Voltage drop at coil when cranking - JeepForum.com

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post #1 of 18 Old 05-21-2017, 10:07 PM Thread Starter
93zjer
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Voltage drop at coil when cranking

Voltage at coil drops to 6 volts when cranking, but battery voltage stays at approx 12 volts.
Conductors to ecu test good and a new ecu did not fix the problem.

1993 ZJ 4.0/AW4

Thanks!

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post #2 of 18 Old 05-22-2017, 07:23 AM
xtrattitude4u
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What issue were you troubleshooting? Crank no start? Voltages will normally drop when the battery is under heavy load such as cranking.

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post #3 of 18 Old 05-22-2017, 07:28 AM
PlainoldCheroke
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Voltage drops to zero if Auto-Shut Down is activated, so that does not seem to be your problem. Do you have fuel pressure? Is fuel pump working.. you can put a pressure guage on the shrader valve on the rail. If not firing you might need to test those other components first. Need more details to help you.

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2011 GC Overland Hemi
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post #4 of 18 Old 05-22-2017, 09:06 AM Thread Starter
93zjer
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Cranks, hard to start, runs rough, weak yellow spark. Again, the battery voltage does not drop, the voltage coming out of the computer to the coil harness drops when cranking.

I have fuel pressure, ASD is closing. I have a snap-on red brick scan tool.

Thought it was bad capacitors in the computer but a new computer and replacing caps in the old one did not fix the issue.
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post #5 of 18 Old 05-22-2017, 09:48 AM
HighLonesome
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It is normal. If you are using a DC voltmeter, what you are seeing at the coil primary is the DC voltage equivalent of an AC signal. You would need an o-scope to properly view the waveform at the coil primary.

I would try another coil.
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post #6 of 18 Old 05-23-2017, 06:14 AM
Joshuatd77
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I had a bad battery was a constant 12.4v would charge but quickly lose voltage under load. If your battery is constantly less than 12.6 it is bad. If it is bad swap it then try your test again

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post #7 of 18 Old 05-28-2017, 03:46 PM
xtrattitude4u
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If it runs rough it sounds more like the alternator. Make sure you are getting 14+ volts from the alternator when running.


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post #8 of 18 Old 05-29-2017, 03:36 PM Thread Starter
93zjer
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Quote:
Originally Posted by HighLonesome View Post
It is normal. If you are using a DC voltmeter, what you are seeing at the coil primary is the DC voltage equivalent of an AC signal. You would need an o-scope to properly view the waveform at the coil primary.

I would try another coil.
I am using a Fluke RMS meter. Can you explain what you mean by "the DC voltage equivalent of an AC signal"? Are you saying Im seeing the root-mean-square voltage? I do have a lab scope but I dont have the adapters to see primary ignition. I dont normally use the scope for automotive work.
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post #9 of 18 Old 05-29-2017, 06:13 PM
HighLonesome
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Quote:
Originally Posted by 93zjer View Post
I am using a Fluke RMS meter. Can you explain what you mean by "the DC voltage equivalent of an AC signal"? Are you saying Im seeing the root-mean-square voltage? I do have a lab scope but I dont have the adapters to see primary ignition. I dont normally use the scope for automotive work.
Yes you are reading the RMS voltage. I've read it's around 8v when running. So 6v when cranking seems acceptable.
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post #10 of 18 Old 06-15-2017, 10:47 AM Thread Starter
93zjer
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A shop (supposed to be the best in town) had their best guy on it for days. He said that the pcm ignition timing is off. Suspected transient voltage from the alternator or a sensor. He tried installing the pcm with replaced caps. Eventually observed an intermittant problem in the cam sensor waveform. Brand new dist had come with a bad cam sensor in it.

(this was on my father's jeep, hundreds of miles away. Thanks for the help!)
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post #11 of 18 Old 06-15-2017, 03:31 PM
PolkaPower
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My mds coil and plug wires worked great until I got mud and water all over them.
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post #12 of 18 Old 06-18-2017, 12:36 PM Thread Starter
93zjer
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Ran for two days then would not start after being turned off. This was the original symptom before it went to not starting at all. The defective cam sensor caused a second problem that masked the original. Ive tried to suggest that the coil is getting heat soaked but it seems to be falling on deaf ears. CPS was just replaced. My father has a lab scope with limited automotive function and a snap-on red brick.
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post #13 of 18 Old 06-18-2017, 04:40 PM
HighLonesome
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Maybe the cam sensor connector. But try a coil, they are cheap and if it doesn't help you have a spare to carry.
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post #14 of 18 Old 06-19-2017, 10:28 AM Thread Starter
93zjer
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Tried three coils, one brand new, symptoms the same for all of them. Cam sen connector appears to be ok, but he's going to give it another look.
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post #15 of 18 Old 06-19-2017, 02:14 PM
PlainoldCheroke
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We had a similar NO START, and replaced the coil, then Cam, Throttle position, and Crank Sensors. I never really knew which was the cause, but I think it was the cam sensor and/or wire connections. Wire brush all ground straps you can find in the engine compartment. especially the ground for the ECM. knock on wood, we're going strong 10k miles later on our 1995. I can't stress the importance of good grounds, and good voltage where needed on these cars.

1995 Cherokee 4.0 with no power options.;
2011 GC Overland Hemi
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