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shorting out 5 prong ECU's? (solved)

4K views 28 replies 6 participants last post by  Unfortunate_Sam 
#1 ·
hmm well after having delve into my 75 ramchargers whole no spark issue It seems to be that the truck is eating ECU's. At first I though that my ECU was not the problem after having tested a known to be good 5 prong ECU. However The truck shorted out the known to be good ECU. I also tried a known to be good 4 prong ECU but it made no spark and it definatley didn't short out.

I took apart my engine wiring harness to see if there were any shorts or anything, but its all factory clean so all that did was waste some electrical tape and allow me to route a couple things more efficiently.

After the ECU's short out they back-feed power down the blk/ylw wire that runs to the negative side on the coil.

Does anyone have an idea of what is going on with this and any possible fixes.

After going through several ECU's and having them all short out I am not sure what to think or how to fix it since it only seems to happen with the 5 prong ECU's and I get no response one way or the other from the newer 4 prong ECU's.

(bad coil, was shorting out my ECU's with five prongs and it was giving no spark replaced coil threw in new ECU, got spark gained and gained a pretty bad short somewhere so I'm onto the next issue)
 
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#2 ·
Had the same problem on the 57 after installing the electronic ignition conversion. I thought it had to do with Accel super coil I installed but , my brother found I hadn't wired it properly to the ballist resistor. Have no clue what he changed, but my ECU's would melt the silicone off the back. I would start by looking at the harness from the ballist resistor to the ECU
 
#3 ·
88raminator said:
I would start by looking at the harness from the ballist resistor to the ECU
Agreed.

The second side of the 4 pin ballast, provides a lower voltage for the ECU's internal workings. The 4 pin ECU's eliminated the need by having an internal voltage regulator. A 4 pin ECU should work in your system. Many aftermarket 5 pin ECU's are really a 4 pin internally, and have the 5th pin as a dummy, and charge a higher price.

Since you say they keep shorting out, I suspect your ballast might be feeding to high of a voltage, Possibly a reversed wire at the ballast.
 
#4 ·
hmm, then why didn't it short out my 4 pin ECU, I'll just have to do some experimenting i guess..... good thing my new fried ECU has a lifetime warranty.
 
#5 ·
warranty is good for shit,especially if something external fried it.you can try,dont expect them to keep replacing the part til you find the problem.
coil could be incorrect for the application.
short circuits can be a real pain to trace.
maybe dodgeboys will chime in with a wiring diagram.
he's usually good with this stuff.
i'm more of a hands on type of diagnostician.
the 5 pin ecu's should be replaced with the 4 pins,in my opinion they are less trouble.
 
#6 ·
Unfortunate_Sam said:
hmm, then why didn't it short out my 4 pin ECU, I'll just have to do some experimenting i guess..... good thing my new fried ECU has a lifetime warranty.
I'm suspecting the 5th pin is getting too much voltage, possibly from a bad or mis wired ballast. The 4 pin ECU would not care about the voltage on the 5th pin.

Why won't the 4 pin work? that leads me to think more of a wiring problem with the ballast.
 
#7 ·
Ok well my ballast hook ups are the cleanest they have ever been the whole engine harness is unused OE mil surplus, so everything is where it is supposed to be could they just be on the wrong prongs maybe? I'll try them reversed when i pick up my replacement ECU tomorrow afternoon.
 
#8 ·
Ok well I burnt up another ECU this afternoon, and the only progress made is that the distributor is not the issue, which sucks because I was hoping it was :'(. However That only leaves so many options, most of which revolve around the green wire that hooks up to prong #3 or the 5th prong that isn't there in the 4 prong ECU's.

Since I have no idea why, and I've been pretty thorough when troubleshooting, any idea's of what might be happening here? I and everyone I've talked to including a lot of experienced mechanics are well stumped... which blows because this is as far as I can tell the last real issue with this stubborn RamCharger.
 
