[b]8N Capacitor Questions[/b]

Rusty · Mar 24, 2006

[b]8N Capacitor Questions[/b]

What can cause the capacitor on a 1952 Ford 8N with side mount distributor to fail? Can a failed capacitor cause the tractor to stall while running and not restart? My son has had three recent failures on his 8N. The first was immediately after replacement of sparkplugs, plug wires, points and capacitor. I'm not sure of the details of the second failure. The third failure was with the tractor running. In all three failures, the final fix was replacement of the capacitor. /forums/images/graemlins/confused.gif Thanks in advance for any assistance! Rusty

BobRip · Mar 24, 2006

Re: [b]8N Capacitor Questions[/b]

Like many components capacitors can fail from heat, over voltage, over current and in your case what may be infant mortality. In other words the capacitor was made wrong. It does seem strange to have three fail immediately. If they all came from the same lot, this sounds possible. What symptons indicated that the capacitor failed?
If the coil had a short this might pull too much current which might cause overheating in the capacitor. Since you have replaced everything else you might check the coil.

If the capacitor shorts the engine will stall. If it opens the points will deteoriate quickly.

This is rather rambling, and I am not sure of the problem or that I have helped.

Bob Rip

Bully · Mar 24, 2006

Re: [b]8N Capacitor Questions[/b]

I your 8N a 6 volt system, or converted to 12 volts. To much voltage on the positive side on the coil will burn them up. If the ignition is left in the on position that can lead to failure also

Soundguy · Mar 25, 2006

Re: [b]8N Capacitor Questions[/b]

( over voltage, over current )

( If the coil had a short this might pull too much current which might cause overheating in the capacitor. )

Uh.. do you even realize what role the capacitor plays in a points style ignition?!?!

A shorted coil won't bother the capacitor at all.. actually.. it will make -less- work for the capacitor. Google 'switch debouncer' if you want to know what the capacitor in a points ignition does for a living..

It's quite common to see a bad 'lot' of caps in a row.. I suggest going to a different supplier to get a cap once you get 1 bad one from a store.

Also.. as far as votlage goes.. the 6/12v isn't an issue for a standard automotive type ignition capacitor.. they are rated for nearly 10x that voltage based on their dielectric properties.. if not more.

When caps 'open' , points burn.. typical symptoms are a tractor that starts.. but then stalls out very soom, and then you need to worko ont he points after that.

When caps 'short' you get weak spark, or no spark, as the points are 'across' the pointsgap, and without the points opening and closing.. there is no pulsed dc and colapsing magnetic field created byt he coil primary, thus inducing a voltage ont he coil secondary.. thus no spark.. or very weak spark, in the case of a partially shorted cap.. etc.

This kind of ignition theory has been around since the early 1900's.. no magic hear.. just E=IR basic electronics 101..

Soundguy

Fredex · Mar 25, 2006

Re: [b]8N Capacitor Questions[/b]

no magic hear.. just E=IR basic electronics 101..

Or as we used to say in the test room: "Nuthin to this 'lectrical work - It's all just battery and ground."

Rusty · Mar 26, 2006

Re: [b]8N Capacitor Questions[/b]

BobRip: Thanks for the reply! I can't say all three capacitors failed "immediately". The first capacitor did not allow the engine to start at all. The third capacitor survived multiple starts and the engine ran for about one hour. A new coil was installed between capacitors one and two. The sympton was no or very weak spark until a new capacitor was installed. Rusty

Rusty · Mar 26, 2006

Re: [b]8N Capacitor Questions[/b]

Bully: Thanks! The tractor has a 6V system with an 8V battery. My son does know to not leave the ignition switch "on", but does anyone know what that means? How long "on" is too long? Are we talking seconds, minutes, hours or what? Rusty

Rusty · Mar 26, 2006

Re: [b]8N Capacitor Questions[/b]

Soundguy: I was hoping that you would see this post and reply. Thanks for the comments! My Google search found mostly solid state circuitry for switches and counters. One article showed a plot of transient opening and closing, which I remembered, but might not have been able to explain without a refresher. The points have been OK after each failure. I guess my real question is, other than leaving the ignition "on", is there another component or operator action that shortens the life of a capacitor so drastically (assuming the capacitor is not bad off the shelf)? Rusty

Keoke · Mar 26, 2006

Re: [b]8N Capacitor Questions[/b]

Rusty, Soundguy has it right, all the capacitor does is provide for a clean collaspe of the magnetic field tha is caused by the flow of current through the primary wiring. In short, without the cap you still would have a spark but it would be a weak one.
However, since you are running 8v instead of 6v you may need to resize your cap. There is a simple rule for checking cap sizing, it is called the "minus, minus rule".
It goes like this, minus material (hole) on the minus point (usually the fixed contact) means minus capacity.
So if your old points had a hole on the gnd point and a peak on the movable point then the cap is undersized. If the reverse is true then cap is to large. Caution gnd point could be pos polarity if tractor has a pos gnd system. Caps are rated in microfarads and can be measured with a meter that won't be easy to find. If present cap turns out to be undersized suggest you try a 12v cap instead of a 6v cap for starters. Caps are not rated in volts but get one from a 12v vehicle.
Also, never mount cap with pig tail facing up and if cap is mounted externally then put a band of RTV around pigtail and seal washer. Most cap failures occur when moistrue enters between plates in cap through pig tail end.

good luck,

Fredex · Mar 27, 2006

Re: [b]8N Capacitor Questions[/b]

I also think trying a cap from a 12v system is worth a try. George is correct about caps being measured in microfarads and the minus minus rule. However caps are also designed with a MAXIMUM voltage rating which is not often available info for ignition system caps. So by using the 8v battery your coil is receiving more current, thus a larger magnetic field, & therefore a higher voltage induced by collapse of the magnetic field when the points open. (Unless you added ballast resistance to the coil circuit when you went with the 8v batt.) Might be enough to break down a marginal or poorly made cap. Many 12v systems do have ballast resistance in the coil circuit, except when starting, but maybe are built to withstand a bit more peak voltage. Don't know for sure, but it is worth a try.

When you went to the 8v battery, did maintain the original ground polarity?

fred