Diagnosing Bad Coil

Tech tips and how to's

Moderators: notoptoy, S Allen, Solex68

Post Reply
Randalla
Roadster Fanatic
Posts: 213
Joined: Fri Jul 20, 2012 8:30 pm
Location: Scottsdale, AZ

Diagnosing Bad Coil

Post by Randalla »

I've developed an issue that has left me stranded briefly, which I believe is ignition related. The symptoms are that when the car sits idling for 10 to 20 minutes in traffic, all of a sudden it starts running rough just before it quits. When I try to start the car again it pops but quickly dies again. After no more than 5 minutes the car starts and runs fine (getting me home both times without further incident). While looking under the hood after one such incident, I noticed the coil was quite hot to the touch (140 degrees measured with my scanner). I'm wondering if I have a bad or incompatible coil, or if there is something else that's causing the coil to heat up and then fail. Waiting 5 minutes may be enough time to cool the coil to the point of starting again. Can anybody shed light on this issue?
User avatar
Linda
Fraternal Den Mother-RIP
Posts: 7807
Joined: Sat Nov 05, 2005 11:37 pm
Location: Los Angeles
Model: 1500/1600
Year: High Windshield-68-70

Re: Diagnosing Bad Coil

Post by Linda »

Vapor lock?
Otherwise you can test the coil with a multimeter.
Linda
Sadly-Linda has passed away 2022. She was the 311's den mother and drove the first Rare-Parts ball joint project. RIP.
User avatar
Nissanman
Roadster Fanatic-Site Supporter
Posts: 3350
Joined: Mon Apr 16, 2007 8:07 pm
Location: Adelaide, South Australia
Model: 1500/1600
Year: Low Windshield-64-67.5
Contact:

Re: Diagnosing Bad Coil

Post by Nissanman »

I gather this problem has appeared out of the blue, you haven't changed anything that brought this issue on?
Nissanman, just trying to help.
1965 SP310 Engine No. G-93326 Car No. SP310-10817
https://photos.shutterfly.com/album/60141836519
On the road again!
User avatar
spl310
Roadster Guru
Posts: 13202
Joined: Sun Dec 29, 2002 10:38 pm
Location: In front of this keyboard... in Jacksonville, Florida!

Re: Diagnosing Bad Coil

Post by spl310 »

Is there any oily residue on the bottom side of the coil?
"Wow, a Roadster!" Stuart Little

1967.5 2000
1967.5 2000
1964 1500
1964 1500
1967.5 1600
1968 chassis
2006 Acura MDX
2013 Volkswagen Jetta TDI wagon
1995 F350 Powerstroke!
More...
User avatar
notoptoy
Vendor-Site Supporter
Posts: 9674
Joined: Fri Oct 02, 2009 2:55 pm
Location: Winston-Salem, NC and Ocala, FL
Contact:

Re: Diagnosing Bad Coil

Post by notoptoy »

Is this a points ignition or EI?
Check your ballast resistor - if points. If it is allowing an over voltage condition, driving the coil at 12V instead of 8V, that would burn out the coil relatively quickly. Run the car in the dark and see if there are any cracks causing arc's.
"When all else fails, force prevails!" Ummm, we're gonna need a bigger hammer here.

67.5 SPL311 H20 w/5 speed
65 Impala Convertible
2017 C43 AMG
Randalla
Roadster Fanatic
Posts: 213
Joined: Fri Jul 20, 2012 8:30 pm
Location: Scottsdale, AZ

Re: Diagnosing Bad Coil

Post by Randalla »

This problem just started, with no mechanical changes on my part. The car runs a Pertronix ignition without a ballast. Good idea to run the car in the dark and look for arching. I'll try this tonight. Does the 140 degree temperature of the coil itself provide any clues regarding what may be happening inside the windings? I verified this morning that there is no oily residue on the coil. Remember, the other clue is that the car starts running kind of rough before it stalls out. Almost sounds like a miss.
User avatar
fj20spl311
Roadsteraholic
Posts: 5007
Joined: Mon Nov 28, 2005 3:54 pm
Location: San Diego, Ca

Re: Diagnosing Bad Coil

Post by fj20spl311 »

Sounds more like vapor lock, than an overheating coil. 140 degrees is hotter than I would like, but I don't think it too hot.
Could also be the Pertronix unit stating to fail. Coil is a cheap fix. Maybe crack the fuel line at the carb to see if you are getting fuel.
Phil
67.5 SRL311-00148 Blue (FJ cruiser VOODOO Blue)
67.5 SPL311 FJ20E teal SDS EFI
69 SRL311 SOLD
19 Raptor SCAB
User avatar
Linda
Fraternal Den Mother-RIP
Posts: 7807
Joined: Sat Nov 05, 2005 11:37 pm
Location: Los Angeles
Model: 1500/1600
Year: High Windshield-68-70

Re: Diagnosing Bad Coil

Post by Linda »

A sticking fuel valve, or a bad float have given me those symptoms too, though you would likely smell gas from the overflow. But a tiny hole can be deceptive in the brass floats. I have foam floats now.
Hope your lines are wrapped in Scotsdale.

