1985 4x4 720 crank no start, no spark

[97tahoeexpress]

hey you guys, please forgive me if this is the wrong place to post this, i’ve been reading up on the forums about a crank no start issue. i’ve been dealing with this same issue in a 1985 720 4x4 standard transmission. i’ve done all the testing throughout the forums i could find to figure out the problem. i can bump start the truck no problem every time but when i crank the key i get no spark. everything works except spark. here’s what i’ve done so far. 

sprayed starting fluid while cranking to eliminate fuel, still no start 

checked battery

 

replaced red fuse on driver side fuse box for icm

 

took off and cleaned the grounds the one to passenger fender by battery tray, the big one going to engine, the two on the back of engine to firewall, the one underneath the ignition control modules

 

the main ignition relay on the passenger fender i’ve swapped with the heater relay, heater works still no start

 

ignition coils have 12.5-12.6volts on positive side key on engine off, and while cranking both coils read 9.6 volts 

 

all the turn signals and reverse lights and everything works on the truck. it’s carb swapped has a weber on it and all that’s brand new. 

 

my father replaced the distributor previously and i’m wondering if maybe the icm inside has gone out. 

im praying one of you can help me in here and again my apologies if i didn’t post it in the right place i just figured i had the best chance of it being seen if i posted a general thread 

 

 

 

[97tahoeexpress]

no help then

[datzenmike]

Of course there's help. Sometimes people are busy. Don't worry we'll get her going. Already have an idea from your description.

 

 

If it push starts then the ignition works. Something is loading the 12 v down during START. This 9.6 is probably not enough to power the ignition module.

 

Measure the battery voltage on the cable clamps, not the posts, and again when turning the engine with the starter. What do you get?

[Crashtd420]

Also check power going to the coil and distributor/ ignition module while cranking...

The ignition switch could be faulty ...

Definitely something lacking in the start signal or ignition is cutting out while cranking....

Because as you stated it can be bump started.... 

[datzenmike]

He checked power to the coils and the distributor module is connected to the intake coil so... all are 9.6v when cranking. This means all 1-5 fuses turned on by the ignition are also low during cranking. Trying to narrow down if the starter is drawing it down or dirty battery posts or maybe the fusible link to the ignition. It's a process.

[Crashtd420]

58 minutes ago, datzenmike said:

He checked power to the coils and the distributor module is connected to the intake coil so... all are 9.6v when cranking. 

Guess I missed that part....

[Crashtd420]

Would using another running vehicle to jumpstart it supply move voltage during crank? 

 

Not really sure what that would tell you If it worked....

[620slodat]

It would tell you that the start system is working. That would be a beginning.

 

Don

 

.

[datzenmike]

The engine turns with the ignition there just isn't spark while cranking. It push starts so everything works if the starter is out of the equation.

[thisismatt]

Ignition switch going bad? ON position fault while cranking in START position? Will it start if you crank and quickly turn to ON position? Try a few times...

[banzai510(hainz)]

My 510 goes down to like 8-9 volts during cranking  so this still might be normal under load

 

wiggle back of key switch incase dirty. just a hail marry on this one

 

on key switch one gets shorted to the starter for that to work ,maybe the one to the coils is not working.

Maybe run a wire from + batt to the coil then start with key switch and see if runs(another guess)