#9 ·
Check the voltage on that extra pin (With the ignition on), if it is 12 volts then you found the problem.
 
#10 ·
it puts out about 11.75 volts, damn this is messed up.... I think I'm going to go through all my connections again tomorrow and triple check that every thing is right where it should be.

Anybody ever feel like beating your head against a wall, I bet that its something stupid that I have overlooked time and again....
 
#12 ·
there ya go.
follow the diagrams,test voltages compared to battery voltage.
 
#13 ·
Be aware, the 4 pin diagram posted above has the handwritten run, and start labels reversed.
 
#15 ·
Is the back of these modules and the firewall clean of paint and rust? Module grounds through case.

How is coil primary resistance? Low resistance (internal short) can pull excess current through module.

Remove both small coil wires and measure resistance between the 2. There is a spec.


***One more note. If this engine runs for a short period of time before burning modules, have the alternator tested for diode ripple. If it burns w/o starting, suspect coil or case ground problems.
 
#16 ·
ya ECU is grounding fine I'm sure of that its right where they have always been.

My battery is dead now so I need to charge it. Before I do anything else, and the truck is outside and its raining.

I've been thinking the last couple days if I switched over to a HEI could this problem possibly be bypassed? or would it persist still, also i can't think of any reason my coil would be bad but it never hurts to test something.

 
#17 ·
and ya its burning the ECU as soon almost as soon as I plug it in it live jsut long enough for me to test to see if the coil - is getting power and after it doesn't for a second the positive loses power and then they both have power a seconds later, I completely disconnected my distributor to see if that might be the problem after talking with a few people, but it was not. But the coil has always been hooked up so I mean maybe?

I hate having to check the voltage and resistance etc of stuff, but I guess I'm pretty much out options. Although when I go get another replacement ECU I guess I can just plug it in with the coil disconnected and then see if it'll start my little brothers crazy mopar powered jeep.
 
#18 ·
You need to find out why that green wire is getting 11+ volts. unplug the ballast, then check the voltage at the extra pin, there should be nothing there, with the ballast unplugged.


I suspect something is messed up with the ballast, and or the wiring.  do you have the stock connecters?

With a 4 pin ECU, you should be able to unplug the ballast, and it should start, and then die as soon as you let go of the key. This test will not work with the  5 pin, as the 5th pin is what gives the ECU power.

Have you tried a known good coil? I wonder if a coil with a dead short could be your problem.
 
#19 ·
Who ever was claiming the distributor, doesn't understand how mopars work. The distributor doesn't get any power, not on pickup side anyways. The pickup makes it's own power with a small generator that depends on distributor moving. No movement, no electricity from pickup.

Any voltage testing needs to have negative test wire on the engine block or battery, not the coil. Any resistance testing needs to be done unplugged.



Ballast was likely reading nearly full voltage because the ground side was open circuit. No load=nearly full source voltage all the way to the point of high resistance or open circuit.
 
#20 ·
Ya I think I'm gonna try a known good coil today, since it seems to have cleared up a bit. I'm also gonna "borrow" a charged battery so I can do some more testing. Hopefully it will end up being a bad ballast or coil since I'm running out of options....
 
#22 ·
well how hot is ballast supposed to get, cause while doing some tests i started to smell some burning and I looked all over for it and couldn't find it so i just kept testing and then i touched the ballast and burnt three fingers I remember my ballast getting warm before but not that warm...
 
#23 ·
Why work so hard trying to avoid the simplest of measurements? Put a lead on each screw and you know for sure whether it's bad or not, w/o even having to remove it or risking damage from some other short.

How hot the ballast gets depends on how much of a short there is.

Are you actually bolting up these "new" modules you are testing with? Or just setting them on the fender or something?
 
#24 ·
Yes I am bolting them up, i bolt them up them up three times, once to the jeep to make sure they work, once to the truck where they die, and once more to the jeep to see if they still work, every time unbloting and bolting them exactly where and how the previous one was, and ya I'm gonna go test to coil right now
 
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