Low volts to coil, but resistances close to spec


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By peeeot - 15 Years Ago
The shop manual says I should see at least 8.5 V at the battery terminal of the coil and at most 0.1 V drop from the distributor terminal of the coil to ground.  I am getting 6.5 V and 0.16 V, respectively.

As per the manual's detailed instructions, I checked out all of the wiring, found some issues, and tried to deal with them.  It says that the voltage drop to the battery terminal of the ignition resistor should be no more than 0.2 V; the best I could get it was 0.25 V.  Even so, with 12 V coming into the resistor, there are just 6.5V coming out of the resistor, yet the resistor shows the correct 1.4 ohms specified when tested.  If I'm losing all my voltage at the resistor, yet it is of the correct resistance, what gives? 

As for the ground side, about half of the voltage drop occurs between the terminal on the points assembly and the moving breaker arm itself (which is to say, NOT through the contact points themselves) and the other half is through the wire that goes to the distributor terminal of the coil.  I cut several inches of length out of the wire and spliced it back together in an effort to reduce the voltage drop, but it didn't help much.  I tried another wire as well, but even though they show no resistance when tested, they consistently cause a voltage drop.

Any thoughts on how to get the system in spec, or whether it's worth it?

By rick55 - 15 Years Ago
I would think there are several things going on here. Your battery voltage is low. A fully charged lead acid battery should show 13.2 volts. 12 volts is a nominal figure. What current is flowing in the circuit? This will affect the voltage drop across the resistor. Do you have the correct coil fitted? 0.05volts difference in your reading to what you expected is nothing to be concerned about. As you found reducing the length of the wire made no difference as the length of the wire will not be contributing to the voltage drop. It is more likely to be the result of poor connections, undercharged battery, a coil with too much resistane. Disconnect the ballast resistor to see what voltage you have without it in play. You may have to take this reading at the ignition switch and then work forward to the coil.

Regards
By Hoosier Hurricane - 15 Years Ago
I believe the 8.5 reading is with the engine running.  Were you running it when you checked it?
By crenwelge - 15 Years Ago
I agree with John, when the motor is running you will have gen output voltage instead of battery voltage to begin with. Is it not running good or are you just worried about it?
By peeeot - 15 Years Ago
I made the coil voltage test with the engine off, key on, points closed, which is what the manual calls for to make the test.  It's true that the battery charge was a little low from having the door open and other things I was doing before making this test, but I don't think that's the whole issue because if I bypass the resistor I get about .2 volts less than battery voltage at the coil; in other words, the resistor has a voltage drop of around 6 volts.  That being the case, even a 14V battery would only supply 8V to the coil, which is less than spec.

The car runs fine, so I suppose you might say I'm just worried about it Wink  I want it to be tuned properly, and unlike modern cars it can't let you know when things are a bit outside of where they should be, only when they're way outside.  People are always saying that the more powerful spark delivered by electronic ignition improves performance, economy, etc, so I figure my points aren't doing the best they can do if they aren't supplied with the proper voltage.

V=IR, so if V=6 and R=1.4ohms, current must be 4.28A.  V should be 3.5, so current should be 2.5A at 1.4 ohms.  If I'm using that formula correctly, that means that the resistor I have is passing too much current.  I think I bought my resistor from Autozone; would it be worth going to a classic vendor instead?

Or am I chasing a ghost?

By crenwelge - 15 Years Ago
I think you may be chasing a ghost. However, I buy NOS ballast resistors on eBay. I like to see the made in USA on the box.
By peeeot - 15 Years Ago
You know, you are probably right.  I just found a curious specification in the manual, in light of the calculations I just did.  It specifies that the coil, engine stopped, should draw 4.5A, and engine idling should draw 2.5A. 

I haven't checked, but I bet there's 8.5V or more when the engine's running.  Perhaps I have too much faith in the shop manual Hehe

By shakey pete - 15 Years Ago
a mechanic told me once if it aint broke don,t fix it   w00t
By rick55 - 15 Years Ago
a mechanic told me once if it aint broke don,t fix it


This is sound advice.

The coil draws less current when the engine is running due to the choking effect that opening and closing of the points causes with relation to the coil switching on and off. This is why there is more current draw when the ignition is on with the engine off. If the formula of R=E/I is applied then 12/4.5 gives an effective resistance of 2.6ohms. With the engine running the effective resistance is 12/2.5 - about 4.8 ohms getting close to double the resistance.

If the battery voltage is low this will cause more voltage drop, effectively dropping all values, except the static resistance. Every point in the ignition circuit that has a joint or switch contact will have a resistance value of some degree or other.

Hope this isn't too much gobbledegook but the first comment holds true unless you are experiencing problems.

The best rule of thumb is to use the best quality components you can afford and be comfortable in the knowledge that it is probably better than what was fitted from new.

Then just enjoy.

Regards
By peeeot - 15 Years Ago
Thanks guys.  I will take your advice and move on from this.