75 chev 350 cu in with hei no spark

[rocker]

hey guys, well I'm completely baffled. I ve owned this truck since 1991 and have had no spark issues before but was always able to fix it doing the normal replacements in the distributor. last sept I had again no spark  so tested everything and it all tested good so I  replaced, in this order, ignition switch, wiring harness in the distributor,pick up coil and module and finally the msd coil that was less than a year old. now I again have no spark after only 12 hours of use, tested everything again it all tests fine, I replaced the coil, module condenser and wiring harness in the distributor it fired up and ran great. I shut it off put everything back together put away my tools went back to the truck to move it up to the shop........no spark. I try to start it after each thing I replace but im out of ideas what to replace. anyone got any ideas or have come across this? seems to me like a loose connection somewhere or poor ground but this is a pretty simple electrical system.......  but I guess complicated enough to stump this old guy lol

[haroldwca]

Did you retain/replace the ground spade for the coil in the top of the distributor cap?  It grounds the coil and is one of the terminals that your wiring harness to the distributor cap contacts.  If it is left off, it will burn a coil wire inside of a couple of days.  This is just a shot in the dark, but I feel for you with these "phantom" problems.  Good luck !

[VileZambonie]

Verify B+ with a test light at the ignition coil with key on and during cranking. If okay remove the cap, verify the cap and rotor are okay and that the rotor is rotating when the engine cranks.

[rocker]

ya the ground is still there and I checked to make sure the connection is clean. the cap and rotor are msd and less than a year old. I tested the B+ and its good

[blazer74]

B+ while cranking as Vile suggested.? You didn't specify in your response.

Spark at the plugs?
I like using an inline spark checker, so easy and cheap tool.

U can run B+ straight to distributer from battery to eliminate any intermittent power in problems for T/Shooting.

If I new for sure my power in was constant with all the parts u changed I'd be replacing the Distributer and verifying fuel delivery.

[rocker]

yes I have power with the key on and while cranking. everything tests like it should but no spark at the plugs. my tester is like an adjustable spark plug with a clamp that goes to ground. its very easy to see the spark. I ll try running direct power and see what happens. thanks

[rocker]

well blazer74 that worked alright it fired right up. so my problem is where? the ignition switch I replaced not even ayear ago?or my harness from the bulk head? but yet when I tested the B+ like 4 times, key on and when cranking I had juice there.

[VileZambonie]

Define juice? What is the voltage during cranking at the coil + ?

[blazer74]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=11766.0

Wiring diagrams on this site. ^

Replace your fuse first for the sake of it.

Loose/ corroded or burnt connections always possible causing loss or low voltage.

[rocker]

ok with the key on I have 11.2 volts, 8.4 while cranking

[VileZambonie]

Disconnect the B- cable and see if you have a yellow wire connected to the R terminal of the starter solenoid which is spliced into the ignition coil feed. While you are there, check the connections and fusible links at the solenoid.

It sounds like you have a resistance wire feeding the coil and if so it would be a good time to replace that section of the harness but start by checking the above.

[rocker]

I checked all my wiring and its good, tested every wire with an ohm meter. I don't understand how I have sufficient voltage at B+ and there is no spark but as soon as I run a direct wire from the battery to the B+ it fires up immediately. I m  bypassing the ignition switch doing that but what else?

[bd]

I don't understand how I have sufficient voltage at B+ and there is no spark

Because, there really isn't sufficient voltage (or current) supplied to the distributor B+ terminal.  Read the following post again paying particular attention to the text in bold.

Disconnect the B- cable and see if you have a yellow wire connected to the R terminal of the starter solenoid which is spliced into the ignition coil feed. While you are there, check the connections and fusible links at the solenoid.

It sounds like you have a resistance wire feeding the coil and if so it would be a good time to replace that section of the harness but start by checking the above.

Check specifically for what Vile described and be specific in your answer as to what you find.

[blazer74]

75 was the first year for HEI in the Gm trucks. If you have the said yellow resistance wire from the starter you could have an early production 75 without HEI and has been converted to from a points Dist. (Guessing)

I converted my 74K5 to HEI and ran a wire directly thru the fire wall to the spare ignition spade terminal above the glass ignition fuse to the dist B+ providing full 12 volts ignition on power.

You can rewire the harness later if u choose.

[rocker]

the shop manual states 7 volts as being sufficient which is what I have. as for the wiring, its the same wiring that's been in the truck since I got it in 1991. I have a 1985 parts truck but I don t think it has the same harness does it? if its the same I would change it just to rule out any broken wires; altho I just checked each wire with an ohm meter