a partially shorted circuit to ground exist after resistor and before splice connection with high speed wire? Using Ohm's Law at resistor(0.9 ohms) and beyond....V/R=I...so 11-12v/0.9ohms= <15amps fuse ...so even with dead short to ground fuse won't blow. Before relay and partial short to ground.... as voltage drops the MFI acts like circuit breaker and opens before fuse gets hot/amps high enough to blow? I guess if it were me, putting the mental masterbation aside, before I bought a new ECU I would take 15 min and pull the speed relay off the connector, use a 16G piece of wire stripped on both ends, and push 1 end into connector port 3 and the other end into connector port 2..safely set it down so it can't short to metal.....and start her up. If it shuts off buy/fix ECU..if runs make jumper formal,stable and wrapped/insulated.
 

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💪 (y) I always try and fix my stuff myself I will not go to a auto shop or stealership unless the sun is falling out of the sky. An added benifit is you get to learn how everything works.

That Capacitor kit from 3SX seems pretty convenient... I am going to look through my spare circuit boards and components first though before I pull the trigger on that.

How is 3SX for shipping times? I live in Vancouver Canada, just wondering if I did go the 3SX route how long (roughly) of a wait I would I be in for?





One day when I do start adding performance parts (it will happen sooner than later), then I will probably do the CHROME ecu, Wide Band 02, FPR, Speed Density, Cat-Back (or full) exhaust, higher cc Injectors, stand-alone MSI ignition, TIEN coil-over suspension, custom made & shaped intercooler piping, etc, etc... oh I can dream. One day when I have the money to do these things I will, and I can't wait until thar time lol.

Already have a Stage 2 clutch lol. Had it installed after I destroyed the clutch on the car shortly after I bought it 10 or 13 years ago when I was in my early 20's. I didnt know much about cars at all back then, so soon after that I got a truck and put my Dodge Stealth into storage under cover for about a decade. I am glad I 'took it off the road' when I did or else, I may not have still had the chance to drive the car today. probably would have crashed it or something would have went wrong due to my lack of understanding of cars at the time...

But alas! Back to reality!
Sorry I confused you saying FP speed relay. I call it that because most posts here or on internet using just FP relay are referring to the MFI. Remember non-TT 3000GTs don't have the FP speed relay so they say FP relay for the MFI frequently.
As far as your ECU, the damaged area you have involving the C6 capacitor is fairly common in 1st gens pics posted over the years. It can be associated with no starts, shutting down when hot suddenly, and MFI relay just clicking on and off. The C6 capacitor and board pads around it are usually burnt like yours. I doubt its just the acid-electrolyte fluid as that usually runs downhill towards the base of the small upright board you show with corroded leads since that side of ECU is mounted down. When you find pics of rebuilt ECUs of MD159965 -966s you see the C6 capacitor often replaced with different type of (not ceramic) capacitor of the same value...I can't give you value. There are kits for 3000gt ECUs like you mentioned that include the electrolytic capacitors but also include the C6s. 1991-1993 Mitsubishi 3000GT / Stealth ECU ECM Capacitor Kit w/ C6 SMD (N/A + TT) | eBay. This one is like $14.99.
To answer your question on testing relay, I would do as I suggested in my last post...pull the relay off the harness and jump the connector between port 2 and port 5 on the connector and start the car...it is testing the entire low speed circuit at once. If it keeps running then you can go back and test part by part the low speed circuit to repair it or just bypass it.
 

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Sorry I confused you saying FP speed relay. I call it that because most posts here or on internet using just FP relay are referring to the MFI. Remember non-TT 3000GTs don't have the FP speed relay so they say FP relay for the MFI frequently.
As far as your ECU, the damaged area you have involving the C6 capacitor is fairly common in 1st gens pics posted over the years. It can be associated with no starts, shutting down when hot suddenly, and MFI relay just clicking on and off. The C6 capacitor and board pads around it are usually burnt like yours. I doubt its just the acid-electrolyte fluid as that usually runs downhill towards the base of the small upright board you show with corroded leads since that side of ECU is mounted down. When you find pics of rebuilt ECUs of MD159965 -966s you see the C6 capacitor often replaced with different type of (not ceramic) capacitor of the same value...I can't give you value. There are kits for 3000gt ECUs like you mentioned that include the electrolytic capacitors but also include the C6s. 1991-1993 Mitsubishi 3000GT / Stealth ECU ECM Capacitor Kit w/ C6 SMD (N/A + TT) | eBay. This one is like $14.99.
To answer your question on testing relay, I would do as I suggested in my last post...pull the relay off the harness and jump the connector between port 2 and port 5 on the connector and start the car...it is testing the entire low speed circuit at once. If it keeps running then you can go back and test part by part the low speed circuit to repair it or just bypass it.
You said you've got electronics background and multimeter. On your ECU board, you need to put your meter on diode setting and check the silicon diode D104...it looks cooked..surface shiny cooked and all markings burned off...should look just like D104 and read about the same..around 0.6 -0.8V forward. Also,be sure to check resistor R125...950-1050 ohms. If any read off spec,realize you may need to lift one end leg off board and re-check em. Its a 4 layer board: traces on top, on bottom, and a few traces on two other layers between for 5v and ground traces.
 

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EDITED: You said you've got electronics background and multimeter. On your ECU board, you need to put your meter on diode setting and check the silicon diode D103...it looks cooked..surface is shiny with all white markings burned off...it should look just like D104 which is the identical component. Both should measure about the same...around 0.5 -0.7V forward is good. Also,be sure to check resistor R125...950-1050 ohms. Also check D8 surface mount diode If any read off spec,realize you may need to unsolder n lift one end leg of the component off the board and re-check measurement. Its a 4 layer board: traces on top, on bottom, and a few traces on two other layers between for 5v and ground traces.
 
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