Thread: HEI voltage drop ?

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I have been tring to track down a miss on my small block chevy and while checking to make sure everything was getting pllenty of juice I found that on my hei hot wire from the key switch has steady 13 volts at the cap,But after the motor is running the volts jump all over the place.As high as 14 and as low as 1 or lower.How can this be right?Shouldnt it have a steady 13 volts or more at the coil all the time it is running?Could it be in my tester?(which is new)Is there something I am missing or is there a problem in the system?I even ran a hot wire right from the battery to the hei and it does the same thing. Voltage is fine untill I crank it then it goes up and down no matter the rpm,and voltage at the battery is fine.
PLEASE HELP IM STUMPED.

The motor doesnt try to die when the voltage goes low but it bounces so fast that it might not have time to die before the voltage goes back up.Just some more info....

The frequency of the spark is probably faster than the reaction time of your voltmeter. To see that voltage accurately you would have to look at it with an oscilloscope.

Finally a techie question I actually know something about.

So there might not be a problem.What you said makes since.I was hoping I found an easy fix to the miss I cant find.Oh well........Ill keep checking,thanks for the info.

A couple of easy and cheap things you could do would be to make sure there are no cracks in the porcelain on the plugs and the resistance/impedance of each plug wire. I'm not real fond of any plug wire with over 1k of resistance.

when in doubt OHM it out... would need osciliscope to see the firing line. cylinder inhibitor ( shorts out cylinder ) could find a miss. 50-100 rpm drop per cyl is nominal. when I took an auto class, we had a osciliscope that had an inhibitor function, would kill the cyl's automaticly and create a balance percentage. hooked it up to a junker tempo ( was sombody's car tho ), ran the test like 5 times, almost killed the motor.

Before we go off into an alternate universe, let's go back to basics. Primary voltage can and will affect secondary voltage, afterall it is what produces the secondary side of the circuit to fire the plugs. BUT! secondary voltage does not and cannot affect the prmary side. The voltage problem being read is not a reaction of bad wires or plugs feeding back to the primary side of the coil. It's not withstanding that these items are not causing the miss or poor run conditions. It's just that plugs and plug wires do not make primary voltage functions do what these readings are doing.

One thing you mite check is the ground strap that hooks to the coil under the cap. I have seen a lot of people with the same problem when it can be as simple as a bad ground.if your hei has been worked on it could be the strap was not even put back on, or the black wire is broken.The engine will still run but will miss at high rpm or under a big load.And if this is the problem, check the leads to the winding on the coil they are probably melted and could short on the field frame.Jeff

I'd go back to where Nitrowarrior was taking you. It sounds like it might be a case of "garbage in, garbage out". If you have a fluctuation in the hot lead voltage then you WILL have a miss on the secondary side.

Voltage fluctuations on electronic motors are worse than on an old fashioned points based system. In both systems when the voltage drops you lose spark immediately (or nearly so). When power resumes it take a bit of time for the electronics version to get it's stuff together again, this stretches the event out and makes it more noticable.

Was the engine miss coincident with the measured voltage dropping down to 1V (or less)?

Where was the ground reference probe (black lead) of the meter placed when you were making these measurements? On the block or at the battery post?

With the probes placed in the location where you saw the voltage fluctuations try wiggling the wires/harness to try and simulate the vibrations experienced when the engine is running?

Cheers, Mark

First, you cannot use a volt meter to check the high voltage side...just like Corvette64 said.

Second, remove your cap and rotor. Look at the rotor closely on both sides near the center. HEI rotors typically fail by arcing thru the plastic near the center and allow a ground to the distributor shaft. The symptom begins as a random miss and eventually grounds the thing entirely. If it is a white rotor, the arc hole is easy to spot, on a black rotor, it is tougher to see.

mike in tucson

When I unhooked the lead coming from the switch and hooked up the lead straight from the battery,I tested both wires the one hooked to the coil jumped around.The one from the switch that was not hooked up just hanging there held a steady voltage.There may be nothing wrong with the voltage jumping around while the motor is running but it doesnt seem right.
Corvette64 may be right about checking it with a multitester.