Rookie mechanic: trouble shooting advice requested

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MRGTX

MRGTX

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Hey, guys.
I have always done basic maintenance on my cars, occasionally getting a bit deeper but generally the tasks that I've done were without guesswork. For example, I'm happy to replace brakes, drop the transmission to swap out torque converter, etc. If I know how things are supposed to go, I'll go for it.

I did much of my Magnum swap on my own but I had help from some friends with some of the (few) trickier parts.

When it comes to diagnosing...I'm weak. :D

So my (carbureted) "Magnum 300" 5.9L suddenly won't start. Last week, I fired it up and started immediately (as usual)... stumbled and stalled. Not the first time this has happened. I fired it up again, it ran for a moment, coughed and stalled again. So...I assumed it was flooded. I did the usual "hold the pedal to the floor"/flooded engine start-up procedure that has worked in the past...no luck. No ignition whatsoever.

I pulled out each plug, saw that they were wet with fuel, dried each of them off...tried again. Nothing.

Is there fuel? Yep. In addition to the wet spark plugs, the Demon carb has windows on the float bowls...I can see that they're half full, as usual.

Is there spark? Well, I pulled one of the plugs, hooked it back up to the wire, set it aside and turned the key- nothing- I SHOULD see a spark, right?

The MSD coil is pretty new (~500 miles). I checked the connections to the coil, swapped in a different coil to distributor wire- nothing.

The contacts in the cap looked ok but a bit funky so just in case, I grabbed another cap and tried that one- no difference. The rotor looks ok...didn't try a fresh one yet but something tells me that's not the problem.

The ignition box on the firewall and the ballast resistor are also new (~500 miles), the car has a Mopar Performance "chrome" ignition controller. The connection to the box seems to be solid. I don't have another one to check but I've never heard of one of these spontaneously failing.

What should I check next? The wiring harness is original and pretty crusty but I'm not quite prepared for the cost of a new harness at the moment...so I'd like to avoid a complete harness swap.

Any advice would be extremely welcome.

-Mike
 
I would start checking voltage at the ballast. (Well I guess I'm lazy so I would just hook up my spare ballast hahaha). Then go down the line to the chrome box.
 
Ok...Checking the voltage in brittle old wires isn't something that I am too excited to do but of course, that's what I need to do!! I look at the wires and it just looks like a mess to me.

I don't know what does what...so they should all show ~12v with the ignition switched to the "run" position?

I do have a new ballast resistor sitting on the shelf. It probably wouldn't hurt to just throw that in

Thanks!
 
Ballast resistors fail no doubt. Ignition modules and magnetic pick ups in distributors are more prone to intermittent come and go type failures. Back when I worked for Ford we would rarely diagnose to determine which of the 2 pieces were at fault. Even when we did know which was the fault ( like a stator melted down ), the customers always got stator and module replaced as a pair. Just good practice to avoid comebacks.
 
I thought the fuel was supposed to be at the bottom of the sight glass. at any rate , sounds like excessive fuel to me.
 
I would consider your power source IN to the ignition to be suspect. Get your voltmeter on the wire feeding the box and crank it over.
 
Run a wire with alligator clips from the battery positive post to the positive coil primary connection and see if it will start. This will eliminate ballast resistor problems and problems with the wiring in the coil primary circuit. Don't leave it running this way very long because full battery voltage is hard on the coil primary.
 
RedFish said:
Ballast resistors fail no doubt. Ignition modules and magnetic pick ups in distributors are more prone to intermittent come and go type failures. Back when I worked for Ford we would rarely diagnose to determine which of the 2 pieces were at fault. Even when we did know which was the fault ( like a stator melted down ), the customers always got stator and module replaced as a pair. Just good practice to avoid comebacks.
Click to expand...

Ignition modules- forgive the ignorance but is this the box on the fender or the flat metal box on the firewall? I'm embarrassed to not know which is which! I swapped out the one on the firewall when freshening the car up when getting it back on the road.

Magnetic pick ups in distributor-- I will look into this!

73AbodEE said:
I thought the fuel was supposed to be at the bottom of the sight glass. at any rate , sounds like excessive fuel to me.
Click to expand...

Thanks...but even if that's correct (about the fuel level) wouldn't I see a spark when holding up the (connected) spark plug and cranking it?

cudavert said:
I would consider your power source IN to the ignition to be suspect. Get your voltmeter on the wire feeding the box and crank it over.
Click to expand...

Ok...this is where I'll start.
 
When checking for spark at the plug, it must be grounded. Also check for a wet carb. It should be dry unless you are pumping the throttle.
 
skep419 said:
Three wire box is the voltage regulator 5 wire is the ignition module.
Click to expand...

