Coils, ballast resistor, and 4500 rpm module

[andy]

I asked this in the Bosch coil thread and nobody answered, so I'll ask again because I'm getting all the signs from one of the 4500 rpm module going bad threads.

This is what I asked in the Bosch Coil thread: I'm probably displaying my ignorance here but, if it is a 24v system, why are we using a 12v coil? Is that the function of the "ballast resistor" to reduce the 24v to 12v? I'm asking because all of a sudden I'm getting the 4500 RPM light on the 712 and there is no change in engine sound, speed, or anything else that is noticeable except the very scarey and annoying light going off.

The problem has been discussed here before, and I know I can jumper the module on the middle two pins, but if the coil is going or the ballast resistor is bad, what have I accomplished?

How does one isolate this?

Thanks

Andy

[StuartL]

I can't answer any of your questions except the one about the 12v coil. As far as I know (and I've been known to be unknowingly wrong) the coil is just an inductor. If it is just an inductor then it's a passive component and the risk of blowing it up relates to much more complicated formuli than just the input voltage. I do recall seeing many threads where people recommend running the 12v coil so it can't be all bad...

[andy]

Well, that's a start, thank you.

Andy

[Jimm391730]

if it is a 24v system, why are we using a 12v coil?

While the coils in question are most commonly used in a 12V electrical system, they are really a CURRENT operated device: When the Pertronix or points close, the current starts at zero and (because of the inductance of the coil primary) the current ramps up in a controlled way. When the points or Pertronix opens the magnetic energy is transferred to the high voltage secondary which jumps the plug. (This is all very simplified).

When used with "24 volts" the current would increase twice as fast as in a 12V system. When I say "fast" I am talking about the time, and remember when you used to set a points system to a specific "Dwell" setting? If you dwell on something, you spend some time on it; if the dwell was too short then there won't be as much time to build up the current, and the spark energy will be weaker. Too large of a dwell would let too much current into the coil, making it hotter and shortening the life of the coil, condenser, and points.

In the Pinz the ballast resistor usually allows the average voltage at the + terminal of the coil to be about 17 volts. I found that my RPM module was bad because I only had about 7 volts at the + terminal. With less voltage, the current ramps up slower, so the spark is weaker. This really showed up during the winter months as terrible running and missing due to the richer mixture in the cold weather. Both a richer mixture or more fuel/air in the cylinders (as when you step on the gas) make it harder for the spark to jump the plug, and as my spark got weaker it finally wouldn't jump as I put my foot into the pedal, but seemed to run OK at idle (with a minimal amount of air and fuel in the cylinder). Once I jumpered out the module, wow, what a difference.

So if your RPM light is flashing and you know you arn't anywhere close to 4500 RPMs then we can assume the module is bad. The module operated by measuring the "current ramps" that go to the coil + terminal; and if these exceeded 4500 RPM (150 current ramps per second, at 4500 rpm) then the module was supposed to turn on the light and start shorting out the power to the + coil terminal, cutting off the spark, and therefore preventing the engine from over revs.

[undysworld]

Jim,
Thanks for a great explanation of that.

[andy]

Thanks, I guess the first thing I'll try is to jumper the module and see what happens.

Cheers,

Andy

[Winter Beater]

Is there any way to reduce the number of parts (that can fail) ie: remove 4500 rpm module, ballast resister ? It's been a long long time since I worked on ignition systems and cannot remember the schematic now.

My module periodically acts up and somewhere near 4200 rpm the tach moves up to 6000 rpm and the sputtering starts until it drops below 4000 rpm , then all goes back to normal.

[Jim LaGuardia]

Just unplug the module and jumper the ceter terminals. I find a lot of iffy modules, just best to remove it from the circuit if you suspect problems.

[ElectraPinz]

Jim,

Your discussion came in handy today as I was trying to figure out a sick engine issue. Only problem is that the coil voltage (+ to ground) with the 4500 rpm module connected was 10volts, and with the connector jumpered (middle to middle), it was 9.8 volts.

The full details are posted in another thread: viewtopic.php?f=12&t=6698

Any suggestions as to what next?

Thanks

Ben

[Jimm391730]

Just to check, you are measuring the voltage from chasis/engine (either location should give very similar results, or the engine ground strap is bad if they are different) to the + side of the coil which should go back into the wiring harness, not the (-) side of the coil that goes to the points/Pertronix, right?

With the RPM module removed and the center two pins jumpered well (the jumper will carry a few amps) I get about 17 volts on the + of the coil; you can try using a wire to short the (-) side of the coil to chassis and see if the + side goes up to ~17V. If so, the engine ground is bad or missing. If the voltage doesn't change then there is a wiring issue or the ballast resistor is going bad. The ballast resistor looks sorta like a bicycle bell (silver, round, with a curved top, and a thumbscrew that holds the "bell" cover on) that is mounted to the front of the truck under the dash. I can't remember exactly how many wires attach to it, but one should have the same voltage that you get on the + of the coil, the other (assuming just two connections) should have 24V. If you get these values then either the resistor is bad (having higher than normal resistance, which makes the voltage out lower) or it could be a bad coil that is drawing too much current.

Let us know what you find.

[ElectraPinz]

Ok - I think we're making progress.

I get the same coil voltage (10.5) between the (+) terminal and ground regardless of which ground I use (chassis or engine) and regardless whether the (-) terminal is grounded (chassis or engine) or not.

I checked the 4500 rpm module and got 25 volts to ground (really good news, as that means I don't have to go looking at the rest of the supply side wiring).

I took off the cover of the resistor. There are three wires. One wire read 25 volts to ground, and the other two read 10.5 volts to ground. Looks like the resistor is the culprit.

By the way, I thought the thumbscrew would fall off after loosening. Not so. It is attached to the cover. The cover was stuck so hard that I broke the thumbsccrew off of the cover. Finally managed to pry the cover loose (it has a little lip), however. No big deal. Just thought I would mention this for future adventurers.

Ben

[ElectraPinz]

Now that I've just popped for a $109.00 ballast resistor, I'm wondering what might have made it crash?

I would hate to install a new one only to have it crash also.

Anyone have any ideas or theories?

Thanks,

Ben

[ElectraPinz]

Well, it apparently wasn't the ballast resistor (ouch!)

Installed the new resistor and tried to start the engine - I got a very slow crank and a sputter, that's all.

Having read some more posts, the voltage out of the ballast resistor with engine not running can be in the 10.5 neighborhood. Can anyone confirm that?

I've checked voltages and resistances of every other component in the circuit. Everything seems normal (coil, noise suppressor, 4500 module shunted), except that I'm getting 25 ohms from + to ground at the resistor, which translates to 27 ohms at the coil + terminal and 30 at the coil - terminal. That doesn't seem right to me, but not knowing what else is in that circuit, I really wouldn't know.

Barring anything suspicious relating to the above continuity between the ignition circuit and ground, my next guess is the Pertronix ignitor. Anyone know of a clever way to check that? I'm still thinking strength of spark is the issue. By the way, I checked the spark plugs, and they are blackened, but in good condition.

Thanks,

Ben

[edzz]

My solution for testing the igniter, would involve putting in my spare igniter or converting back to the points. I keep both in my field spares kit; never know when we may get hit by an EMP.

Or you could follow the Petronix advice on their web site, if that fails their tech support line may be able to help.
http://www.pertronix.com/support/tips/default.aspx

also http://www.ttalk.info/Tech/PerNegGndTest.htm