Low Voltage at Coil

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ak_garner
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Low Voltage at Coil

Post by ak_garner »

Hey gang:

My Wag left me stranded the other day. Was on a drive and she backfired, and died hard. Wouldn't start.

Got a tow home, and tested for spark with an inline bulb tester. No spark. Tested the coil, an MSD Blaster 2, and tested ok. Tested for 12v at primary. It's only about 2.5V. ??

This ignition is pretty much stock. I see there's a voltage regulator on there, but not sure how to test that. Ballast resistor measures 2 Ohm.

I also wonder if the ignition switch has something going on.

I'm pretty green with the electronics here. Catching on, but don't have a full command of the systems.

Thanks for any help!

Adam
-1966 Wagoneer : Vigilante 327 - 3 speed manual T-85 to Dana-20 T case.
-1960 Ford Falcon
-1993 Range Rover Classic SWB
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tgreese
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Re: Low Voltage at Coil

Post by tgreese »

You have points ignition? Points open or closed? If closed, the ballast resistor forms a voltage divider with the coil internal resistance, and any leakage through the condensor. Points open should read full battery voltage if the condensor is good (should be no leakage).

Every Jeeper should have a spare coil, IMO. You can borrow a coil from another car to test. That's a better test than you can do with a multimeter of whatever. No spark means both your coil and condensor are suspect. You can also jumper from the battery to the coil to test for a problem in the ignition switch and ballast resistor. Won't hurt to bypass the ballast resistor for a few minutes, but don't leave it like that.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
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ak_garner
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Re: Low Voltage at Coil

Post by ak_garner »

I have a Lucas from my old Rover I may pop in to try. Thought about it, but wasn't sure if I'd blow the thing up.

I do have a points ignition. I graphed the voltage at the coil when trying to start it, and it showed a square(ish) wave only peaking around 2.7V. Not starting, just ignition on, was about that.

I can't find the resistor value in the service manual, and in fact it's not mentioned at all. I found a part listed that says it should be .05 Ohm. Not sure it's the right one, but bypassing makes sense for a test!

I'll report back asap.

Thanks Tim!
-1966 Wagoneer : Vigilante 327 - 3 speed manual T-85 to Dana-20 T case.
-1960 Ford Falcon
-1993 Range Rover Classic SWB

sierrablue
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Re: Low Voltage at Coil

Post by sierrablue »

I would guess it's your ignition switch. I had a similar thing where it was having more and more trouble starting, and then I was on my way up a hill on the way out of town when it surged a couple of times and died. Wouldn't restart, ended up waiting for my dad to bring the truck and tow me home.

When I got home, I tested and there was no voltage at the coil. Ran a wire off a prong from the switch and it took right off and ran perfectly.

Eventually I took out the switch, and when I pulled out the key cylinder, it was full of little bits of metal. Not sure what happened but it wasn't good.

Since mine's a '71 and yours is a '66, should be the same switch.
'71 Wagoneer (DD)
-B350 (HEI, iron 4-barrel, Edelbrock 1406), TH400, D20
-'74 D44 front (nonpower discs)
-custom headliner
-Front shoulder belts (rears eventually)

viewtopic.php?t=23070

There are 2 major differences between new Wranglers and FSJs. FSJs are meant to be both utilitarian and capable, not just capable. FSJs are also rarely initially recognized as Jeeps by the average American.
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tgreese
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Re: Low Voltage at Coil

Post by tgreese »

ak_garner wrote: Mon Mar 06, 2023 2:01 pm ... I graphed the voltage at the coil when trying to start it, and it showed a square(ish) wave only peaking around 2.7V. Not starting, just ignition on, was about that. ...
When you say "graph" did you use an oscilloscope? Electrically it's complicated, and your multimeter won't tell you much. However, you should see full battery voltage when the points are open. That's a circuit back to the battery with no significant voltage drop.

Try it hot wired (a wire direct from the coil to the battery). How did you evaluate the coil? Coils can suddenly fail; working fine on previous start, and nothing. BTDT.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
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ak_garner
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Re: Low Voltage at Coil

Post by ak_garner »

Screen Shot 2023-03-06 at 9.19.38 PM.png
Tried with both coils, and each was the same. I have this graphing multimeter that gives good basic info. Oddly, my coil only gets about 3V when cranking.

I feel like a shmo, but after I traced all of this, I noticed that the battery was pretty tired, and if I jumped it, it would spark a TINY bit... not very bright. I popped a different battery in and it sparked, but wouldn't start. Checked fuel and... uh, there wasn't any.

I ran out of gas.

Sorry for the circle. I smelled gas earlier in the afternoon, and swore my fuel pressure gauge was showing a couple PSI, but I guess I was sloppy. Probably need a fuel sender that works with my gauge! I usually just feel the tank, but forgot I'd tightened it more when looking for what sender to get. SORRY.

