what voltage should I have at the coil

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yellowdart72

yellowdart72

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I have a 76 dodge truck with a 440/727 and everything is original. I drove it out of the garage and it was having an issue staying alive but would start up so I then tried to set the timing and then I moved the distributor around and made it improve. Now I go to start it this morning and no spark. after fiddling with for a few hours, I tried a new ECU, and ballast and coil and I even set the engine to TDC and made sure the rotor was pointing in the right direction and still no Sparky.

I did notice that it only has 3.47 across the coil with the key on.

What am I missing, is that even right to have 3.47 volts at the coil??
 
This could be considered "normal" and really does not tell you very much

The current path from the battery to the coil is quite long. Far more important is what you have on the "switch side" of the ballast which SHOULD be but probably is NOT within a very few tenths of battery.

Some things to do:

Very first things:

Remove the ECU clean the mount and re-mount with star lock washers. The ECU MUST be grounded MUST

Remove the system connectors, ECU, ballast, and distributor, examine them with a light for corrosion, and work them in/ out several times to scrub clean and to "feel" for tightness.

Run a heavy clip lead from the coil+ terminal to a battery source like the starter relay battery stud and see if you have spark

On a related note, clip your meter to coil + and crank the engine USING THE KEY. This should result in very close to "same as battery" voltage and in no case less than 10V

DO NOT forget!!! that the coil HT wire could be bad.

Set you multimeter on low AC that's right, AC volts, and hook the probes to the distributor connector. Crank the engine.........it should generate about 1 volt AC

Remove the cap and inspect rotor, cap, for moisture, dirt, carbon, inspect the rotor for damage. Remove the rotor and inspect for "punch through" the center (AKA lightning bolt)

Make SURE LOL that the distributor actually turns!!!

Check the shaft for play and wobble. Check the pickup and reluctor for strike damage, for rust and debri. Get a .008" feeler gauge (O'Reallys) that's inches not metric. Check the gap between reluctor tip and pickup coil.

I never can remember which of the two wires..........try them both. With key on, ground one of the two distributor connector wires on the ECU side. Each time should produce one spark.

If none of the above shows up anything, it starts to be a tossup. Check the resistance of the pickup coil in the distributor, which is "not that important." They vary "somewhere" around 200--400 ohms. If a bunch above, or below, or won't produce the 1V test, replace it.

Ohm the pickup coil from each terminal to ground. Should NOT show continuity....IE it should show infinity. Be careful to keep your "fingers" off the probes, as some meters will register your body resistance.

After this, it's almost a tossup. Cheapest is probably a coil, if you "think" the distributor is OK try an ECU. The dealers were supposed to have expensive testers, but many just had "trial" boxes to plug in!!!
 
It should have full batt voltage with the key turned to start ( cranking ). After start in the run position it should ideally be around 7 to 9 volts depending on the ballast resistor. The ballast resistor shorts out when the key is in the start position this is normal and is why it's getting the full 12v.
 
EL5DEMON340 said:
it should ideally be around 7 to 9 volts depending on the ballast resistor.
Click to expand...

You have to be careful making these statements.

If the engine is NOT running (or not charging)

or the coil is a "heavy" HP coil IE draws a lot of current

or if it's cold weather, and or the battery is somewhat low on charge.

coil voltage can be quite low. And, it depends on RPM, too Some run as high as 11--12 with the engine running and system voltage at 14, depending on RPM

But with the key on / engine off, I've seen coil voltage as low as 3-4 volts

But this is a poor place to start troubleshooting. Coil + voltage with key on really tells you very little about whether "it will work" It DOES tell you "it's getting power", that it's properly drawing current through the coil, and that of course the ballast is good. But the actual specific voltage will vary
 
Ya there are a lot of variables, but sounds like its stock, have someone crank it and you should see near battery voltage at the coil if it's at 8 or lower there is a problem if it starts you should see 7 to 9 running, a few more volts will cook the coil a few less will cause a bad running motor. Checking the coil itself if you have correct voltage is the next step by a spark check or ohm out the coil.
 
67Dart273 said:
This could be considered "normal" and really does not tell you very much

The current path from the battery to the coil is quite long. Far more important is what you have on the "switch side" of the ballast which SHOULD be but probably is NOT within a very few tenths of battery.

Some things to do:

Very first things:

Remove the ECU clean the mount and re-mount with star lock washers. The ECU MUST be grounded MUST

Check

Remove the system connectors, ECU, ballast, and distributor, examine them with a light for corrosion, and work them in/ out several times to scrub clean and to "feel" for tightness.

