HELP - hot ignition resistor

[pcolette]

I noticed a hot electric smell coming from under the dash and traced it to the ignition resistor on the front cowl. Almost too hot to touch! I had only been running at slow speeds and low rpms and as I understand it this should only get warm if the rpm limiter cuts in and shorts the power to the coil using the resistor. Is this correct?

When I turn on the ignition key the 4500 dash indicator flashes once which is supposed to indicate that the rpm module is working.
I drove it multiple times with the same results - the longer the drive, the hotter the resistor. The truck runs and drives great.
So, what's anyone suggest as my diagnostic path?
I was going to jumper the 2 center pins on the rev limiter plug (thanks Jim L.) but assuming this fixes it I'm not sure if this tells me the rev limiter is bad or the resistor is bad.
Or should I start by measuring voltage at the coil?

Thoughts, comments, suggestions are appreciated!

[Jimm391730]

If you can touch the resistor and hold your fingers on it for 5-10 seconds then it is not too hot. The resistor's purpose is to "burn off" excess voltage by turning it into heat.

Cranking the engine to get it started draws the battery voltage down and this is the time when a good ignition is critical. So the ignition is designed to run on 17-20 volts as might be seen during a hard cranking. Once the engine starts, the alternator kicks in and starts charging the batteries at ~28V which would cook the ignition system - hence the ballast resistor is needed to reduce this voltage. When you push the starter button, the ignition is fed directly from the battery and when you release the button then the ignition is fed through the ballast resistor.

As best I know the 4500 RPM module interupts the current flowing to the coil (to stop the ignition completely) if the speed goes up too far; it doesn't "short" out the coil. So this would not heat the resistor more than normal; it actually would decrease the heat generated.

If water or spit sizzles when touched to the resistor, THEN it is too hot. They are really designed to be able to get quite hot without damage. The ballast resistors are pretty bulletproof and don't fail easily.

But if the smell is new then obviously something has changed. Check the battery charging voltage (should be 27-29 volts), the voltage on the + side of the coil to chassis (should be 17-20 volts). Have you changed coils lately? Is the point dwell correct (check this even with a Pertronx) as this could allow too much current through the coil? Is your coil getting too hot to touch?

[pcolette]

Jim, thanks for the information and pointers. I'll start by measuring the voltages and checking the coil temp. The resistor never got so hot that I could burn my hand on it but there was a noticeable smell of hot wires/metal. If the resistor wasn't so expensive ($175) I'd put a new one in.

[Jimm391730]

The resistor never got so hot that I could burn my hand on it but there was a noticeable smell of hot wires/metal.

I'd look carefully under the dash for signs of wires that had overheated/melted; there may be something else wrong that is totally unrelated to the ballast resistor.

[David Dunn]

Check your system voltage while the engine is running at speed. Your voltage regulator may not be doing it's job and cooking electrical items.

[pcolette]

UPDATE

All voltages were within specs and after comparing the resistor temp (by feel) between both of the 710's I found that it was not overly hot.

I took it out for another test run and there was a "pop" and the smoke genie appeared! The pop was breaker #12 and the smoke came out of the brake light switch. Turns out the hot, electrical smell was the switch slowly shorting out. I've got a new one arriving today and that should take care of the problem.

Thanks for the help!

[Jimm391730]

You must have asked the Genie to help you find the problem, and he did! You know what they say, be careful what you wish for...

Glad you got it figured out.

[M Wehrman]

I had one switch burn thru and light up brake fluid ,flames from under the dash really bother me.

[pcolette]

Fortunately no flames!

The switch insert (plastic?) was definitely loose and the brake fluid must have been seeping past it. All is now working properly again and the Pinz is happy!