All Small Six Diagnosing with air/fuel meter

[sixtseventwo4d]

The MSD 6A box uses 0.7 amps per 1000 rpms.

Where is this measurement taken?

 

[pmuller9]

Where is this measurement taken?

Current draw from the battery.

 

[DON]

A parallel (there and back) circuit helps to prevent a problem from being introduced the way triangulated ground points do.

I think that is the way I was testing. One lead at battery and the other one at the terminal at the “ appliance “.

 

[DON]

I think I can test most of them with the engine running.
Still would like to know about testing the ac. Is the meter on a/c? Just don’t want to blow a fuse or worse, in the meter

 

[DON]

My neg battery cable goes to the block, then a cable from the block to the engine mount base

 

[DON]

What gets me is the installation has been fine for years and corrosion is not a factor. Rpm’s to 4k frequently and occasionally to 5.5 ish. It would seem to me it would have started at higher rpm if it was a wiring issue. But abruptly starting at 2k?? Electronics are not involved in common sense!!

 

[sixtseventwo4d]

Current draw from the battery.

I am getting a draw of 2.8-2.9 amps at idle (below 1k) and it will climb to 4.6 amps or so at 2500-3000. I'm using a clamp meter.

 

[Bronctopia]

Still would like to know about testing the ac. Is the meter on a/c? Just don’t want to blow a fuse or worse, in the meter

If you are trying to test for ripple, you will set the meter for AC-voltage, and then test across the battery leads with engine running.
What you want to see is less than 0.1V.
Anything higher is usually bad alternator diodes, but with big caps misbehaving, much strangeness is possible.

Some cheap multimeters may not be able to measure this properly. You also won't get frequency information like with an oscilloscope.
As long as you keep your multimeter in voltmeter mode, you should be safe.
It is ammeter mode that usually smokes the fuse. Those with clamp ammeters are usually safe as houses, if a little less precise.

 

[pmuller9]

I am getting a draw of 2.8-2.9 amps at idle (below 1k) and it will climb to 4.6 amps or so at 2500-3000. I'm using a clamp meter.

How much does it draw with the ignition switch on without the engine running?

 

[hodaka100]

Isn't a clamp meter just for AC and you would use an inline meter for DC? If the MSD uses less than 10 amps at high RPM, why does MSD recommend a heavy 10 gauge wire to the battery?

 

[Bronctopia]

Isn't a clamp meter just for AC

Used to be the case, when clamp meters were limited to detection by induction.
But modern Hall sensors can detect the magnetic field produced around a DC wire.

 

[pmuller9]

If the MSD uses less than 10 amps at high RPM, why does MSD recommend a heavy 10 gauge wire to the battery?

Because the average current is low but the peak current is high.
Think about what the MSD power supply is having to do.
It is charging a capacitor up to 520 volts.
The initial charging current is high and decreases exponentially until the capacitor is charged. Then there is no power consumption until the capacitor is discharged across the ignition coil and needs to be charged again.

 

[DON]

Maybe amp draw is affected by the coil being used??

 

[sixtseventwo4d]

How much does it draw with the ignition switch on without the engine running?

barely .4 amps

 

[sixtseventwo4d]

If you are trying to test for ripple, you will set the meter for AC-voltage, and then test across the battery leads with engine running.

The accuracy of this check will be better if taken at the battery post on the alternator.

 

[Bronctopia]

The accuracy of this check will be better if taken at the battery post on the alternator.

True, but frequently hard to access.

 

[pmuller9]

Maybe amp draw is affected by the coil being used??

No
The capacitor has about 140 mJ of energy when it is fully charged and discharges that 140 mJ of energy into whatever coil is being used.
The energy draw to charge the capacitor doesn't change.

 

[DON]

Ok, check voltage several places running and not running, starting with the battery
Not running: at battery 12.73
At MSD terminal at solenoid 12.73
At terminals just going into MSD box 12.73
At frame MSD ground terminal and battery + post: 12.73
At alt post. 12.73
Rechecked all the above with engine running
Battery voltage 14.5
All the rest were no more than 0.15 v less

Amp draw, key off: 0.43 at + cable
At idle. 4.9 Amp

At MSD + cable at idle 5.3 a
At 1800 rpm. 5.7 a
AC check :
At battery terminals: it did very, it scrolled through auto modes. Best I could tell 7 v
I was suspicious so unhooked the alternator completely, and it did about the same reading, maybe a little less
First video (IMG4271)it with alternator running, second (IMG4274)is with it unhooked.

 

[Bronctopia]

Doesn't look like standard Alt ripple, because that would cause a fairly steady AC overlay at some multiple of the Alt RPM.

But that Auto ranging sure is annoying.
And it 'might' be hiding something in the constant display refresh.

I would force that with the 'range' button to somewhere in the 20V range.

Most likely what it was reading are the transient fast spikes that happen in most spark ignition 12V systems.

 

[pmuller9]

With the ignition key off, how would you have .43 amps at the battery cable without the battery going dead after a while?