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Tom, ASE Certified Technician
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1991 Isuzu Rodeo: Blew ecm fuse and died along side road..hooked

Customer Question

We have a 1991 Isuzu Rodeo. Original symptoms: Blew ECM fuse and died along side road. Runs with both wires hooked to alt. and #7 fuse removed in the cab..but does not charge and guages do not work. When you have both wires hooked to the alternator with fuse in it starts and dies. When you unhook the small (Exciter) wire it runs fine off the battery but doesn't charge. We have replaced: ALternator, coil, Ignition Control Module, Pick up coil, Throttle Position sensor, Idle Air Control Valve, and etc..Could this be a ECM problem? Please advise It is not poping the ecm fuse after replacing.
Submitted: 2 years ago.
Category: Car
Expert:  Walt-mod replied 2 years ago.

Hi,

 

I'm a moderator for this topic and I wonder whether you're still waiting for an answer. If you are, please let me know and I will do my best to find a Expert to assist you right away. If not, feel free to let me know and I will cancel this question for you. Thank you!

Customer: replied 2 years ago.

 

Yes i am still waiting for an answer :( Thank you!

Expert:  Walt-mod replied 2 years ago.
Sometimes, finding the right Expert can take a little longer than expected and we thank you greatly for your understanding. We'll be in touch again shortly.
Expert:  Amedee replied 2 years ago.
Hello and Welcome to Just Answer. My name is XXXXX XXXXX I will try my best to assist you and your 100% satisfaction is my goal. I have been working on these vehicles for over 10 years so I am very familiar with this system and how they work. However, keep in mind, that I need some solid concrete facts/evidence/clues from you in order to give you a good answer about your vehicle. Please realize that all my answers are based solely on the info that you provide me with.

I need to get a bit more information from you before I can point you in the right direction. Is this ECM fuse inside the cab or under the hood? Is it labeled "ECM"? Can you tell me the number fuse this is and or the amp rating this fuse is?

Also, I can I get your VIN number?
Customer: replied 2 years ago.

The ECM fuse (10a) that blew is under the hood.It is the #1 fuse.

Expert:  Amedee replied 2 years ago.
I see two number one fuses under the hood. An 80 amp fuse that powers the alternator and power distribution center and a 15 amp fuse that powers the ECM relay??
Customer: replied 2 years ago.
Your right it is a 15a fuse sorry guy its dark here and im lookin with a lighter at the moment couse i have monster kids that wont leave my lights alone.It was not and has not been the 80.
Expert:  Amedee replied 2 years ago.
Ok.. just for grins, remove the PCM power relay and insert a new fuse. Then turn the key on and see if the new fuse blows with this relay removed.

Let me know!
Customer: replied 2 years ago.
I have the fuse cover diagram and do not see a PCM relay? I do however see a ECM relay. It is no longer blowing ECM fuse; however, whenboth wires are hooked to alternator the vehicle starts and dies. When you unhook either wire, the Exciter or the charging wire it will run off the battery. If youremove the #7 fuse from the dash panel which is the guage turn fuse, leaving both wires hooked up it will run off battery but won't charge and guages don't work. VIN for this vehicle is 4s2cy582xm4328261
Expert:  Amedee replied 2 years ago.
Ok.. don't jumper anything.. we want to check this when the vehicle is acting up. Just do as I instruct and nothing more.

YES.. this is the same relay. PCM relay is the ECM relay.

If the fuse is no longer blowing when this happens, disconnect both connectors going to the fuel injectors and see if the fuse continues to blow.

Let me know!
Customer: replied 2 years ago.
The fuse is not blowing anymore. The rig just starts and dies. i forgot to add i replaced ECM relay and swapped other relays of same amperage. New fuel injectors in throttle body
Expert:  Amedee replied 2 years ago.
Does the TPS have 5 volts with the key on?

