Crank but no spark

VMAX  Forum

Help Support VMAX Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Chasej94

Member
Joined
Oct 9, 2019
Messages
13
Reaction score
1
Location
West Jordan, Utah
Hey all,
ive been having some weird issue lately with my bike (92). i started up my bike and it started like normally, no issues. i then go back out a few hours later, and it just cranks. i come back the next day and it starts normal, and the same amount of time later, just cranks.
i pull a spark plug out, ground it and no spark, went all the way around and still no spark.
what should i check?
 
I'd say either the ignition box or the pick-up coil. The pick-up coil has to be checked when it happens. 90-120 ohms for the pick-up coil is what it should read.


VMax primary coil-pick-up coil resistance.png
 
When the bike runs will it come up to operating temperature and continue to run?

If so then I would be looking for a bad connection. Check that you have power to the coils. If no then check if you have power at the ignition fuse. If yes then there is a wiring problem between the fuse and coils.
If you dohave power to the coils then check the pick-up coil and associated wiring.
If OK then I'd suspect the ignition control unit. However if it will run normally I wouldn't think this likely as from what I have read they tend to stop working during the warm-up period.
 
When the bike runs will it come up to operating temperature and continue to run?

If so then I would be looking for a bad connection. Check that you have power to the coils. If no then check if you have power at the ignition fuse. If yes then there is a wiring problem between the fuse and coils.
If you dohave power to the coils then check the pick-up coil and associated wiring.
If OK then I'd suspect the ignition control unit. However if it will run normally I wouldn't think this likely as from what I have read they tend to stop working during the warm-up period.
Is there a way to check if the ignition box is bad?
 
I guess if you have the relevant test equipment and knowledge I assume you could. I recall reading threads a long time ago regarding replacing the diodes (?) which seemed to be a causal issue.

It seems to boil down to if everything else checks out OK then all that is left is the ICU.
The only sure fire way would be to substitute with a known OK unit and see what happens.
 
BTW when engine is up to temperature and running, if you stop it, does it restart immediately - in other words does the engine start ok when cold?

at least 3 possibilities:
1. coils have gone bad when hot
2. CDI box failure when hot
3. pickup coil fails when hot

Kinda unlikely for all 4 ignition coils to go bad but .....

There is 1 wire from the ignition unit going to each of the coils, so 4 wires in total: yellow/orange/white/grey

If you can monitor the voltage any one those, say using an oscilloscope, then you would be able to distinguish between a CDI fault or the coils not firing.

Note that the grey wire (to one of the front two coils) also goes to the tacho so when the engine turns over, you should see the tacho needle jump a bit - each time that cylinder fires. Note I just tried in on mine = the ignition is good but engine not running due to carbs needing a clean from standing.

Also If there is no spark on any cylinder, then it would be unlikely for all 4 output stages for the ignition unit to fail so does point to the pick up coil - see below.

the spec is: 81 - 121 ohms @ 20 C


1618833290006.png
 
Also If there is no spark on any cylinder, then it would be unlikely for all 4 output stages for the ignition unit to fail so does point to the pick up coil - see below.

If memory serves well (or perhaps not) I recall reading of IGU's that worked fine when cold but failed when hot i.e. the motor stopped at some stage during the warm-up phase.

If I have remembered correctly then on that basis I don't think you can rule out the ICU if you have no spark.
 

Latest posts

Back
Top