Got it...thanks!

furyus2 said:
When checking for spark at the plug, it must be grounded. Also check for a wet carb. It should be dry unless you are pumping the throttle.
Click to expand...

Ah...good point. When the plug is installed, it's grounded through the block, right? So if I was just holding it up by the boot (hoping to not get a shock through the wire!) it wouldn't be grounded...is that correct?

As for the carb being wet-- it might be. I can smell fuel (a sign of being flooded, hence my original thought) but this is to be expected in the case that the motor was cranking with no spark- and drying off the plugs should alleviate that, no?
 
Demons have a large portal for fuel level viewing, they got smart and made it the middle of the glass instead of the bottom lip. Why make them glass if you cant see the level. Sctratch the negative lead of the coil to ground and check for spark leaving the tower, rest the lead off the exhaust manifold and make the conductor very close to the manifold. Coil test.
 
You seem to think? that you are running ? BOTH a Mopar ignition AND an MSD. IF this is the case I would rewire to run ONLY the MSD. The MSD can be triggered directly by the Mopar distributor. You need to look up the MSD destructions for your box

PLEASE post some photos, showing how/ where the distributor pickup is wired, a photo of what you think? is the Mopar ECU

1.....Troubleshooting a system with Mopar box feeding an MSD could be difficult

2.....Troubleshooting methods for MSD ARE MUCH DIFFERENT than for the Mopar ECU

Example.........YOU DO NOT want ANYTHING at all hooked to the coil EXCEPT the MSD box........no tach......and no radio suppression cap. You DO NOT want to feed 12V to the coil "for testing" with an MSD box.
 
he has a MSD coil and a moparperf chrome ignition box-no problem. what is the compression ratio and spark plug number? try a hot wire to coil as was suggested. moparperf chrome box overheats at extended use below 3000 rpm. check resistance of ballast resister
 
Marcohotrod said:
he has a MSD coil and a moparperf chrome ignition box-no problem. what is the compression ratio and spark plug number? try a hot wire to coil as was suggested. moparperf chrome box overheats at extended use below 3000 rpm. check resistance of ballast resister
Click to expand...

Thanks- you got it. MSD coil, Mopar ignition...sorry for the confusion.

Interesting info about the chrome box...if this is true, you may be onto something. My car with the 2.73 final drive (until I have the cash for a proper 8.75" with 4.10s) means that this thing is running below 3k all the time! Granted, I haven't put that many miles on it....but that certainly could be a problem.

Not sure what plug number that I'm running but I will take a look. Compression ratio is stock for a Magnum...and pretty low- 8.5:1?
 
OK so we have Mopar.

FIRST .........a "chrome box" does not mean you have what is "known as" chrome box. There are lots of aftermarket boxes that are not a "chrome" box.

Testing.........

The box MUST BE grounded. Start there. Dismount the box, clean and scrape the bolt holes, and remount using star lock washers.

Remove all the connectors.........box, ballast, and MOST especially the distributor, and inspect with a light for corrosion, and "work" them in/ out several times to 'scrub' the terminals and to feel for tightness. Again, the distributor is a particular problem

Once you get it running measure coil + voltage and report back

For now, check coil + and - voltage with key on/ engine stopped.

Coil + should be "around" 5-9V or so. Coil - should be very low, around 1 or 2V

Next, clip your meter to the coil + and crank the engine. With the starter in operation read the meter. Then run the starter and see what battery + reads. The two should be nearly identical, and above 10.5V

Another way is as said above.........to run a TEMPORARY clip lead from coil + to battery and see if it runs. Do NOT leave this connected any longer than to test.

Check the interior of the distributor around the cap and roter, and around the pickup / reluctor. Look for strike damage, rust, and debri. Get a BRASS feeler (were available from O'Reallys) and set the reluctor gap at .008" (inches).

Another test you can make is key in "run." Remove the distributor connector, and take up the engine bay side of the connector. "Tap" the bare terminal on ground, it should make a single "snap" spark each time.

I wrote this up some time ago, as a way of detail explanation You have to click at the bottom to expand

67Dart273 said:
Maybe this helps, and maybe not

Here's a post I made about the simplest way I know to get across testing the ECU/ ignition parts

74 duster electrical problems


You need a coil, the ECU and the distributor

Lay it out on the bench. Follow the diagram. Find the two distributor pickup terminals on the ECU. Hook them to the distributor

Hook the ECU case to battery NEG

Coil does NOT need grounded

Distributor does NOT need grounded.

Hook coil + to the power lead terminal on the ECU. Get a clip lead hooked there and let dangle. This is your battery "hot" when you are ready

Hook something from coil "case" to a probe for testing spark.

Hook up your power clip lead. Twist the distributor shaft while holding the test probe near the coil tower. The thing should make sparks

If not, unhook distributor. Take first one, then the other pickup clip leads, and "tap tap" ground them at the battery connection. Coil should make 1 spark each time you do so.

