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Unread 11-07-2013, 05:11 PM   #1
jdb_us1
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1979 CJ7 
 
Join Date: Jul 2005
Location: Leesport PA
Posts: 112
bad ecu?

Quick question...I hope. If I were to forget to disconnect the battery and did some welding on a frame that held a 4.0 would the current damage the ecu so as to cause a no crank situation? The fuel pump runs with the key in the on position, but twist a little further to start and I get nothing. I have a slick little relay bypass switch that when plugged into the starter relay position will crank and start the engine, so I have power into and out of the relay to the starter. I have traced wires and checked continuity and tested switches and plugs and on and on. Any thoughts?

Jim

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Unread 11-07-2013, 06:35 PM   #2
Firefyter-Emt
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1995 YJ Wrangler 
 
Join Date: Aug 2005
Location: Northeast CT
Posts: 592
Yes, it could... this is why you should always disconnect when you weld. I once saw a guy blow the roof air bag on a BMW while using a resistance spot welder to install a quarter panel (with the battery disconnected!!)

You have an odd problem for sure, but if all you did was welding and were no where near the wiring doing so, I would suspect it myself.

First up, check each and every fuse... Then I would swap out relays for the ASD and Fuel Pump with other relays and see if it changes before you go any further.
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Unread 11-07-2013, 07:14 PM   #3
jdb_us1
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Join Date: Jul 2005
Location: Leesport PA
Posts: 112
yup, swapped all relays, then bought some new ones thinking maybe my spares were bad. Fuel pump primes with the key rotation, not sure I would know about the ASD though. No go. All fuses are good. I am really miffed, I can start the engine with a remote starter and with the aforementioned relay bypass, but no key. The starter relay diagram in the Haynes manually actually shows a lead going to both the "ecu", and the"engine controller". I can only think that those are one in the same although haven't traced the wire to the "engine controller". Does anyone know if those two wires are delivering signal to or from the ECU? Although I guess it wouldn't much matter, if the unit is bad, it is bad.

I guess my real question still is will a bad ECU prevent the engine from cranking?
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Unread 11-07-2013, 07:14 PM   #4
jdb_us1
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1979 CJ7 
 
Join Date: Jul 2005
Location: Leesport PA
Posts: 112
oh, you did answer that question. Sorry
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Unread 11-08-2013, 08:14 AM   #5
Siva283
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1995 YJ Wrangler 
 
Join Date: May 2011
Location: Falling Waters, WV
Posts: 3,698
No the starter is on its own circut. If you can start it with a remote starter and it runs your ECM is fine. I do not know how to troubleshoot your issue but I do know your ECM is fine since the engine runs and the actual starter has its own circut seperate from the ECM. You should talk to QUe89YJ. He is our electrical guru.
__________________
1995 YJ. 2 inch BDS Spring lift. 1 inch shackle lift. 1.25 inch JKS Body Lift 33x12.5x15's. Engo 10,000 pound winch.

[QUOTE=Magnum;14117863] I gave the Jeep the required offering of $$, sweat, and blood, and everything works fine now. -- Jim[/QUOTE]

[QUOTE=ldso;16498409]. It started with a $200 axle, and a few thousand dollars later I was done :)[/QUOTE]

[QUOTE=Luuca;16122017]diagnose the real issue before you start going all Obama on it - spending mad cash you'll need for other important things.

Ask me how I know...[/QUOTE]

[QUOTE=Overhead;17658665]this is also my second set of RC springs this year. I'd rather spend the money again and get something that will last.[/QUOTE]
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Unread 11-08-2013, 08:21 PM   #6
jdb_us1
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1979 CJ7 
 
Join Date: Jul 2005
Location: Leesport PA
Posts: 112
ok. the jeep runs again. And before I tell everyone who might stumble upon this thread, or those who might be watching, or who have commented, I need to make a statement.

No matter how good you think you are, how great your mechanical skills are, for petes sake, always retrace your steps when you encounter a problem that seems to have come out of the blue. It very well may be the simplest thing that you thought could never go wrong with the job. That includes checking your fuses and swapping relay's, and as much of a pita it is, check your grounds.

Now to the solution to my problem. I had a no crank situation after I welded some new shock mounts to the frame. I was pretty sure I unhooked the battery but not 100%, and so was afraid I fried the ECU. Come to find out, after peeling the loom off of the entire engine wiring harness just to trace a wire that went from the starter relay to the ECU, I found a splice in the wire and it wan to another circuit. A circuit that ran through a plug. Well....in my earnst to make the swap look OE, I unplugged the harness at said plug because it did not power any components that I was using with the swap. Apparently the wire in question double back through the plug to complete a circuit. PLug the discarded harness back in and voila!

THanks for the input from all who tried to help. Now if only someone could come up with an acronym to relate my alcohol consumption to my Jeep ownership, I would have a slick ending to this three day ordeal. Thanks again
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