Check

Run a heavy clip lead from the coil+ terminal to a battery source like the starter relay battery stud and see if you have spark

Check and still no spark and still had only 3 and a half volts across the coil even with the jumper connected to the + battery post.

On a related note, clip your meter to coil + and crank the engine USING THE KEY. This should result in very close to "same as battery" voltage and in no case less than 10V

Check and had 2.98 volts while cranking and no spark

DO NOT forget!!! that the coil HT wire could be bad.

Not familiar with "HT" wire what does "HT" stand for? (excuse the ignorance)

Set you multimeter on low AC that's right, AC volts, and hook the probes to the distributor connector. Crank the engine.........it should generate about 1 volt AC

Check my multimeter is a cheap one and only has "200" and "750" settings, when on "200" and the dizzy was turning it did show an alternation on the meter screen.

Remove the cap and inspect rotor, cap, for moisture, dirt, carbon, inspect the rotor for damage. Remove the rotor and inspect for "punch through" the center (AKA lightning bolt)

Check New stuff, really looks ok

Make SURE LOL that the distributor actually turns!!!

Check, sure does

Check the shaft for play and wobble. Check the pickup and reluctor for strike damage, for rust and debri. Get a .008" feeler gauge (O'Reallys) that's inches not metric. Check the gap between reluctor tip and pickup coil.

Check plus I compared to a spare dizzy and all seems fine

I never can remember which of the two wires..........try them both. With key on, ground one of the two distributor connector wires on the ECU side. Each time should produce one spark.

If none of the above shows up anything, it starts to be a tossup. Check the resistance of the pickup coil in the distributor, which is "not that important." They vary "somewhere" around 200--400 ohms. If a bunch above, or below, or won't produce the 1V test, replace it.

Ohm the pickup coil from each terminal to ground. Should NOT show continuity....IE it should show infinity. Be careful to keep your "fingers" off the probes, as some meters will register your body resistance.

After this, it's almost a tossup. Cheapest is probably a coil, if you "think" the distributor is OK try an ECU. The dealers were supposed to have expensive testers, but many just had "trial" boxes to plug in!!!
Click to expand...

Tried a different ECU, coil, ballast. changing only one at a time and observing for any change then moving on to the next one, still the same result of NO spark.
 
when in start position full battery voltage will be applied.In the run position the circuit to the coil is through the ballast resistor and will probably be 7 - 9 volts if things are in good working order.
3 volts will cause hard starts,missing and flooding until warmed up.Check all your connections and clean them with a scotchpad.
my money is on the bulkhead connections.
 
67Dart273 said:
I never can remember which of the two wires..........try them both. With key on, ground one of the two distributor connector wires on the ECU side. Each time should produce one spark.
Click to expand...


I tried this and you can hear a spark but you cant see it in the spark tester I have. I remember this same vehicle producing a visible spark before with the same tester, so that means there is a spark but a very weak one.
 
also checked the bulkhead and there was no real corrosion but I still sprayed the terminals with some "metal ready", that usually does the trick to clean up the terminals and promote electric conductivity. The bulkhead condition didn't surprise me because I live in a very dry climate and cars don't usually rust here so the corrosion is kept to a minimum with the climate.
 
Time to head to the auto part store, or sears and brake down and buy a multimeter! Or do you have one?
 
Check and still no spark and still had only 3 and a half volts across the coil even with the jumper connected to the + battery post.

Ummmm, are you checking the voltage across the coil by putting the voltmeter leads directly on both coil terminals? You need to check for the recommended voltages with the voltmeter - lead to engine ground (And you can then check to chassis ground, then battery - if you like; this will make sure you have good grounds throughout.)

It is sounding like the distributor is not triggering the ECU, since you changed the coil and ECU (which we presume are good; are they known good spare parts?). Have you checked the wires from the distributor to the ECU thoroughly to make sure nothing is open or grounded? Look for odd kinks or really 'floppy' spots in these wires, indicating that the wire is broken on the inside of the insulation.

Or, maybe the ECU is not getting good power. Make sure you have 12V to pin 1 of the ECU connector with the ignition 'on', and check each ECU connecor pin for not being corroded or any broken/suspect wires.

Where does your spark tester hook up to? From the "HT" lead?

HT= high tension which is British for high voltage, and is the lead from the coil to the distributor cap.

And when you say your VM has 200 and 750 settings, is this AC volts?
 
