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Distributor rotor resistance 56 Buick question


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I am having trouble with engine starting. I have no spark at the spark plugs. Disconecting the coil lead at the distributor cap I do get spark. So to me that means the issue lies with the distributor rotor, cap, or spark plug leads. Using a multimeter to check resistance, I have around 0.2 ohms for each of the 8 plug terminals and 1.0 ohm on the centre terminal. That seems okay to me. But putting the multimeter across the rotor gives around 11000 ohms and that may be the issue?

 

Any thoughts appreciated, thanks.

 

Drew

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I would be more concerned it might have burned through and is leaking current through the center to the distributor shaft. If you have spark at the coil wire where could it be going? 11,000 ohms isn't that much compared to a spark gap. It could be normal if there is a resistor built into the rotor. If not, it's bad. Either way, put a new rotor on. Make sure the carbon brush in the distributor cap (that contacts the rotor) still touches the rotor and hasn't disintegrated. How strong is the spark at the coil wire? If it's kind of weak there, you might want to try a new condenser, too.

 

 

Edited by Bloo (see edit history)
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Thanks Bloo. No not burned through. The rotor and cap all look good. No cracks or burning. The rotor does have that carbon rod or similar in it and to see what happened I pulled that and wrapped it in aluminium foil. That changed the resistance to around 0.2 ohms. That change also started the engine. If that is a carbon rod? then that should have little or no resistance. Anyway, seems a new rotor is needed.

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Googling some more on the Internet it appears that carbon rod in the rotor is supposed to have resistance but not sure how much. Wish I had a spare sitting around to do a check. Anyway I have no idea why Buick/GM would want to have resistance across the rotor. I did see some write ups where people had ditched that carbon rod and epoxied in a nail cut to length or similar.

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Interesting. You could backtrack Ohms Law. The voltage would be around 3,000. Current is minimal. My wife is standing here giving me the evil eye to go for groceries but it's just a couple minutes. And Ohm's law always works.

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The carbon contact spot might have a certain amount of resistance in it, but I suspect its main purpose is a metallic conductor contact that can also self-lubricate for the turning rotor contact surface.  Which can mean that epoxied nail end will not live long in there, wearing into the rotor contact?

 

In the HEI distributor cap, we've seen some instances in higher mileage vehicles where that carbon contact wears and disappears.  Leaving the spark to jump from the output side of the coil to the rotor's contact area.  That carbon contact is available separately,  and is inserted into the cap before the internal ign coil is installed into the cap assy.

 

As I recall, resistor spark plugs have an internal resistance of about 5K ohms?  Which makes 11K ohms not that much, by comparison.

 

I'm thinking that there should be minimal resistance in the rotor's metallic items, BUT the length of the metallic end and how close it gets to the terminals in the cap can be important.  It was discovered that in the ;ater 1960s, that the length of that contact tip end was shortened a tad for a larger gap.  That larger gap makes the coil develope a higher voltage in order to jump that larger gap.  Seems like the rotors built this way had an "E" stamped into that contact end?

 

NOW, the coil will only develope enough juice to jump the gap there and ultimately at the spark plug end.  Which happen in unison, or in nanoseconds of each other.  It's easy to get a spark to jump "in open air", but a bit harder to get it to jump in the compressed atmosphere of a running engine.

 

A side issue might be the polarity of the charging system as that can relate to how the sparks jump, too.  With "negative ground" being the most powerfuil of the two choices.

 

I've observed that normal ignition coils have become much more generic (since the later 1970s) than anybody might suspect.  Even the ones in the GM-brand packaging!  When I suspected a particular performance issue might be coil-related, no change with one of the new coils in place of the older one, for example.   As much as I might not like the large, gaudy "50KV" aftermarket ign coils, they CAN be used for testing purposes.  As I mentioned, that 50KV coil will only produce that lofty output when needed, NOT with every spark in a normal system.

 

Make sure all of the terminals in the system are clean and tight, starting with the battery posts AND terminal contact areas.  There can be a thin layer of gunk between the posts and terminals that can not be easily seen until the terminals are removed, from my experiences, which limits voltage to the charging system.

 

Please keep us posted on your progress,

NTX5467

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41 minutes ago, NTX5467 said:

As I recall, resistor spark plugs have an internal resistance of about 5K ohms?  Which makes 11K ohms not that much, by comparison

Solving Ohm's Law for resistance would be R=V/R. My experience would plug in 3000 for V and a minimal estimate of 1. About 3000 ohms would agree. I would look for something around that.