Thanks for the good insight! I think I figured out how to trouble shoot the ignition system pretty well for NEXT time.

:roll:
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-1966 Wagoneer : Vigilante 327 - 3 speed manual T-85 to Dana-20 T case.
-1960 Ford Falcon
-1993 Range Rover Classic SWB
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Stuka
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Re: Low Voltage at Coil

Post by Stuka »

So are you up and running now then?

And yeah, the newer multimeters with a built in scope are pretty cool. Though they tend to be too slow to fully register something like coil voltage when running, or even turning over.

But as an FYI, you should only have 12V going to the coil when the key is in 'start'. With the key in run, the voltage should drop down to like 7-8V.
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ak_garner
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Re: Low Voltage at Coil

Post by ak_garner »

YES, up and running.

But, this has been an eye opener - it appears that the coil is only getting about 3 volts. I can see that on the multimeter when the ignition is on, and it shows when cranking, it's pulsing the 3V at the primary.

I will try jumping the coil with a 12V supply and see if it changes. Electrical question - should I disconnect the wiring from the distributor that is supplying the 3V, or will the 12V jump be additive to that?

I've calculated I'm getting about 4 mpg, and the truck does run rich. I wonder if my spark weak on account of low voltage. And, the only thing I can find is that the ballast resistor is 2 ohms while the ones I can find as a replacement are .5ohms, but that doesn't make sense since a higher resistance would produce higher voltage.

So that might leave the other spots the other posts target - ignition switch, points condenser.
-1966 Wagoneer : Vigilante 327 - 3 speed manual T-85 to Dana-20 T case.
-1960 Ford Falcon
-1993 Range Rover Classic SWB
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tgreese
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Re: Low Voltage at Coil

Post by tgreese »

The ballast resistor limits current to the coil when running.

You could look up the part number in the '62-73 parts book on the Tom Collins site. https://oljeep.com/edge_parts_man.html Then search for that number online and see if there are any matches.

Next I would look at what part an online listing like RockAuto shows for the resistor.

Lacking that, we sold a lot of these ballast resistors for a '70s Dodge as a universal application back in the day.
https://www.rockauto.com/en/catalog/dod ... istor,7052
Universal-ish for passenger cars from the era. It'll work. I'd get the RU-11.

Any two wires in parallel will share the current, splitting it in proportion to their resistances. This is current, not voltage. Ohm's law, V=IR. Some current will go through the resistor, some through the hot wire. Five feet of 14 ga wire has a resistance of about 0.013 ohms. Your ballast resistor has a resistance of about 100 times that, so roughly 99% of the current will go through the hot wire.

Your graphing multimeter is making this more difficult than it need be. Any floating wire (open circuit) will have no voltage drop at its end, so the voltage you measure at the coil should be the same with or without the hot wire, as long as the points are open. The condensor is in parallel with the points, and should be an open circuit at DC, when the engine is not running. If it's bad it becomes a resistor to ground at DC and forms a voltage divider with your ballast resistor. Turn the engine so the points are open, turn the key on, and measure the coil voltage. Compare that to the battery voltage.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

sierrablue
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Re: Low Voltage at Coil

Post by sierrablue »

The ballast slowly bleeds off voltage--when you first flip the key on, it should be getting about 12V to the coil anyway. Then once it starts it should be slowly bringing that voltage down to about 7-10 volts, depending on the resistor. This way to make things catch you have full power, but you're not going to burn everything out after 200 miles because it's not constantly getting the 12 V. I dunno what voltage it should be getting while cranking, as the starter is drawing directly off the battery.

I suggest you go Pertronix or some other electronic ignition system (dunno if you have the 6 or the V8), as modern points just don't last. I burned through the set I had on the Jeep in less than 5k miles, with a brand new resistor and condenser. It's not worth it to keep the points, if you wanna drive it more than to shows and stuff.

I wouldn't rule out the ignition switch either, could be that it's just worn out.

I'd also check that you don't have a stuck needle/seat or anything like that in the carb. That combined with weak spark can make it incredibly hard to start, and would both contribute to awful mileage.
'71 Wagoneer (DD)
-B350 (HEI, iron 4-barrel, Edelbrock 1406), TH400, D20
-'74 D44 front (nonpower discs)
-custom headliner
-Front shoulder belts (rears eventually)

viewtopic.php?t=23070

There are 2 major differences between new Wranglers and FSJs. FSJs are meant to be both utilitarian and capable, not just capable. FSJs are also rarely initially recognized as Jeeps by the average American.
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Stuka
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Re: Low Voltage at Coil

Post by Stuka »

sierrablue wrote: Tue Mar 07, 2023 11:21 am The ballast slowly bleeds off voltage--when you first flip the key on, it should be getting about 12V to the coil anyway. Then once it starts it should be slowly bringing that voltage down to about 7-10 volts, depending on the resistor. This way to make things catch you have full power, but you're not going to burn everything out after 200 miles because it's not constantly getting the 12 V. I dunno what voltage it should be getting while cranking, as the starter is drawing directly off the battery.
The ballast doesn't bleed the voltage over time. The coil has two feeds going to it from the starter solenoid. The 'I' terminal from the solenoid goes through the ballast resistor. The 'S' terminal bypasses the ballast resistor and goes straight to the coil. So when the key is in start, the coil gets a full 12V for easier starting. When the key is moved back to run, only the feed going through the resistor gets to the coil.