Linda
Sadly-Linda has passed away 2022. She was the 311's den mother and drove the first Rare-Parts ball joint project. RIP.
User avatar
spl310
Roadster Guru
Posts: 13202
Joined: Sun Dec 29, 2002 10:38 pm
Location: In front of this keyboard... in Jacksonville, Florida!

Re: Diagnosing Bad Coil

Post by spl310 »

The roadster style coil is an oil cooled unit. It is filled with oil. Typically the recommendation is to mount them vertically so that they will never leak the oil. This one may have leaked out the oil and is overheating as a result. That is why I asked about the oily residue.
"Wow, a Roadster!" Stuart Little

1967.5 2000
1967.5 2000
1964 1500
1964 1500
1967.5 1600
1968 chassis
2006 Acura MDX
2013 Volkswagen Jetta TDI wagon
1995 F350 Powerstroke!
More...
Randalla
Roadster Fanatic
Posts: 213
Joined: Fri Jul 20, 2012 8:30 pm
Location: Scottsdale, AZ

Re: Diagnosing Bad Coil

Post by Randalla »

Just got off the phone with Pertronix and they asked two questions. Is the coil mounted vertically or horizontally (to your point spl310)? Are your terminals at 6 and 12 or 3 and 9? On my car the coil is mounted horizontally and the terminals were at 6 and 12. He claims they need to be at 3 and 9 or you're likely to get an air bubble inside preventing the coil from cooling properly. I've made the change and will report if this solves the problem. Anybody else ever hear that the terminals need to be properly oriented???
User avatar
notoptoy
Vendor-Site Supporter
Posts: 9674
Joined: Fri Oct 02, 2009 2:55 pm
Location: Winston-Salem, NC and Ocala, FL
Contact:

Re: Diagnosing Bad Coil

Post by notoptoy »

The orientation of the terminals is a new one to me. I do question the horizontal versus vertical positioning, as OEM has them mounted Horizontal and I have seen them mounted in every possible direction with other manufacturers - upside down, slanted, horizontal and vertical.
"When all else fails, force prevails!" Ummm, we're gonna need a bigger hammer here.

67.5 SPL311 H20 w/5 speed
65 Impala Convertible
2017 C43 AMG
User avatar
spl310
Roadster Guru
Posts: 13202
Joined: Sun Dec 29, 2002 10:38 pm
Location: In front of this keyboard... in Jacksonville, Florida!

Re: Diagnosing Bad Coil

Post by spl310 »

"Wow, a Roadster!" Stuart Little

1967.5 2000
1967.5 2000
1964 1500
1964 1500
1967.5 1600
1968 chassis
2006 Acura MDX
2013 Volkswagen Jetta TDI wagon
1995 F350 Powerstroke!
More...
User avatar
Linda
Fraternal Den Mother-RIP
Posts: 7807
Joined: Sat Nov 05, 2005 11:37 pm
Location: Los Angeles
Model: 1500/1600
Year: High Windshield-68-70

Re: Diagnosing Bad Coil

Post by Linda »

There is one MSD coil that can be mounted anyway needed, the high vibration one.
Otherwise,I believe an MSD coil is not exactly correct for a Pertronix unit.
I see that MSD recommends the ballast resistor for points.

Linda
Sadly-Linda has passed away 2022. She was the 311's den mother and drove the first Rare-Parts ball joint project. RIP.
C.Costine
Roadster Nut-Site Supporter
Posts: 1731
Joined: Fri Feb 13, 2015 8:08 pm

Re: Diagnosing Bad Coil

Post by C.Costine »

It could be either slow fuel or a bad coil. I have experienced both. They will both recover temporarily after sitting for a few minutes. I Have never experienced a coil getting anywhere near 140 when still good. To determine whether it is fuel or gas remove a plug and secure it where it will ground and check the spark. Take your spark plug wrench and whatever you used to secure it with you when you go for a drive. When it dies immediately stop and remove a plug and check for spark. Then you will know which it is.
located in Chester NH
1967 1600 in restoration
2013 Arctic Cat F-1100 turbo
Ford F-350 6.0
Ford 9000 puller, Ford 960 puller, Ford 901show, Ford 971 worker, Oliver 70 waiting its turn
User avatar
redroadster
Roadsteraholic
Posts: 2383
Joined: Tue Mar 12, 2013 10:58 am
Location: KCMO
Model: 1500/1600
Year: High Windshield-68-70

Re: Diagnosing Bad Coil

Post by redroadster »

What engine ?
1600
fuel boiling in the bowl a possibility exhaust close by
Only does it idling for 15 min?
K D. Makes a spark testor. adjustable to see coil output
It's a great trouble shooting tool
Datsun dealer tech 76 to 87
Mitsubishi tech 9 yrs
Volvo, Kia, Toyota too
6 month - Rolls Royce
ASE MASTER TECH 96. - 11
70 SPL 86 Z31 T , Sportster
Post Reply