 

44 minutes ago, NTX5467 said:

It's easy to get a spark to jump "in open air",

This is very important. A dangling secondary wire will have a resistance of infinity if it were fired. All available coil capacity will be generated. You could over tax the coil and add a collateral problem to the one you already have. I always ground any secondary terminal when I need to crank an engine with a plug wire or coil wire disconnected. I have a couple leads with alligator clips on a peg just for that.

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Forget the rotor if intact.  Many years ago I had one pop the carbon rod out.  It started and idled, but backfired when rpm raised.  Later in a misguided attempt to "improve" on the ignition, I installed a Pertronix along with the recommended 'flame thrower coil' .  Same backfiring after 500 miles until it caught fire and burned off completely.  Change the coil (NAPA have given good service).  If no joy change points and condenser and you will have a spare coil.

If the spark plugs are platinum or other exotic electrode variety, replaced with common plugs....Even with a good running engine those plugs will foul and never recover...not compatible with carbureted engines.

 

 

 

 

 

rotor.jpg.8d280299fb8451a3c2b57c8062e9c0d3.jpg

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1 hour ago, old-tank said:

Even with a good running engine those plugs will foul and never recover.

Up until 1988 I was under the illusion that a fouled plug could recover.

 

Illusion has its place. Keep it there.

Penn & Teller Fool Us - Never Again - Review of Penn & Teller, Las Vegas,  NV - Tripadvisor

 

 

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I still ponder why that carbon rod is being used in the rotor at all rather than just a strip of metal.

 

Willie, I hazard a guess the engine backfired given that the rod had become slightly displaced. This displaced rod was interrupting the voltage flow, which was exaggerated at higher rpms with resulting  irregular spark at the plugs. And eventually the rod popped out altogether with that gap then causing the voltage to arc or jump from the centre of the rotor and that large arc or spark melting the rotor.

 

The carbon rod is certainly not used with many rotors. However, certainly I see rotors being used for Buick and Cadillac in the mid 50s do use this setup. I an not sure what other manufacturers were using.

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Having a quick look at part listings on the Net, the rotor with the rod is described as a resistor coil. It is listed as applicable to all early-mid 50s GM cars. Still no idea why you need the resistance as it will only serve to cause a voltage drop. Maybe there was a need to reduce the current, I don't know.

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It might have been there to lessn radio interference with "police band radios" of back then, which were on the FM band spectrum.  Just as using resistor spark plugs and resistor spark plug wires together or resistor spark plugs and "wire" spark plug wires might have performed the saem purpose?

 

Just some thoughts,

NTX5467

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Just to clarify . . . where is this "carbon rod" located?  In the dist cap or on the rotor?  Reason I'm wondering is that when I went into www.rockauto.com and looked at rotors and dist caps, other than the fact the dist cap is "windowless", it looks just like any other dist cap for as long as they used distributors.  Same with the dist rotor, too.  Only two listings, one for "WVE", which used to be "Wells" and Standard Motor, both OEM-level suppliers.

 

Curiously,

NTX5467

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Don't get hung up on the resistance. Assuming it was built into the rotor intentionally, it was PROBABLY there to reduce AM radio interference. It could have been there also to raise the spark voltage AFTER initial firing (during the burn period), and some people used to think that was a good thing. I think it's dubious at best but probably doesn't hurt anything.

 

You can analyze this with ohms law if you like, but don't forget kirchoff's voltage law. In order to fire, a spark will have to be initiated across both the gap from the rotor tip to the distributor cap terminal, and the spark plug gap. These are in series with the 11,000 ohms and are orders of magnitude higher in resistance. All the available voltage will appear across the combination of these gaps, and the poor tiny 11,000 ohms will be left with really no voltage to drop, for the same reason all the supply voltage for a light bulb appears across an open (off) light switch. The resistance of a light switch that is shut off is extremely high, just like a spark gap that has no spark across it.

 

After the gaps ionize and a spark is initiated, the resistance of those gaps drop drastically, the spark current flows, and the 11,000 ohms becomes significant. But now the circuit is operating at a far lower voltage overall, maintaining a spark rather than trying to initiate one. Since the supply voltage is so low now compared to what the system can deliver to initiate a spark, the 11,000 ohms probably still doesn't matter. The coil will run at a higher voltage than it would without the 11,000 ohms, but still way less than what it took to initiate a spark. How could this cause a problem?

 

I suspect if you can tell a difference, then the available voltage from the ignition is extremely weak, and the 11.000 ohms is a red herring. My guess is you have a bad condenser.

 

I would replace the rotor though any time there is the least bit of doubt about it. It is pretty common for old used ones to be leaking some energy down through the center even when they look fine. There isn't much plastic or bakelite between the spark initiation voltage and the grounded center shaft. It causes rust and extreme wear in the advance mechanism. and on some GM distributors the centrifugal weights will literally cut their pivot pins off. Distributor caps on the on the other hand can last practically forever as long as they don't crack or carbon track.

