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Re: No spark - multimeter troubleshooting
Phak1 #1402440 Mon Mar 22 2021 12:00 AM
Joined: Jan 2014
Posts: 1,623
J
JW51 Offline OP
'Bolter
Originally Posted by Phak1
Try jumping out the resistor and if it starts, you’ll know that is the problem or at least part of the problem. It won’t harm anything! If it doesn’t start you’ll know that it’s down to distributor, coil or the wiring.

I don’t know if you know this or not but a 12v conversion that has ballast resister is also supposed to have a starting bypass circuit. Usually a wire from the starter switch that bypasses the ballast resistor on starting to supply a full 12v to the coil. It almost sounds to me that you are missing this bypass circuit. Many people run their trucks without this circuit but it’s not recommended especially if you live in colder climates.

Good luck!


Yes, this has that bypass circuit.

Re: No spark - multimeter troubleshooting
JW51 #1402442 Mon Mar 22 2021 12:05 AM
Joined: Feb 2004
Posts: 23,002
H
'Bolter
Originally Posted by JW51
Not sure what the deal was yesterday.

Flooded, maybe? If it happens again, try holding the throrrle wide open and crank it for several seconds. Since the advent of "instant start" fuel injection, flood clearing has become a lost art!
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: No spark - multimeter troubleshooting
Hotrod Lincoln #1402447 Mon Mar 22 2021 12:39 AM
Joined: Jan 2014
Posts: 1,623
J
JW51 Offline OP
'Bolter
Yeah, I definitely didn’t have spark yesterday. Probably some gremlin that will haunt me later. Flood procedure is handy with small engines too.

You think the low voltage downstream of resistor is coil and not the resistor itself?

Re: No spark - multimeter troubleshooting
JW51 #1402453 Mon Mar 22 2021 12:58 AM
Joined: Feb 2004
Posts: 23,002
H
'Bolter
Coil failures are few and far between, but it's more likely that you might have a coil with the wrong resistance in the primary winding. Try measuring the resistance of the primary with both wires disconnected. A coil that will flow approximately 5 amps with 9.5 volts applied to it will have somewhere near 1.9-2 Ohms of resistance. 5 amps is the maximum a set of points will handle reliably without burning.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: No spark - multimeter troubleshooting
JW51 #1402498 Mon Mar 22 2021 12:35 PM
Joined: Feb 2019
Posts: 2,087
P
AD Addict
Originally Posted by JW51
If I’m low at the + side of the coil, and the downstream side of ballast resistor...is that a sign of bad ballast resistor?

FYI, the symptom for a failed resistor, is when the engine starts but stops as soon as the key/foot starter is released.


Phil

1952 Chevrolet 3100
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Re: No spark - multimeter troubleshooting
Hotrod Lincoln #1409322 Mon May 10 2021 02:24 PM
Joined: Jan 2014
Posts: 1,623
J
JW51 Offline OP
'Bolter
Originally Posted by Hotrod Lincoln
Coil failures are few and far between, but it's more likely that you might have a coil with the wrong resistance in the primary winding. Try measuring the resistance of the primary with both wires disconnected. A coil that will flow approximately 5 amps with 9.5 volts applied to it will have somewhere near 1.9-2 Ohms of resistance. 5 amps is the maximum a set of points will handle reliably without burning.
Jerry

Back to this saga. Engine has been run successfully several times since this thread began, but the weird voltage has been bugging me so I tried a new balance resistor last night and checked voltages just for experimentation purposes.

Old ballast resistor:
12.5 volts at battery terminals
11.7 volts at resistor input (I swear I used to have full battery voltage here, but I guess I didn’t document it)
5-ish volts at resistor output

New ballast resistor:
12.5 at battery
11.7 at resistor resistor input
3.5-ish volts at resistor output

So...if 9 volts to the coil input is considered about right...the new resistor took me the wrong direction.

Jerry mentioned it’s possible I have a coil with improper resistance. I did measure the primary last night and it was 1.3 ohms. What else would I need to do to test the coil? Or just should I just try new one and test the same spots for voltage?

Edit: I did forget to disconnect wires before checking the coil primary voltage.

Last edited by JW51; Mon May 10 2021 02:33 PM.
Re: No spark - multimeter troubleshooting
JW51 #1409333 Mon May 10 2021 03:21 PM
Joined: Jul 2000
Posts: 1,310
G
Insomniac
My understanding is that the old ballast resistor readings were made before you cleaned the points. Reducing the resistance across the points by cleaning them will increase the voltage drop across the resistor. (Kirchhoff’s Voltage Law)

If you want to compare your old and new resistors, set your multimeter to ohms and check them that way.


Gord
----
1954 1/2 ton 235 4 speed
Re: No spark - multimeter troubleshooting
Gord&Fran #1409346 Mon May 10 2021 04:39 PM
Joined: Jan 2014
Posts: 1,623
J
JW51 Offline OP
'Bolter
Originally Posted by Gord&Fran
My understanding is that the old ballast resistor readings were made before you cleaned the points. Reducing the resistance across the points by cleaning them will increase the voltage drop across the resistor. (Kirchhoff’s Voltage Law)

If you want to compare your old and new resistors, set your multimeter to ohms and check them that way.


Good comment....I should clarify.

1) I don’t recall at what point I cleaned the points. Probably before taking any of the readings in the original thread....some weeks ago.

2) The voltage readings of the two different resistors took place just last night (on the firewall of the truck). All other variables should be the same. The old was taken before removing from the truck. The new was taken after installing on the firewall. I will measure the resistance of each disconnected from the system as well.

Re: No spark - multimeter troubleshooting
JW51 #1409425 Tue May 11 2021 02:38 AM
Joined: Jan 2014
Posts: 1,623
J
JW51 Offline OP
'Bolter
Had a small breakthrough this evening. Long story, but the coil that’s on the truck is apparently a 6v coil. I had a guy do the brakes for me before even bringing it home after buying it. He installed the coil and all the verbiage was facing the engine and I never had it off until tonight.

From what little reading I’ve done tonight, sounds like a 6v coil will still function ok in a 12v system (it’s about ohms and not volts) but at least I can understand now why I was getting 3.5 or 5 volts on the resistor output.

Re: No spark - multimeter troubleshooting
JW51 #1409521 Tue May 11 2021 04:35 PM
Joined: Mar 2019
Posts: 115
T
'Bolter
It seems like you are dealing with lots of variables and inconsistencies. Troubleshooting these situations can drive you crazy.

Whenever I get into a mess like this I step back and organize myself and my test plan. You have at least 3 variables you need to reconcile. Which resistor produces what results, which coil should you be using, and is the wiring and ignition switch working correctly?

As a starting point I would connect a jumper wire directly from the starter terminal/battery to the coil after disconnecting the wire from the resistor. Determine if the engine starts and runs as you expect. Change only one variable at time as you troubleshoot.

Remember that the major reason for the resistor is to reduce the voltage across the points when the engine is running. The lower voltage reduces the heating and arcing of the points so they last longer.

The idea behind a "resistor bypass circuit" for starting is to simply give a slightly hotter spark to get things going. In my experience if everything is in good condition that circuit is not essential.

I will not even start the discussion about whether a coil is a 6 volt or 12 volt and if one will operate properly on the opposite voltage.


Rusty
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