No spark when warm

[ReefRaider]

5.7 Mercruiser. Starts fine cold. No spark after it warms up. New coil, new cap, rotor. It will run as long as I leave it running (does not die on own), but as soon as I turn the key off, no restart, no spark from coil. So my question is, what other safety or otherwise switches are downstream from the coil that kill the spark? I heard maybe oil pressure switches? The one on the 'shifter'? Do my symptoms match anything in particular?

More......I put a test light on the coil. It lights bright on the positive, dimly on the negative side. As soon as I turn the key, light goes off. ?

While cranking, and not starting when warm, my oil pressure gauge only jumps up to about 20. That's still in the 'red'. If I'm either not getting the oil pressure, or the oil pressure sensors aren't working, will that prevent spark to coil?

What about the thunderbolt IV module thing inside the distributor. Except that it's upstream of the problem, right? Or could it still prevent spark when warm or failing. No idea what its job is.

Thanks in advance!

 

[alldodge]

Re: No spark when warm

5.7 Mercruiser. Starts fine cold. No spark after it warms up. New coil, new cap, rotor. It will run as long as I leave it running (does not die on own), but as soon as I turn the key off, no restart, no spark from coil. So my question is, what other safety or otherwise switches are downstream from the coil that kill the spark? I heard maybe oil pressure switches? The one on the 'shifter'? Do my symptoms match anything in particular?

More......I put a test light on the coil. It lights bright on the positive, dimly on the negative side. As soon as I turn the key, light goes off. ?

While cranking, and not starting when warm, my oil pressure gauge only jumps up to about 20. That's still in the 'red'. If I'm either not getting the oil pressure, or the oil pressure sensors aren't working, will that prevent spark to coil?

What about the thunderbolt IV module thing inside the distributor. Except that it's upstream of the problem, right? Or could it still prevent spark when warm or failing. No idea what its job is.

Thanks in advance!

It would help to know the year of the 5.7, a serial number works good. If the engine will crank there are no switches down stream so to speak to stop the spark. The oil pressure switch will not stop you from starting.

You mention Thunderbolt IV so here is the trouble shooting guide but it sounds like you have a bad connection somewhere. You will need to use a volt meter in place of a test light.

 

[achris]

Re: No spark when warm

We do need a serial number, also what drive you have...

5.7 Mercruiser. Starts fine cold. No spark after it warms up. New coil, new cap, rotor. It will run as long as I leave it running (does not die on own), but as soon as I turn the key off, no restart, no spark from coil. So my question is, what other safety or otherwise switches are downstream from the coil that kill the spark? I heard maybe oil pressure switches? The one on the 'shifter'? Do my symptoms match anything in particular?

If you have an Alpha style drive then you have a shift interrupt, but if that was faulty, then it would kill the engine, not just cause a no spark on cranking.

More......I put a test light on the coil. It lights bright on the positive, dimly on the negative side. As soon as I turn the key, light goes off. ?

Can't use a test light on the ignition system, needs to be a voltmeter, and follow the procedure EXACTLY.

While cranking, and not starting when warm, my oil pressure gauge only jumps up to about 20. That's still in the 'red'. If I'm either not getting the oil pressure, or the oil pressure sensors aren't working, will that prevent spark to coil?

There's nothing to connect the oil pressure (or water temperature) to the ignition system.

What about the thunderbolt IV module thing inside the distributor. Except that it's upstream of the problem, right? Or could it still prevent spark when warm or failing. No idea what its job is.

The TB-IV system consists of 3 parts (mostly). First the coil.. Obvious what it does (transformer low voltage into high voltage, easy). Next the sensor inside the distributor. It sensors where the reluctor wheel position and switches so the last part, the ignition amplifier/module (mounted on the exhaust elbow in earlier models and on the outside of the distributor on later models, hence the request for a serial number) can generate the fast rising pulse for the coil to transform and create a spark.