If not, try another coil. If that does not fix it replace the ECU

IF you hook it all up and it WORKS, then there is something AFU in the car harness. SUSPECT a bad ECU connector OR a bad DISTRIBUTOR connector
=============================================
This is all you need to test the basics of the ignition. You can easily test the ballast separate. A battery, the ECU, distributor and a coil, and of course some test leads

2ai0wsj.jpg


Below, the basic diagram for a 4 pin ECU

166lmj7.jpg


Below, the wire for testing spark. I use my 12V test light. No, LOL the spark won't blow up the bulb

166lmj7.jpg


Below, the ground connection. ALL you need is one wire from batt NEG to the ECU case

xawjl.jpg


Below, the two distributor connections. In the car these are polarity sensitive, but for testing does not matter

b3opvm.jpg


Below, the coil NEG connection

6jfywp.jpg


Below, battery PLUS connection, one wire to this terminal of ECU and jumpered over to + side of coil


11lqu5l.jpg


Below, all hooked up and ready to test (except for battery ground). Should produce sparks at least 3/8" and typically 1/2" long

1zzoya0.jpg


Below, distributor "one wire" test. I have removed the other distributor wire for simplicity. Take the bare connector end or this clip lead (the yellow) and with everything hooked up, ground it repeatedly. Each grounding should result in a spark (In this photo you need to hook up the ECU ground wire, I left it off for the photo)

vgitld.jpg











_________________
Click to expand...
 
Maybe this helps, and maybe not

Here's a post I made about the simplest way I know to get across testing the ECU/ ignition parts

74 duster electrical problems


You need a coil, the ECU and the distributor

Lay it out on the bench. Follow the diagram. Find the two distributor pickup terminals on the ECU. Hook them to the distributor

Hook the ECU case to battery NEG

Coil does NOT need grounded

Distributor does NOT need grounded.

Hook coil + to the power lead terminal on the ECU. Get a clip lead hooked there and let dangle. This is your battery "hot" when you are ready

Hook something from coil "case" to a probe for testing spark.

Hook up your power clip lead. Twist the distributor shaft while holding the test probe near the coil tower. The thing should make sparks

If not, unhook distributor. Take first one, then the other pickup clip leads, and "tap tap" ground them at the battery connection. Coil should make 1 spark each time you do so.

If not, try another coil. If that does not fix it replace the ECU

IF you hook it all up and it WORKS, then there is something AFU in the car harness. SUSPECT a bad ECU connector OR a bad DISTRIBUTOR connector
=============================================
This is all you need to test the basics of the ignition. You can easily test the ballast separate. A battery, the ECU, distributor and a coil, and of course some test leads

2ai0wsj.jpg


Below, the basic diagram for a 4 pin ECU

166lmj7.jpg


Below, the wire for testing spark. I use my 12V test light. No, LOL the spark won't blow up the bulb

166lmj7.jpg


Below, the ground connection. ALL you need is one wire from batt NEG to the ECU case

xawjl.jpg


Below, the two distributor connections. In the car these are polarity sensitive, but for testing does not matter

b3opvm.jpg


Below, the coil NEG connection

6jfywp.jpg


Below, battery PLUS connection, one wire to this terminal of ECU and jumpered over to + side of coil


11lqu5l.jpg


Below, all hooked up and ready to test (except for battery ground). Should produce sparks at least 3/8" and typically 1/2" long

1zzoya0.jpg


Below, distributor "one wire" test. I have removed the other distributor wire for simplicity. Take the bare connector end or this clip lead (the yellow) and with everything hooked up, ground it repeatedly. Each grounding should result in a spark (In this photo you need to hook up the ECU ground wire, I left it off for the photo)

vgitld.jpg











________________
 
Sounds like either module or pickup. My money's on module. Pickups tend to misfire and studder a bit before they tank. It used to be std practice to carry a spare module and ballast in the car.
 
If you are not going to do testing, and just throw parts at it, you might get lucky and you might strike out. You'll feel pretty "funny" if it turns out to be a power / wiring issue............
 
67Dart273 said:
Another test you can make is key in "run." Remove the distributor connector, and take up the engine bay side of the connector. "Tap" the bare terminal on ground, it should make a single "snap" spark each time.
...
Click to expand...

Thanks again to all for the suggestions here. I finally had some time to spend on the ol' Dart and discovered that not only would the connector to the distributor not "snap," it was only reading a few tenths of a volt when I checked it with the meter...

So pretty darned likely that this is the electronic ignition module...does anyone disagree?

If so, any suggestions on what I should replace it with? I gather that the Mopar Chrome box might have been a poor choice for a car that sees a mix of cruising and WOT runs.

Thanks again!!
 
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