Weak spark problem

[Oldcomguy]

VP 5ltr with weak spark and won't start.
Changed coil, plugs, leads, cap but still get poor spark.
Is the 12v to the coil via the ignition or ecu?
Possible crap earth?
Possible short in wiring?

 

[wildfiremaint]

The 12V to the primary (plus) on the coil comes from the ignition switch.
The secondary on the coil is connected to the Ignition Module.
Check that secondary connector at the coil. I had a nostart which was caused by that connector/wire being corroded.
The coil is a bugger to get at - best to relocate to the firewall near the strut tower

 

[Oldcomguy]

I will check that out when I get some of it back together.
I decided to check continuity between the positive terminal (not connected to battery) to engine (earth) and there is continuity between them.
Tracked it back to a fuse, number 1 in the pic

 

[Oldcomguy]

So just tested the power to the coil, it was 12.42 the same voltage as the battery but when I cranked the motor it dropped to 8 something volts.
Trying to confirm if this is normal but I would doubt it.

 

[krusing]

That would possibly be correct, Voltage Drop when Cranking,
Pull a plug out and place it in the SP Lead, and lay it on the motor, and see of its got a healthy spark.
IF so, possible could be a fuel issue,
Is the Pump Priming when you turn the ignition on ?

 

[Oldcomguy]

It has fuel but the spark is small and weak I'm hoping the voltage drop is the issue will run some more tests tomorrow

 

[krusing]

Maybe check your Ignition/Engine Relay,
And swap it with another one, possibly a headlight relay or similar, to run some tests.

 

[Oldcomguy]

Yep swaped it over with one less used some time ago.

 

[Not_An_Abba_Fan]

12.42 is not that great, put the battery on charge, a healthy battery is around 13.5v. Try jumping from another battery.

 

[Skylarking]

So just tested the power to the coil, it was 12.42 the same voltage as the battery but when I cranked the motor it dropped to 8 something volts.

12.42 is not that great, put the battery on charge, a healthy battery is around 13.5v.

Open circuit voltage of a 90% charged battery will be 2.1 volts per cell, or 12.6 volts, see here.

13.5 volts is when the battery is full and the engine is running with thealternator in float charge mode, that is, no real load on the battery or alternator.

As op stated, 12.42v was with engine off which indicates the battery does need a charge. The crank voltage seems to be a little low at 8v so I’d charge the battery fully and get a load test on it to verify it’s good…

In any case, spark strength relates to coil design, lead resistance and earthing so all aspects should be checked.

Is primary and secondary coil resistance within spec?
Is spark plug lead resistance within spec?
Is engine earth good (no resistance)?
Is earth on the switching side of the coil good (not sure how the spark is controlled on crank but the circuit that does this needs good earth)?
Are the spark plugs new?

Many ignition systems have specs on what gap can be jumped by the spark and can be tested using a spark tester (wind out to open gap type tester)… Spark should be strong and blue at specified gap…