 

Edited by Bloo (see edit history)
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I believe you are all correct in that the resistance of the carbon rod in the rotor is for protection against interference. I think the carbon rod in mine has been fried or something else. I also removed it and attempted to clean it up with sandpaper and that just made it worse with the resistance now some 70,000 ohms. I suspect the initial 11,000 ohms reading was not as it should be either but I can't be sure without measuring a replacement. Anyway, thanks to all for the considered thoughts and information. New rotor on the way.

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Little late to the party.  The carbon rod spec is 10K ohms and concur it’s for suppressing radio interference as the car originally had non resistor plugs and wires.  The new rotors resistance vary wildly.  Excerpt from my 12 volt coil problem thread  below, I chucked the 140K ohm one.

 

Posted May 13, 2021 (edited)

The two on the left were purchased within the past 2 years.  They measure .5 and .6 ohms.  Like a dead short.  The .6 ohm one is in the car.

 

The two on the RIGHT are older ones - the 12.7 K ohm one is very old and out of the 55 distributor with many miies on it but still works ok.  The 143K one is new and never installed.  These two were purchased probably 10 years ago.  You can see the carbon resistor is almost potted in the newer ones and inserted in the two older ones.  These are all standard motor parts.

 

The spec says 10K ohms.

 

5007DBAB-6CE2-4E28-A837-39D8576EEEF5.jpeg

AB194A59-FFE4-4255-AEA1-8F51841B7614.jpeg

Edited May 13, 2021 by KAD36 (see edit history)
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1 hour ago, old-tank said:

All this resistance discussion does not explain why...

Agree no smoking gun yet.  Doesn’t seem like it’s the rotor resistor IMHO unless it’s electrically opening up somehow under operating conditions.  The 140K ohm example in my post still ran my car FWIW.

 

If the absence or presence of the 11k rotor resistance is the difference between a start and no start condition as reported, might it suggest a weak spark condition?  In other words the system is on the threshold of having sufficient primary or subsequent secondary voltage to successfully jump the ionized air gap at the plug with .2 ohms in the secondary path, but not with 11k ohms.  As others pointed out usual suspects are dirty connections on individual components anywhere in the primary circuit path, the coil (measure resistances compared to spec or replace), condenser (replace or do a static charge/discharge check with a DVM to get a rough idea of its integrity), try jumping across the ballast to make absolutely sure it’s out of the circuit on startup, measure/replace points, measure ignition/coil wire resistances.  Curious what you find out


 

Edited by KAD36 (see edit history)
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1 hour ago, KAD36 said:

the absence or presence of the 11k rotor resistance is the difference between a start and no start condition as reported, might it suggest a weak spark condition

Alas, I am going to have to keep looking. So far, the coil measures resistance to spec but I will look at all the other aspects.

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Voltage check of primary circuit of coil when engine running shows 12.6 volts. The plug wires all have resistance between 12000 and 18000 ohms. In the dark I can see no flashing at spark plug boots to suggest spark leakage to the engine block. I can't see any cracks or damage to the distributor cap. I suppose that despite the coil being in spec there still could be an output problem...what colour should the spark be coming from the coil lead...I assume blue-white rather than red-orange?

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Look for a carbon track inside the cap.  If there's strong spark jumping from the coil tower but the plugs aren't firing, there has to be a secondary short to ground somewhere between the coil and the plugs.  As an aside, I remember replacing dozens of rotors on the early GM HEI cars (especially Pontiacs, for some reason).  The HEI spark was so 'hot' it would burn right through the rotor and short to the distributor shaft...

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Would you happen to have resistor wire for both coil & plugs (vs solid wire), plus resistor plugs plus a resistor in the rotor?  And then there is the condition of the rotor tip and each cap terminal. Wonder if that’s too much secondary resistance stacked up if other parts of the system are out of spec, unless others have this configuration working well.  Any recent part changes to the ignition or did this degrade over time or just happen..

 

Hot spark is blue/white and will give a crisp snap sound at the plug.  Orange is usually weak and often sounds more like a pop.  Was “no spark” at a plug gap or end of ignition wire.  If the spark seen at end of coil wire was weak it is unlikely sufficient to fire the plug considering the downstream resistances.
 

12.6 was probably bouncing around if car was running but it’s an indicator.  If troubleshooting the primary circuit my preference is to start the measurement with the engine off, check the resistance or voltage across each component and connection from ignition switch to distributor ground to make sure there’s no stray resistances that could rob current or voltage.