I have tried to connecting the spark plug tester between the cap and HT wire as well as the coil to HT wire side and nothing on either side. Also had the tester connected between number one spark plug and wire.

Also just tried the spare dizzy and still nothing.

yes that was AC volts
 
OK, just checking on the location of the tester. Did you clean the inside of the cap with a good clean rag? Button is good, right?

Try this: disconnect the - lead from the coil (disconnect the ECU from the coil). Take a short wire jumper and ground the - coil lead for a short moment with the ignition 'on', and then immediately unground this test lead, and see if you get a good spark out of the HT lead from the coil HT post; you are imitating a set of points this way. Don't be holding the wire lead when you remove it from ground; it might bite a bit. This will tell you if the coil is OK.
 
I didn't quite understand how the spark tester is hooked up. Best if one side is to the HT wire on the coil and the other side grounded to BATT-. That takes the distributor out of the spark path. If you still hear a clicking but see no spark, the coil might be arcing internally (bad). I recently had that with a coil pack in my 3.8L and changing it fixed my mis-fires.

Since deep into it. Remove the distributor (mark rotor & housing position first) and spin it by hand. You might need to clip the body to ground. Without engine cranking noises, you can better hear any sparking or arcing.

Nobody suggested checking the reluctor gap in the distributor. Should be 8 mil max and best to use a brass feeler gage. It sounds like you are getting a magnetic signal, but your multimeter isn't exact. The pickups are cheap. I paid ~$7 for a new one for my small block (could be same PN).

Clip a ground wire to the body of the ECU. Indeed, I would just run a permanent ground wire. I never rely on a rusty sheet-metal screw like the factory did.

Finally, let us know what fixed it. Too many fly-bys here, or people who say, "sold the vehicle, that fixed it".
 
yellowdart72 said:
I tried this and you can hear a spark but you cant see it in the spark tester I have. I remember this same vehicle producing a visible spark before with the same tester, so that means there is a spark but a very weak one.
Click to expand...

This might be a clue. Abandon the spark tester for now. "Rig" a probe out of what you can. That is, use a grounded clip lead to something like a screwdriver, grounded to the block. Hold the probe near the top of the coil tower. Either spinning the distributor, or grounding the proper wire at the distributor harness connector should produce spark
 
yellowdart72 said:
also checked the bulkhead and there was no real corrosion but I still sprayed the terminals with some "metal ready", that usually does the trick to clean up the terminals and promote electric conductivity. The bulkhead condition didn't surprise me because I live in a very dry climate and cars don't usually rust here so the corrosion is kept to a minimum with the climate.
Click to expand...

The way to test and temporarily eliminate this as a problem is to clip a test lead to the coil PLUS and to the starter relay battery stud. This "hot wires" the ignition and eliminates the car harness, giving you battery voltage to the ignition system. If it won't run this way, it's right there in the small isolated section of the harness to the ignition, or the ignition components themselves
 
yellowdart72 said:
Run a heavy clip lead from the coil+ terminal to a battery source like the starter relay battery stud and see if you have spark

Check and still no spark and still had only 3 and a half volts across the coil even with the jumper connected to the + battery post.
Click to expand...


This seems to be a problem. Let's start over right here!!!! What you are attempting to do here is to provide good solid 12V TO the coil "hotwired" So let's do that over and check ---------maybe your clip lead is bad, IT HAPPENS!!!!

Measure from starter battery stud to engine block. You should have battery voltage, IE something about 12V. Clip your wire lead to the relay stud, and to coil POS connection. You should see a small spark if lighting is subdued. Now take your meter and make these measurements

1..........From Coil PLUS to block, should read same as battery

2.........From Coil NEG to block should read very low, less than one volt

3........From Coil PLUS to coil NEG should read 10V or more

If you leave this clip lead hooked up for a short time, 1 minute, you should feel "some warmth" at the transistor on the ECU and maybe some warmth in the coil


yellowdart72 said:
On a related note, clip your meter to coil + and crank the engine USING THE KEY. This should result in very close to "same as battery" voltage and in no case less than 10V

Check and had 2.98 volts while cranking and no spark
Click to expand...

This is a BIG problem if you ran this test correctly. However, the first test above is intended to temporarily bypass this part of the problem. This would indicate you are not getting voltage during start. There is a separate contact in the ignition switch, and goes through the bulkhead connector to the ballast resistor to the plus side of the coil. That contacts entire job is to provide start voltage while the key is in "start"
 
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