Help - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 12 (permalink) Old May 13th, 19, 1:56 PM Thread Starter
 
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Help

Had a mental lapse and reversed battery cables during battery replacement, fries the fusible link, got that all replaced and everything seems to work except the motor wont fire, I have power going to the msd box, motor cranks but no fire? Could I have fried the msd box or coil? Help!!
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post #2 of 12 (permalink) Old May 13th, 19, 2:08 PM
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Re: Help

What vehicle?
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post #3 of 12 (permalink) Old May 13th, 19, 2:47 PM Thread Starter
 
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1972 chevelle
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post #4 of 12 (permalink) Old May 13th, 19, 8:25 PM
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Re: Help

This is the non-chevelle automotive tech page- does not compute.
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post #5 of 12 (permalink) Old May 13th, 19, 9:03 PM
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Re: Help

Does it have spark?

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This is the non-chevelle automotive tech page- does not compute.
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post #6 of 12 (permalink) Old May 13th, 19, 11:04 PM
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Re: Help

From MSD..."The Digital 6A and 6AL have a built in reverse polarity protection circuit. This will protect the ignition
in the event of wrong connections." So hopefully the box is not fried.

Is there an LED on top of the box? Does it give you a long flash right after the ignition is turned on? Then ok and rev limiter is active. A long flash and five short flashes means ok but rev limiter not active. When you crank the engine does it flash? It will flash every time the spark is triggered. When running, flashing will be so fast that the light appears to be on continuously.

If this checks out check for spark. Inspect the Primary Coil Wire connections. Because the MSD is a Capacitive Discharge ignition and
it receives a direct 12 volt source from the battery, there will not be any voltage at the Coil Positive (+)
terminal even with the key turned On. During cranking or while the engine is running, very high voltage
will be present and no test equipment should be connected. WARNING: Do not touch the coil terminals during
cranking or while the engine is running.

https://documents.holley.com/6201.pdf

Which box do you have and which distributor are you running to trigger the box?
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post #7 of 12 (permalink) Old May 14th, 19, 6:42 AM Thread Starter
 
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The box is a 6a, but it is over 25 years old, the dist. is a matching msd unit, there are no lights on the msd box I have
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post #8 of 12 (permalink) Old May 14th, 19, 7:08 AM Thread Starter
 
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Do you know if that polarity protection was there that long ago?
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post #9 of 12 (permalink) Old May 14th, 19, 7:28 AM
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Re: Help

Quote:
Originally Posted by ABcroozin72 View Post
Do you know if that polarity protection was there that long ago?

I would call MSD and ask them. Have the serial number off your box when you call. Their tech support has always been pretty decent. They can also tell you other things to check.



1-888-258-3835

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post #10 of 12 (permalink) Old May 14th, 19, 12:41 PM Thread Starter
 
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Thanx
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post #11 of 12 (permalink) Old May 16th, 19, 10:35 PM
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Re: Help

As already mentioned, but not answered... Spark?
Why not check for voltage pulses at the coil when cranking? Might try both AC and DC since a pulsating DC will look a lot like AC.
Ohm out the coil. It should be very low, but have some resistance.
You can also check the coil the caveman way with a plug and applying, then removing 12V to the coil.
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post #12 of 12 (permalink) Old May 17th, 19, 8:10 AM
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Re: Help

The voltage pulses at the low voltage (primary) terminals of a coil connected to an msd box should be several hundred volts, the newer stuff advertises about 530 volts if I remember correctly. Best way to see what's there would probably be an oscilloscope with a voltage divider probe to keep the voltage from blowing the input amplifier away on the scope. Or, you could simply do what's already been suggested-check for actual spark. Even though I have the equipment to do what I described above, I would check for spark. An awful lot of stuff gets eliminated as possible problems if you have a good strong spark. If not, then get scientific and start checking individual things.

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