Bad ECU? How do I check my brainbox... - JeepForum.com
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post #1 of 19 Old 12-30-2019, 01:07 PM Thread Starter
Gypsy_Botanist
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Bad ECU? How do I check my brainbox...

I was driving the other day and shortly after I got fuel, my tac, speedometer and all of my gauges went dead. I thought/hoped it might be a loose fuse & continued to drive her for about an hour and then I started to loose a little power, she just couldn't maintain 75, I was driving on the highway, and slowed to about 60 with the same amount of throttle on level ground. Then all of a sudden she starts kind of chugging or gives a shudder, a few seconds later she backfires and I lost all power. She died when i took her out of gear, or maybe I turned the key off. After I got her stopped I tried to start her up, and got nothing. The fuel pump turns on and the engine cranks and turns over but no fire (I've not checked the plugs for fire yet so only 90%). I also have no check engine light, not even when you turn the key or for the few seconds it turns on after starting. I tried turning the key thrice to see if she is throwing any codes but I got nothing. So i think the brainbox is dead I just don't know how to test it. I thought I could plug an OBD computer into it but I don't know where to plug it in at, or any other ways to diagnosis it. I'm going to get a relay checker and check all the relays and fuses under the hood. If anyone has any other ideas about whats wrong or how to check your brain box please let me know.

Thanks


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post #2 of 19 Old 12-30-2019, 01:38 PM
jkbrereton
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If there are no codes then I'm thinking fuel delivery. Check fuel pressure, check for vacuum leaks, especially the one going from manifold to fuel pressure relief switch on the fuel rail. I recently had a cracked boot on themat vacuum line cause a similar issue.

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post #3 of 19 Old 12-30-2019, 01:54 PM Thread Starter
Gypsy_Botanist
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Quote:
If there are no codes then I'm thinking fuel delivery
The check engine light isn't lighting up at all what so ever. I just redid the vacuum lines but I'll check again when I go over the relays and fuses.



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post #4 of 19 Old 12-30-2019, 02:07 PM
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I'm not much on electronics, but I swapped in an XJ ECU of the same year/engine and it worked perfectly. If you have a PnP near by might be a cheap try.

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post #5 of 19 Old 12-30-2019, 02:18 PM
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As long as you get one without the VSS.

Another thing it could be is a bad crank position sensor

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post #6 of 19 Old 12-30-2019, 02:33 PM
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I would not condemn the brain when you stated that all the guages, etc. went dead. You should not trust the CEL when the rest of the cluster does not work.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #7 of 19 Old 12-30-2019, 02:59 PM
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Originally Posted by Boojo35 View Post
I would not condemn the brain when you stated that all the guages, etc. went dead. You should not trust the CEL when the rest of the cluster does not work.
This. Definitely check for codes using an OBD scanner and see what you can find.
What is the year/make/model/engine you've got? Is it your wrangler that you're having the problem with?

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post #8 of 19 Old 12-30-2019, 03:14 PM Thread Starter
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Quote:
What is the year/make/model/engine you've got? Is it your wrangler that you're having the problem with?
Yea it's my wrangler, 1993 stock, its got the AMC 2.5L 4 cylinder.

Quote:
You should not trust the CEL when the rest of the cluster does not work
What exactly does this mean? I don't know what CEL stands for, and is cluster like all the instruments and gauges?

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As long as you get one without the VSS.
Another thing it could be is a bad crank position sensor
What does VSS mean?
I'll look at the crank case sensor. I had one go out last year and I think I remember how to check it with a meter. The plug is easy to get to, I'll add it to the list when I check the vacuum lines and relays.



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post #9 of 19 Old 12-30-2019, 03:30 PM
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CEL= Check Engine Light
Yes, "cluster" is a generic term for the instrument panel.....gauges, idiot lights, etc.

Roger
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post #10 of 19 Old 12-30-2019, 04:38 PM
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Vehicle Security System. Some Cherokees had them linked to the door lock or something. If you get a computer with that system, the engine will start and immediately shut down

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post #11 of 19 Old 12-30-2019, 05:11 PM
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Originally Posted by Boojo35 View Post
I would not condemn the brain when you stated that all the guages, etc. went dead. You should not trust the CEL when the rest of the cluster does not work.
True words.
But you need to verify power to the ecu first up. There’s another fuse panel (a PDC) under the hood that has stuff.

Im not convinced it’s your computer (although there’s a chance it might be) by the symptoms and this early in the diagnostic.

I’m not going to write a book right now &#x1f642;

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #12 of 19 Old 12-30-2019, 06:04 PM
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Mine does this occasionally. There's a fuse under the dash that controls the gauges (and other things). Those fuses get corroded and with the vibration of a running Jeep, they'll stop letting electrons flow.

Pull all your fuses out and clean off the corrosion. Or just replace them.

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post #13 of 19 Old 12-31-2019, 11:06 AM
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The main 50 amp fuse in the PDC will cause this as well.

If your gauges stopped working your check engine light wont eithier. I am betting a bad fuse or wire somewhere. Especially since most of the gauges dont get thier signal from the computer

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post #14 of 19 Old 02-14-2020, 07:37 PM Thread Starter
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I checked all the fuses and relays, visual inspection of the fuses if I really have to test them with a tester I will but... they all looked good. I checked fuel pressure it is good, fuel pump sounds strong (it should it's new). All the relays tested okay. I checked for spark, had none. Checked the crank sensor, thought it was bad and replaced it, it was not bad I was using my meter wrong, woops $40. I found one bad fuse under the dash and that one got the instrument cluster working again. When I have it throw codes it does, however the battery died and the computer reset its only giving 12,33,55 now, like a healthy jeep should. The ignition coil, cap and rotor and plugs and wires are all new, so I don't think it is that. I also checked over all the vacuum lines and they are all solid. When she sits the battery dies and its got a new battery so that also makes me think it could be the ECM as I've heard that is a symptom. Whether or not thats true I'm not sure. I plugged it up to a code machine and it is only giving the 33 code for AC fault, because the AC isnt there.
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post #15 of 19 Old 02-15-2020, 07:33 AM
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Just to confirm, you're still having trouble with the engine not starting?

It's hard to tell what the right next step might be without being there. A blown fuse under the dash and a drain on the battery suggest a problem with wiring or with another electrical component. Whether or not that's related I won't guess.

With the ecu able to "talk" again you could work on diagnosing the no-start. If the engine has compression, spark, and fuel at the correct time it should start. You could check for spark first. That's a biggie. No spark means moving to check for power at the coil, then check for the coil negative signal as the engine is cranked over.

If you have spark the next step might be to check for fuel. If you have starting fluid you could try spraying a little bit into the throttle body then cranking the engine. If it starts and stalls you should try to determine why fuel isn't getting into the cylnders. Are the injectors firing? Check power and ground circuits at the injectors for proper operation. If you have spark but starting fluid won't start the engine shut everything down and pull the plugs. Are they wet? Covered in black carbon? Do they smell like gasoline? Is the stuff you put into your tank not really gasoline? If the plugs are wet try cleaning or replacing them (do not clean plugs with a wire wheel) and try starting the engine again. If you have fuel and spark and the plugs are good I think diagnosis is going to be a little tougher. You may have a sensor giving wrong information to the ecu so you're getting bad spark timing or wrong fuel. Or you may have a mechanical problem.
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