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Renix 4.0l No spark no injector pulse


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Long time listener, first time caller!
Ive got a lot to explain, so ill get right to the point:
87 Comanche
HO Swap
Ran after swap fine for a month

Problem:
A short in the dash powerside wiring (the small orange/black wire) caused a no start problem
The dash light wiring has been repaired

Symptoms:
No spark from coil
No injector pulse
Fuel pump primes but not running during cranking
Intermittent Voltage gauge operation
Tachometer not working

Diagnostics:
CPS is newish and tests at 235 ohms and puts out .5v ac current during cranking
5v at tps and map
TPS tests puts out 0.8v
MAP tests good
O2 tests at roughly 400 ohms
Both temp sensors test good
Replaced coil, ICM, coil lead
12v and good ground at ICM 3 prong plug
Ground at ICM 2 prong plug (not sure if this is supposed to be, the wire is yellow)
12v to coil from ICM
Opened up ECU, looks good and all testable components tested good
The four relays behind the battery all test good, same with the relay on the drivers side engine bay
Cleaned C101 connector (was filled with goopy sticky black stuff)
Cleaned plug going to fuse box 9 filled with same stuff)
Checked the CPS resistance at ECU pins D1 and C1, came back same as at CPS
Checked all fuses and fusable links
Cleaned ground strap and connections at the starter relay

I thought I had no power at ECU but I think I am testing the wrong connectors (D10 and C11) my wiring diagram seems to be wrong, what pins should i check for power at?
I am honestly stumped.
Any help would be greatly appreciated, Ive read through nearly every forum I can find

 

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Checked the pins on the ecu using the diagram provided. everything checked out except for the connections relating to the latch relay. the #3 on diagnostic connector #2 reads only 1.8v on the dvom and a whopping 4M ohms. #12 on diagnostic connector 2 reads negative also, even though it says it is the coil feed. Also, all ecu pins came back correctly, except for B10, the power supply. where is this latch relay? what does it do and how do i replace it? thanks a bunch in advance

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I traced the #3 on diagnostic connector #2 to the negative on the latch relay. It is the frontmost of the 4 relays by the battery. I also found that it has 12 volts neg with the key off but 12v positive with the key in the on position. whenever i pull the relay, the current goes back to negative, regardless of key. also, if i "jump" negative current to that side with the key on, the relays click. I know I'm looking for a short now, but where does that feed? I also checked the voltage at the other three pins on the latch relay and they all come back with 12v positive, even with the relay removed

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turns out the ground on the latch relay was weak, i ran one straight to the battery and the relay started working, and i now have power at the ecu. the car still does not start though. still no spark no pulse. i retested the cps at the ecu and it came back with 0.4v ac. is this enough? i also tested the pickup coil, and between the blue and grey/black wire it has 0.3v ac. i was reading the guide on how to test sensors and it said the pickup coil should read 5v ac and pulse when cranking. mine did neither, is it bad? thank you in advance

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Does the ecu provide the 5v ac to the pickup coil (sync signal generator)? I think i read that the ecu provides the negative to the latch relay too? is this correct? if so, is my ecu blown? 

Correct on 2 counts. And, maybe.

 

Its starting to seem like a blown ecu. I always try to save that for the very last option though. What else could be causing these problems though? besides just catastrophic wiring damage? Also, what ECUs are compatible with it? 87-91? I have a manual transmission too, can i use an automatic's ecu? thanks a ton, youve been ridiculously helpful! 

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Does the ecu provide the 5v ac to the pickup coil (sync signal generator)? I think i read that the ecu provides the negative to the latch relay too? is this correct? if so, is my ecu blown? 

Correct on 2 counts. And, maybe.

 

Its starting to seem like a blown ecu. I always try to save that for the very last option though. What else could be causing these problems though? besides just catastrophic wiring damage? Also, what ECUs are compatible with it? 87-91? I have a manual transmission too, can i use an automatic's ecu? thanks a ton, youve been ridiculously helpful! 

 

Get an 89 or 90 ECU regardless of trans. Instant 9 horsepower increase. 

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There are two things I would look at, especially with dash shorting and old age connectors.

 

1.  The 201 and 202 connectors (the ECU plugs)  may need to be rebuilt.  The contacts tend to open up over the years allowing corrosion to block connection with the ECU pins.  Try taking them apart and filing and recrimping the contacts.

2.  With dash shorts, you may have burnt out parts of your ECU...buy a new one if above doesn't work.

 

On my last - no primary signal to the ignition coil problem - it was the 201 and 202 connectors, not the ECU.  Driving voltages from the ECU were blocked by open contacts.

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There are two things I would look at, especially with dash shorting and old age connectors.

 

1.  The 201 and 202 connectors (the ECU plugs)  may need to be rebuilt.  The contacts tend to open up over the years allowing corrosion to block connection with the ECU pins.  Try taking them apart and filing and recrimping the contacts.

2.  With dash shorts, you may have burnt out parts of your ECU...buy a new one if above doesn't work.

 

On my last - no primary signal to the ignition coil problem - it was the 201 and 202 connectors, not the ECU.  Driving voltages from the ECU were blocked by open contacts.

I will definitely check that out! The ECU plugs look to be about the best condition plugs on the whole truck though, so I wouldn't bet any money on it! lol 

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Does the ecu provide the 5v ac to the pickup coil (sync signal generator)? I think i read that the ecu provides the negative to the latch relay too? is this correct? if so, is my ecu blown? 

Correct on 2 counts. And, maybe.

 

Its starting to seem like a blown ecu. I always try to save that for the very last option though. What else could be causing these problems though? besides just catastrophic wiring damage? Also, what ECUs are compatible with it? 87-91? I have a manual transmission too, can i use an automatic's ecu? thanks a ton, youve been ridiculously helpful! 

 

Get an 89 or 90 ECU regardless of trans. Instant 9 horsepower increase. 

 

Awesome! Thanks for all the help! With a new unit being just over $100, even if the ecu isnt blown, it still makes for a pretty monetarily sound performance mod!

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  • 1 month later...

nobody is probably following this anymore, but the new ecu hasnt solved the issue though, also got a new ignition switch on it as the old one was sticking. I'm going to take a second look at the ecu connectors. the c101 connector and fuse box connector have been cleaned multiple times now but there is still burnt goo in them. I'm thinking about deleting the c101 connector. don't really know what it could be now besides bad wiring 

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nobody is probably following this anymore, but the new ecu hasnt solved the issue though, also got a new ignition switch on it as the old one was sticking. I'm going to take a second look at the ecu connectors. the c101 connector and fuse box connector have been cleaned multiple times now but there is still burnt goo in them. I'm thinking about deleting the c101 connector. don't really know what it could be now besides bad wiring 

Delete the C101.

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  • 4 months later...

I'm having the same issues as i rebuilt my 4.0 tractor engine on my MJ. its been a while anyone has posted but.... did you ever find the source? 

Well, if you quit describing it as a tractor engine, which it isn't and never has been, maybe you would get some positive results when working on it. 

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