I would check the coil positive terminal voltage while cranking the engine. It needs to stay at battery voltage. If it drops below, then you will have no spark while cranking.

Chris....

 

[Speakrdude]

Re: No spark when warm

I've had the exact same problem before. The problem was the "automotive" coil I bought at the parts store.
After running, it would get VERY hot. Then try to start, nada.
Put an old, crusty, merc or sierra coil back on, problem gone forever.

 

[achris]

Re: No spark when warm

I've had the exact same problem before. The problem was the "automotive" coil I bought at the parts store.
After running, it would get VERY hot. Then try to start, nada.
Put an old, crusty, merc or sierra coil back on, problem gone forever.

Was that a coil with an internal resistor? (Would have had 'Don't not use with external resistor' or maybe 'Full 12v' on it)...

Chris....

 

[ReefRaider]

Re: No spark when warm

Thanks everyone. I just read your replies here at home, boat is down at moorage.....but understand what you are saying and appreciate the replies. Will continue to investigate. Boat is '88 Trophy, has new(er) engine block and new SEI (Alpha) outdrive. But other things, such as wiring harness etc are original. My guess now would be wiring / connections, after I do some checking with the voltmeter.

 

[ReefRaider]

Re: No spark when warm

So.......my voltage at coil does not stay at 13. It drops to 8,9....levels off at 10.5 when I crank it over. What does that tell you? I followed the TB-IV troubleshooting and have concluded it is the ignition amplifier, assuming my new coil is good. Does the voltage drop while cranking also point to the amplifier?

 

[alldodge]

Re: No spark when warm

If you followed the TB-4 troubleshooting exactly then you need to replace the amplifier. So you removed the coil wire, struck the white/green wire aganist ground and did not get a spark, even with new coil, the it's the amp if everything else was followed.

 

[achris]

Re: No spark when warm

So.......my voltage at coil does not stay at 13. It drops to 8,9....levels off at 10.5 when I crank it over. What does that tell you? I followed the TB-IV troubleshooting and have concluded it is the ignition amplifier, assuming my new coil is good. Does the voltage drop while cranking also point to the amplifier?

No, it indicates the starter is drawing power. But 8.9 volts is just below spec. Try another battery.

Chris....

 

[ReefRaider]

Re: No spark when warm

So.....I replaced the ignition amplifier. Still no spark. On to the left column of the TB-IV troubleshooting chart. Problem is, can't seem to find the harness (Mercruiser part number 84-805363A1). All say no longer made, out of stock. Any suggestions or a link to where I can find one? Thanks again.

 

[achris]

Re: No spark when warm

Have you disconnected all the 'non-essential' gear from the negative side of the coil? I would start there. remove the wires that go to the tacho (gray) and the shift cutout switch...

I would also remove the spark plugs so the engine spins quicker, and the voltage drop on the battery isn't as high....

Chris....

 

[ReefRaider]

Re: No spark when warm

Have you disconnected all the 'non-essential' gear from the negative side of the coil? I would start there. remove the wires that go to the tacho (gray) and the shift cutout switch...

Chris....

Yep. Followed another guide on that.

I found the harness, it's on order. It was somewhat corroded, tried to clean it, but I'll try the new one when it arrives. Went ahead and got the sensor for inside the distributor while ordering. That should do it, according to the TB-IV troubleshooting guide.

 

[achris]

Re: No spark when warm

Yep. Followed another guide on that.

I found the harness, it's on order. It was somewhat corroded, tried to clean it, but I'll try the new one when it arrives. Went ahead and got the sensor for inside the distributor while ordering. That should do it, according to the TB-IV troubleshooting guide.

Cool , just checking. Had another thread with a similar problem. The poster went through the chart, concluded it was the module. Decided it was too expensive, and bought a new Delco system (dissy, module, coil...). Went though all the pus and pain of changing the engine wiring to suit the Delco, still didn't work... Came here to find out why, only to discover that the shift cutout switch was activated, and that had been the problem all along..... :facepalm:....

Chris......