This is done because a straight 12V will cause the points to wear excessively fast.
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Stuka
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Re: Low Voltage at Coil

Post by Stuka »

ak_garner wrote: Tue Mar 07, 2023 7:58 am YES, up and running.

But, this has been an eye opener - it appears that the coil is only getting about 3 volts. I can see that on the multimeter when the ignition is on, and it shows when cranking, it's pulsing the 3V at the primary.

I will try jumping the coil with a 12V supply and see if it changes. Electrical question - should I disconnect the wiring from the distributor that is supplying the 3V, or will the 12V jump be additive to that?

I've calculated I'm getting about 4 mpg, and the truck does run rich. I wonder if my spark weak on account of low voltage. And, the only thing I can find is that the ballast resistor is 2 ohms while the ones I can find as a replacement are .5ohms, but that doesn't make sense since a higher resistance would produce higher voltage.

So that might leave the other spots the other posts target - ignition switch, points condenser.
When you are measuring the voltage, where are you putting the ground probe? You should have it on the negative post of the battery, and NOT the negative post of the coil. The fact that you see it pulsing in your graph makes me think you are measuring across the coil. This is going to give you a false reading.

Put the negative probe on the negative terminal on the battery, and then put the positive probe on each side of the ballast resistor. You should see about 13-14V when running on the input leg, and around 7-8V on the output leg.
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Re: Low Voltage at Coil

Post by ak_garner »

@stuka: "You should have it on the negative post of the battery, and NOT the negative post of the coil"

OH! This is that sort of thing that I don't fully understand in the circuit, but you're right, that was causing a lower voltage. I have a healthy 12V square type wave at the primary coil. That's great!

Also, with a few fresh gallons of gas I got her warmed up and she purrs again (a grumbly smokey purr, but she's back to normal).

Thank you guys for the pointers on how to understand the ignition system. It's SO important, and probably the most complex of the "Fuel, compression, spark" equation. I'll do more reading/youtubing on the subject and if I have any more questions I'll ping the gang in a different thread.

Thanks!!

Adam
-1966 Wagoneer : Vigilante 327 - 3 speed manual T-85 to Dana-20 T case.
-1960 Ford Falcon
-1993 Range Rover Classic SWB
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Re: Low Voltage at Coil

Post by Stuka »

ak_garner wrote: Tue Mar 07, 2023 5:36 pm OH! This is that sort of thing that I don't fully understand in the circuit, but you're right, that was causing a lower voltage. I have a healthy 12V square type wave at the primary coil. That's great!
12V at the coil with it running? If so, they is actually a bad thing if you have points as a straight 12V will burn them up. The point of the ballast resistor is to lower the voltage so that they don't arc when opening and closing.

It sounds like you may not have the resistor in place.
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Re: Low Voltage at Coil

Post by tgreese »

Points open is an open circuit, after the condensor charges. An open circuit will show full battery voltage to ground at the coil positive or negative terminal. The ballast resistor is a voltage divider, but only after the points are closed and the condensor discharged. If you are watching the waveform on a 'scope (here the "graphing multimeter") I would expect peaks at full battery voltage on the time-varying waveform, maybe even above 12V due to the reactance of the coil.

Easy to see if you have a ballast resistor installed. You should be able to find it; the parts book shows a specific part for a 327, if that's the engine here. Look in the alphabetical index and find group/line 3.065-1. Jeep PN 938734 https://www.rockauto.com/en/moreinfo.ph ... 65&jsn=269 - I pointed this out above.

Here I would measure resistances, not voltages. If you can't find the ballast resistor, measure the resistance between the ignition switch and the coil positive terminal. Low resistances are hard to measure with inexpensive equipment, but you should measure a few ohms if the ballast resistor is present. Be sure to zero your leads. A wire will have a resistance so close to zero as to be unmeasurable without exotic equipment.
Last edited by tgreese on Wed Mar 08, 2023 4:14 pm, edited 5 times in total.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.
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tgreese
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Re: Low Voltage at Coil

Post by tgreese »

To the OP Garner - you should put the equipment of your Wagoneer (232? 327? 3-speed? 4-speed? Automatic?) in your sig along with the year.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.
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