88 truck, runs great with the map unplugged

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Schurkey

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I use an ancient Snap-On MTG2500. Similar to the MT2500. They're dirt-cheap on eBay, and getting them with software and cables old enough to scan pre-'99 domestic vehicles is pretty easy.

Mine's getting a little flaky, though. I may be looking for a newer scan tool in the next year or so.
 
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I use an ancient Snap-On MTG2500. Similar to the MT2500. They're dirt-cheap on eBay, and getting them with software and cables old enough to scan pre-'99 domestic vehicles is pretty easy.

Mine's getting a little flaky, though. I may be looking for a newer scan tool in the next year or so.

yeah, I've used one of these before. was a heck of a lot nicer than what i have. I am curious though, what would make mine produce a false code like was suggested on another forum? I've never gotten false codes off her before and has always lead me in the right direction for diagnosis.
 

Schurkey

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I can't help you with "false codes" except that it's either in the ECM or the scan tool.

Given that the scan tool gives you MAP codes, and the vehicle won't run right with the MAP plugged in, I'd tend to trust the scan tool over the flashing light.

Find another scan tool, one that requires some VIN input, and see what happens.
 
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That's what my thoughts were. guy on another was more insistent on it being the scan tool and that's why i was asking. I should have something,whether bought or borrowed within the week. I'm actually thinking the ecm myself. Signals were getting to the pin out once i tested to check for shorts.
 

L31MaxExpress

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Just a word of advice when it comes to these trucks. Over the years I have run across some of them doing very whacky things electronically. Even had a friend throw an ECM in his truck at the advice of an independent shop. Most of the time I would find a dirty or broken ground connection on the top of the intake manifold by the thermostat or on the back of the head. Make sure the ECM has the proper grounds getting to it before condemning it.
 
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Just a word of advice when it comes to these trucks. Over the years I have run across some of them doing very whacky things electronically. Even had a friend throw an ECM in his truck at the advice of an independent shop. Most of the time I would find a dirty or broken ground connection on the top of the intake manifold by the thermostat or on the back of the head. Make sure the ECM has the proper grounds getting to it before condemning it.


I made sure of the grounds before even considering. They're clean and i checked for shorts everywhere in between. my auto instructor always said to rule out everything before condemning any computer. i'm sure once i finally get something to view live data i'll be able to determine whether or not it's an ecm issue or something else. my biggest concern was whether i looked something over or not. i know something is telling the ecm to cut off the fuel when everything is plugged in. while running i can see the fuel cut off right before it dies plugging the map in.
 
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finally got her hooked up to an mtg2500 scan tool. got everything is situated and then when i turned the key on, the scan tool died out. also found a cheap 7747 ecm i picked up just to have but i have not messed with it yet. the grounds on the ecm checked out fine as mentioned before.
 
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I made sure of the grounds before even considering. They're clean and i checked for shorts everywhere in between. my auto instructor always said to rule out everything before condemning any computer. i'm sure once i finally get something to view live data i'll be able to determine whether or not it's an ecm issue or something else. my biggest concern was whether i looked something over or not. i know something is telling the ecm to cut off the fuel when everything is plugged in. while running i can see the fuel cut off right before it dies plugging the map in.

There are 5 ground wires that terminate near the coolant temp sensor on my 89 are all 5 there on yours?
 
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