Inconsistent spark from ignition

[Kris P.]

Hi guys, I have a question with regards to inconsistent spark distribution from the coil to the distributor cap.  My engine is an m110 djet.  My car wouldnt start now because the sparks from the coil starts everytime I turn on the ignition and no sparks are made during cranking.  And once I turn of the key, a spark would come out again.   This bothers me since I haven't experienced this problem before.  Would it be the transistor box at the right side most part of the engine bay that's malfunctioning or some other electrical part causing this.  Would this also be the cause of it running rough idles before? 

Any thoughts on this guys?

[koan]

If you're getting sparks at any time the switching box is probably OK.

There are two ballast resistors in the ignition circuit, possibly one has gone open circuit or has a bad connection.

(pretty sure all engines with transistor ignition have two resistors, tell me if Ive got it wrong)

koan

[Kris P.]

Yup there are two... with different ohm readings. How would I check if one is not functioning properly?

[koan]

Yup there are two... with different ohm readings. How would I check if one is not functioning properly?

Take them out, one at a time if you can't identify them. Measure with multimeter on Ohms range if you have one. Alternately connect the resistor in series with 12v 6W tail lamp bulb and a battery. Lamp should light but not at full brightness.

They are low resistance, about 4 or 5 Ohms.

koan

[Kris P.]

Yup, took them out already.  its  0.6 ohms and 0.4 ohms resistors installed.  Will check them in a bit.  I have a multitester here, will check if its faulty or not.

[Kris P.]

Hi Koan, got the resistors out.  One of them was faulty. 0.6 ohms resistor.  Did change them with new ones but I have still the same problem.  Spark at the turn of the key to ignition, no sparks during cranking, spark when I turn off the key.  THere's current going (positive) going to the positive side of the coil (if key turned to ignition) and negative on the negative side of the coil.  I believe you need a pulsating ground coming from the dizzy back to the coil to produce the current needed for the sparks right?  It seems that the coild doesn't give out that current during cranking.  I also checked the contact point of the dizzy that touches the shaft that signals the firing.  As for the condenser, I haven't checked it yet if its faulty or not coz I don't know how to test it.  What could the problem be? any thoughts on this?

[Big_Richard]

still can be the switching box.

[koan]

One of them was faulty. 0.6 ohms resistor.  Did change them with new ones but I have still the same problem.

Bit odd.

The transistor box is what switches current through the coil to ground, it's triggered by the points in your car (later models have magnetic trigger with a different box).

Not sure what can be wrong.

I misunderstood the first post MT, you are right.

You get a single spark when ignition turned on and same when turned off but nothing during cranking, is that correct?

If that is correct, it's possible the switch box is dead.

Can you verify the points are opening and closing and  that they are generating ground pulses at pin 7 of the switching box when the engine is cranked?

Should also see low voltage pulses at pin 4 of the diagnostic plug

koan

[Kris P.]

Yup, spark at key in the ignition position, no sparks at cranking, spark when key is turned off.  I look for pin 7 at the switch box.  As for the diagnostic plug, what should it look like? Im not familiar with it... How about the plug going to the switch box from the dizzy (green wire with a plug at the end).  I did check it if it produces the ground from the dizzy... it does ( using the test light with it lighting up when pointed in the inner part of the plug with the other end of the test light pointed at a positive line... when cranking, the light turns off and on as it passes through the lobes of the shaft of the distributor).   However, the outside part of the plug surrounding the inner part doesn't show any continuity to any part of the condenser nor the contact point of the dizzy... hmmm... dont know what it does actually.

Anyways, I have another switch box that I got and Ill try this one.    The previous owner told me he had a reoccuring problem like this before in which he replaced the switch box and the car ran but with a bit of rough idle.  But on the road the car ran normally .  Now, this problem again. Is there a possibility my computer box might be faulty also? Im a bit scared that I might fry this switch box or this might be  a continuing problem.

I have a set of computer box with harness, pressure sensor and switch box that I could get, would it be wise to change them all together?

[koan]

As for the diagnostic plug, what should it look like? Im not familiar with it

Round black thing just to the rear of ignition switch box with a screw off cap.

However, the outside part of the plug surrounding the inner part doesn't show any continuity to any part

That's OK

Before trying the new switch box find the diagnostic plug and check pin 4 for pulses when cranking.

The D-jet control box doesn't control the ignition, I wouldn't touch anything related to injection.

if the previous owner had similar a problem and a new switch box half fixed it and now the problem has surfaced again sounds like the new switch box didn't really fix it and the fault is somewhere else.

Is the spare box you have the same part number as the one on the car?

At this stage find the diagnostic plug and check pin 4 for pulses when cranking.

koan

[Kris P.]

Hi Koan,  you lost me on the diagnostic plug, do you have a pic to show me onto where it is located? Im not yet that familiar with it.

Thanks

[TJ 450]

I'm not sure where the diagnostic plug is on the M110, but on a D-Jet 450 it's right next to the distributor. It's a cylindrical black plastic device with a screw-on cap.

Tim

[Kris P.]

Hi guys, I saw the diagnostic plug already.  I think I saw the problem.  I got the positive lead of the coil and took it off.  Made a posiitive lead coming from the battery and connected it to the positive side of the coil.  The car started and ran.  It ran a lot stronger than before.  It was a bit confusing because everytime I checked the positive lead of the coil it had a positive current in it.  But I didn't check if it were 11-12 volts going thru.  Is it possible that the turn key of the ignition is faulty or a relay? Now that we know that the positive lead going to the coil seems to be faulty, what could be causing this?

[s class]

Yes, the switch contact assembly behind the ignition lock is known for giving trouble sometimes.  Also to consider - do you have an aftermarket immobiliser/alarm system?  They usually interrupt the supply to the coil, and this can lead to poor performance when the alarm system electronics have degraded. 

[wbrian63]

I could be completely wrong, but I think ballast resistors are used for the "run" cycle in the ignition system. On cars that I've owned in the past, there were always 2 wires to the positive side of the coil. One came from the ballast resistor, and the other came from the crank side of the ignition switch.

During cranking, you want max voltage to flow to the coil, for a hotter spark.

Once the car is running, a lower voltage is passed (flowing through the resistor) which extends the life of the points.

When a ballast resistor starts to fail, it usually involves a break in the resistor which can be hard to trace. When cold, everything works fine. Once the resistor warms up, it gaps and you've got no spark and the car dies. After a brief cool-down period, the car will run again, but it will usually start, but not stay running during this event. Or, you'll get periodic "drop outs" on the road as bumps, etc cause the resistor to gap and voltage to drop.

Of course, these cars had no manner of electronic ignition, just points, a coil and plugs.

Also, just because you see 12v+ at the coil, that's 12v+ with no load. When the engine is running, that's the true test of load through the coil. If there's a way to load test the 12v+ circuit you disconnected, you may find that any load results in a massive voltage drop.

Could a faulty ignition switch be to blame? Good voltage passing to the coil when in the crank position but bad conductivity in the run position resulting in poor voltage to the coil? It would seem that the crank contacts would likely have a lot less wear than the run contacts...

I'm still learning, so please correct me if I'm wrong.