If the car started reliably with all these “extra” resistances before, when checking the caps integrity include ensuring clean terminals where the wires insert and check condition between cap and rotor contacts, condition of the coil secondary (could be breaking down which resistance check may not show),  condenser breaking down/leaking, and ensure low resistance readings across points, any primary wires inside the distributor (fraying/corroded) and across ballast/ignition switch to ensure good current flow through primary.  Think we all beat up the rotor pretty good.  Keep us posted.
 

 

Edited by KAD36 (see edit history)
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4 hours ago, EmTee said:

Look for a carbon track inside the cap.  If there's strong spark jumping from the coil tower but the plugs aren't firing, there has to be a secondary short to ground somewhere between the coil and the plugs.  As an aside, I remember replacing dozens of rotors on the early GM HEI cars (especially Pontiacs, for some reason).  The HEI spark was so 'hot' it would burn right through the rotor and short to the distributor shaft...

 

Not just Pontiacs. The first HEI (black) rotors were not up to the job and did this quite a bit. I was taught to replace black ones on sight. They were hardly ever seen. The white rotor usually won't stop running due to a complete burn through, but still gets leaky when it gets old and the spark energy tears up the mechanical advance mechanism. GM's "window" distributor is also highly prone to this. There just isn't much plastic between the spark and ground.

 

GM by no means has a lock on this problem. Most cars don't have enough plastic to hold all of the spark energy back indefinitely.

 

 

19 hours ago, 56 Buick said:

So far, the coil measures resistance to spec but I will look at all the other aspects.

That test won't really find anything except a completely burned out coil. If you really want to test a coil, you need an old fashioned coil tester. Almost no one has one. Few had them back in 56. I don't have one. While it would be nice for sure to eliminate the coil as a possible problem, I would look at other things first.

 

4 hours ago, 56 Buick said:

I suppose that despite the coil being in spec there still could be an output problem...what colour should the spark be coming from the coil lead...I assume blue-white rather than red-orange?

 

Usually yes.  More to the point it should make a powerful snap. Try to draw it out and see how long it will go. Off the top my head, it probably should do 7/16" or more.

 

If you have spark at the coil wire, but not at the plugs, one of two things is happening. The first possibility is that the spark is going directly to ground through failed insulation in the rotor, or a track or crack in the cap as @EmTee suggested. It usually isn't the cap because there is so much more plastic between the center tower and ground compared to the rotor. It does happen though. A really close inspection under a bright light should sort that out. The second possibility is that the spark is just not strong enough to jump the rotor gap and the spark plug gap. That has to be trouble with the supply voltage from the key, or the coil, or the condenser. Hint: it's always the condenser.

 

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X2 Condenser.  And how is the resistance across the points?  Another idea is to check the voltage at the yellow wire at both the junction block and coil + to ground when cranking.  It should be pretty close to batteries voltage - if it’s a few volts less it would suggest a poor current carrying connection at the starter solenoid path (which is used to bypass the ballast on startup) or at junction block.  
 

The fact the rotor resistance made a difference in starting when it should be negligible makes it seem like whatever is causing the degradation is unique to starting circuit or it’s not enough to make it fail when the rotor resistance was effectively removed with the jumper you made.  The coil swap will tell us all something for sure.

 

After finding the degrading component weakening the spark, if you keep both resistor plugs and resistor wires, and get a “new” rotor that has less than an ohm resistance, you’ve still got over 20K ohms or 2x the spec on the secondary circuit.  

 

Edited by KAD36
Corrected proposed test approach. (see edit history)
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Thanks for following up with the solution, that’s always helpful; do you carry a spare?  Experienced a run of bad “new” ones in rapid succession about a year ago - keep a watch out….

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1 hour ago, KAD36 said:

Thanks for following up with the solution, that’s always helpful; do you carry a spare?  Experienced a run of bad “new” ones in rapid succession about a year ago - keep a watch out….

I am certainly carrying a spare now. I did end up removing the carbon rod from the distributor rotor given its resistance was extreme and replaced it with a length of aluminium rod that I set in place with a small amount of epoxy.

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On 10/2/2022 at 1:47 PM, old-tank said:

Forget the rotor if intact.  Many years ago I had one pop the carbon rod out.  It started and idled, but backfired when rpm raised.  Later in a misguided attempt to "improve" on the ignition, I installed a Pertronix along with the recommended 'flame thrower coil' .  Same backfiring after 500 miles until it caught fire and burned off completely.  Change the coil (NAPA have given good service).  If no joy change points and condenser and you will have a spare coil.

I tried to save you all this trouble :P.

On 10/14/2022 at 8:45 PM, 56 Buick said:

I did end up removing the carbon rod from the distributor rotor given its resistance was extreme and replaced it with a length of aluminium rod that I set in place with a small amount of epoxy.

I should do that and go back to Pertronix in 30,000 miles when the points need replacing.:D

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