Are there any codes in the computer?
Customer: replied 2 years ago.
yes tps has 5v and is new i stoped working on this last fall and at that time had a code scanner on it cant remember for the exact code but i do remember it was map sensor witch is also new.32 maby.
Expert:  Amedee replied 2 years ago.
Ok.. check to see if the map sensor is also getting 5 volts and a good ground with the key on. If so, check to be sure it is getting FULL manifold vacuum when cranking the engine and just before the engine stalls out. Also, check the signal return voltage to be sure it is within specs.

Another thing you can do is check to see if the engine will remain running if you were to spray starting fluid into the intake when it is about to stall.
Customer: replied 2 years ago.
Relist: Answer quality.
I don't think he is understanding me very well.
Expert:  Tom replied 2 years ago.
Different expert here .Has the alternator ever been replaced or the ECM itself?
Customer: replied 2 years ago.

The alternator has been replaced. THe ECM relay was replaced but not the Electronic Control Module itself.

Expert:  Tom replied 2 years ago.
Ok the most common issue is the ECM itself is shorted internally the printed circuit boards are known to short out and cause this issue .This needs to be checked next.You can get this from ebay or a salvage yard for about 100 dollars to save some money no programming needed.Also make sure there is no signs of rodents chewing through the engine harness these are the next steps.Hoped this helped.Positive feedback and bonuses greatly appreciated.
Customer: replied 2 years ago.
We got the ECM and put it in. The thing is now is ONLY charges off the battery...no charge to the alternator?? We replaced the alternator last fall when this started. We are getting a new alternator and Ignition switch just to eliminate them...we are down to the ECU and thats about all thats left! I discovered the ECM i replaced was the Powertrain ECM so maybe its the ECU I needed? The brain under the dash in the kick panel that ou hook the code reader to.Thats what may need replaced?
Expert:  Tom replied 2 years ago.
Yes that's the one I wanted you to replace originally .
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Customer: replied 2 years ago.
Sorry..the wife ordered the other one. Will do that next. Thanks for all the help.
Expert:  Tom replied 2 years ago.
Anytime . Positive feedback and bonuses greatly appreciated.
Customer: replied 2 years ago.
Well..Bad news. Replaced both ECM, and ECU and Ignition Switch.. Still starts and dies. Now followed by a buzzing noise under hood.
Expert:  Tom replied 2 years ago.
Where exactly is that buzzing noise ? Was the alternator checked and electrical part of the ignition switch?
Customer: replied 2 years ago.

Replaced Ignition switch, define electrical part?

Notsure where exactly buzzing is comin from.

Expert:  Tom replied 2 years ago.
Not the mechanical part the back part the electrical part .
Customer: replied 2 years ago.

wiring portion that hooks into the wire harness off back side of key

 

Expert:  Tom replied 2 years ago.
Yes correct that what was changed ?
Customer: replied 2 years ago.
Yes.
Expert:  Tom replied 2 years ago.
Do me a favor and check the condition of the positive and negative battery cables the entire cables andante sure they are not frayed or damaged .
Customer: replied 2 years ago.
Yes cables are good well grounded.
Expert:  Tom replied 2 years ago.
Had you tried another alternator and voltage regulator to rule out a bad rebuild you possibly have in there now ?
Customer: replied 2 years ago.

Where exactly is the Voltage Regulator?

Expert:  Tom replied 2 years ago.
I'm going to look over the electrical schematics again and post anything to check tomorrow . Has to be something we missed like a bad engine harness or something .
Customer: replied 2 years ago.
Ok. How bad is the wiring harness to work on?
Expert:  Tom replied 2 years ago.
Not bad I've replaced a few from the drivers firewall to the entire engine took like an hour and a half not bad at all to due. All were due to rodents chewing through wires .
Customer: replied 2 years ago.
At our wits end with this thing back to original problem of starting and dying. Unless you unhook the alternator. Then runs but there is no charge to alternator.
Customer: replied 2 years ago.
Relist: Other.
I can't believe no one has had this prob before! Stumped many a mechanic.. now we are down to the Wiring harness it looks like.
Expert:  Tom replied 2 years ago.
Yep that's were you need to start next . Make sure the under hood fuse box is not shorted out also. These are the next steps.

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