Technical New guy with odd electrical problem

Earlier, 19EDDY30 questioned the battery size, which could be the issue. How did you go about choosing the battery size for your setup?

 

Unfortunately I didn't get to choose this one. The original builder who past away made a "battery box" under the car to fit this specific battery. He actually had 2, one on each side of the car and had all the wiring ran across the car and into the cabin under the passenger seat to a battery isolator/cutoff switch for aircraft. I don't know how he originally did it, but he used wire that had a copper core that measured 3/4of an inch thick or larger! Not exaggerating! I am not sure off the top of my head what the group size battery I have in there. This battery is looked to be used on a lot of larger import v6 and v8 suvs ,as well as some domestics. I have no electronics in the car except an electric fan and fuel pump. The car cranks fast and strong. The voltage doesnt even waver when the fan kicks on since the alt is way more than capable for the application. The battery wiring is just one of the reasons I ripped all the wiring out of the car and started completely over.

 

I am not an expert on electronics, but I would think you need a battery size somewhere around group 24. But check other older applications with a sbc and very few electronics. Then compare to what you are running. If your battery is too big or small you may need to swap it out. Good luck

 

I would like to hear more about how the battery could be too big.

 

I have done a fair bit with charging systems, but I too would like to hear how a battery can be too big. If you mean too heavy or too large for the location I can understand. But a battery is like a bank for electricity. It loans the car what it needs when you start it and the charging system pays it back. Of course it doesn't charge interest which is why they eventually fail. But you really can't have too much in the bank unless you can't get to it to use it.

 

First time hearing this analogy but this had always been my understanding as well. Once the car cranks and is running, the alternator should be supplying all power to the components. The battery is just there at that point to complete the circuit.

Talked with the gauge manufacturer and they said there is an issue with the tach. They are going to swap it out for me. Unfortunately it will most likely be a week and a half or two weeks till I get the replacement.

Trying to work with pertronix tech now to see what they will do if anything. Assuming the original coil was at 1.1ohm before I installed it, he did say that would cause the module to burn up. With the other 2 from them testing 1.1 out of the box new, it seems a pretty safe bet that this is the cause of the original module failure. Still waiting to hear more from them.

 

So the builder used too good a cable for the batteries, so you replaced them? Hmmm….

 

yeah...this keeps up--well..it will seem odd!

 

Again, this cable was 3/4in copper strand diameter without the shielding. As big or bigger than my thumb. Most racecars with rear mounted batteries requiring high cranking amps for high compression and big cubic inches will go overkill with 1/0 or 2/0 cable. This was a full 1/4in larger in diameter than 4/0 cable. It could not be looped in a 6in radius bend to get it under the seat in the car. He used the seat to hold the 4 of these wires and the switch down. Also there was no grommet around any of these wires coming thru the sheet metal and the ground shielding was already cut thru. It was just a matter of time before he had the positive cut and short directly to the body.

 

Heard back from pertronix again. Tech confirmed that if the coil reads 1.1ohm resistance it is faulty and will not only damage the coil more if ran, but will kill the module. Explained that by his definition of a faulty coil, I now have had 2 bad ones new out of the box before installation from Speedway Motors and the original that reads 1.1 was probably bad when I recieved it and that is what killed the module. I asked if there was anything we can do to make sure I get a good coil or to make it right and his response was "your getting it from a good vendor, just send it back and they will send another". To me thats basically saying just keep trying till you get a good one, good luck. So I will do some research see if Speedway will exchange it for a different brand. I do now have a 0.8ohm inline resistor to use as well. My only preference is that it is epoxy filled coil, not oil filled. Any suggestions?

 

Having 3 Pertronix coils with faulty resistance doesn't say much for their supplier quality control.

 

Did you verify that your meter is accurate? When multiple items comes up with the exact same incorrect value I would want to verify the measurement with other meters. Especially as low resistance is hard to measure accurately with a two wire meter - a four wire test (where the testing current and voltage drop across the device under test is connected by separate wires) becomes far more accurate. The two wire meter can't see the difference between resistance in the device and resistance in the leads, connectors etc, and zero calibration is often around half an ohm off.

A four wire test can be improvised with two meters (one for current, one for voltage), a power source (battery for example) and something to limit the current (light bulb for example).
Use battery with suitable light bulb in series to run a current through the device under test. Measure the current.
Use the volt meter to see the voltage across the DUT.
(If you only have one meter you can measure current and voltage separately, you don't have to measure current all the time, but accuracy will suffer as the current can change over time.)

Use ohms law to calculate resistance; Volt/Amp=Ohm
If the current was 1.36A and the measured voltage 1.96V the resistance would be 1.96/1.36= 1.441 ohm.

These improvised four wire tests can be very accurate even far down in the milliohm range, as long as the meters are good and the test current is suitable - big enough to produce a easily measurable voltage drop, small enough to not heat the DUT enough to change the resistance value too much (yes, this method will easily detect the resistance change in metal due to changing temperatures - you can hook it up and see the voltage drop rise as it heats up).

 

Yes, multiple meters used and other coils and resistors tested on other cars were dead on what they said they were supposed to be. Holding the leads together had a reading of 0.0ohm.

 

Meter accuracy ran through my mind also, but I don't think that's the issue for multiple reasons. First, the OP states that he checked the meter against other known resistors and got the right readings. Second, he's received three bad coils in a row and Pertronix seems strangely nonchalant about that. Is their quality control that bad? And third, the OP has killed a couple of modules, a fairly sure sign that the coil primary resistance as measured is too low, allowing excess current to the module.

At this point, I'd change horses. At the risk of sounding like an advertisement, I'd recommend an MSD 6A box be added. He can keep both the module and coils he has (although the coils are still pretty suspect in my mind). The module will now be used strictly as a trigger for the box, with very little current to it so burn-out is gone. And many guys don't know how the MSD box differs from all the others. First, there's the Multiple Spark Discharge. At slower engine speeds, it continually sparks (for up to 20 degrees of crankshaft rotation) so this greatly reduces coil heating from extended dwell time. Second, it boosts voltage to the coil primary into the 500V range, contributing to a hotter spark. To prevent coil overheating, it also limits current by RPM. It will support a coil primary resistance as low as .7 Ohm with no ballast resistor needed. I've used OEM coils with these with excellent results.

No ignition system will add high RPM power to an engine, but a properly sized one will enable you to get all it has. With that said, the MSD does offer a few small advantages. Better starting, improved idle quality and a small but measurable increase in low-speed (under 3K RPM) fuel economy.

And because all high-current switching is done in the 'box', much larger (and more robust) components can be used.