hardbugsmattress Posted August 20, 2015 Share Posted August 20, 2015 Long time listener, first time caller! Ive got a lot to explain, so ill get right to the point:87 ComancheHO SwapRan after swap fine for a monthProblem:A short in the dash powerside wiring (the small orange/black wire) caused a no start problemThe dash light wiring has been repairedSymptoms:No spark from coilNo injector pulseFuel pump primes but not running during crankingIntermittent Voltage gauge operationTachometer not workingDiagnostics:CPS is newish and tests at 235 ohms and puts out .5v ac current during cranking5v at tps and mapTPS tests puts out 0.8vMAP tests goodO2 tests at roughly 400 ohmsBoth temp sensors test goodReplaced coil, ICM, coil lead12v and good ground at ICM 3 prong plugGround at ICM 2 prong plug (not sure if this is supposed to be, the wire is yellow)12v to coil from ICMOpened up ECU, looks good and all testable components tested goodThe four relays behind the battery all test good, same with the relay on the drivers side engine bayCleaned 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 CPSChecked all fuses and fusable linksCleaned ground strap and connections at the starter relayI 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 Link to comment Share on other sites More sharing options...
Pete M Posted August 22, 2015 Share Posted August 22, 2015 wait, are you running the renix system or the HO system? Link to comment Share on other sites More sharing options...
cruiser54 Posted August 22, 2015 Share Posted August 22, 2015 B10 is power to the ECU from the B+ latch relay. Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 22, 2015 Author Share Posted August 22, 2015 wait, are you running the renix system or the HO system? I'm running the renix system. Just the HO block and head Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 22, 2015 Author Share Posted August 22, 2015 B10 is power to the ECU from the B+ latch relay. Thank you for the pinout! I'm gonna check for the right imputs right now! Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 22, 2015 Author Share Posted August 22, 2015 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 Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 22, 2015 Author Share Posted August 22, 2015 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 Link to comment Share on other sites More sharing options...
MancheKid86 Posted August 22, 2015 Share Posted August 22, 2015 just throwing it out there, simple as it seems, check your positive connection onto the starter Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 22, 2015 Author Share Posted August 22, 2015 just throwing it out there, simple as it seems, check your positive connection onto the starter Ill check to make sure it isnt grounding, but it cranks over just fine. It just doesnt start once its cranking. Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 23, 2015 Author Share Posted August 23, 2015 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 Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 23, 2015 Author Share Posted August 23, 2015 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? Link to comment Share on other sites More sharing options...
cruiser54 Posted August 24, 2015 Share Posted August 24, 2015 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. Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 24, 2015 Author Share Posted August 24, 2015 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! Link to comment Share on other sites More sharing options...
cruiser54 Posted August 24, 2015 Share Posted August 24, 2015 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. Link to comment Share on other sites More sharing options...
metrictonner Posted August 24, 2015 Share Posted August 24, 2015 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. Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 25, 2015 Author Share Posted August 25, 2015 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 Link to comment Share on other sites More sharing options...
hardbugsmattress Posted August 25, 2015 Author Share Posted August 25, 2015 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! Link to comment Share on other sites More sharing options...
hardbugsmattress Posted October 11, 2015 Author Share Posted October 11, 2015 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 Link to comment Share on other sites More sharing options...
hardbugsmattress Posted October 11, 2015 Author Share Posted October 11, 2015 are there crucial circuits i should check first? bear in mind i already checked all the fusible links Link to comment Share on other sites More sharing options...
cruiser54 Posted October 12, 2015 Share Posted October 12, 2015 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. Link to comment Share on other sites More sharing options...
DaveMj Posted February 15, 2016 Share Posted February 15, 2016 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? Link to comment Share on other sites More sharing options...
cruiser54 Posted February 16, 2016 Share Posted February 16, 2016 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. Link to comment Share on other sites More sharing options...
evanlove Posted November 12, 2022 Share Posted November 12, 2022 So, did you ever solve the issue? Mine is doing something similar right now. 1988 Comanche 4x4 short bed 4.0 manual Link to comment Share on other sites More sharing options...
cruiser54 Posted November 12, 2022 Share Posted November 12, 2022 6 year old thread. Link to comment Share on other sites More sharing options...
jpnjake Posted November 12, 2022 Share Posted November 12, 2022 13 hours ago, evanlove said: So, did you ever solve the issue? Mine is doing something similar right now. 1988 Comanche 4x4 short bed 4.0 manual This guy hasn’t signed back on since 2015, I’m betting he doesn’t have an answer😞 Link to comment Share on other sites More sharing options...
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