Check for spark - the old fashioned way.

 

Tonight's testing report:

When cranking over, plug hooked to a wire & grounded, I get a spark. Tried this on two random cylinders. The other 4 I used the test light because I was by myself and couldn't see the plug while turning the key.

We've got spark. Looks like a normal spark to me.

I pulled all the plugs and turned the engine over with a wrench on the crank. My goal was to see if #1 was firing at TDC. Doing it by hand I never got fire (Key was on & I used my in-line light bulb tester here). Maybe I shouldn't, never had to try this before? I did have the following relays closing-opening as I rotated the engine:

Fuel pump relay
Auto shutdown relay
The one marked:

31 Trans
Rad Fan
(2.5L)
Relay

(I'm guessing just trans on the 3.9L that doesn't have an electric radiator fan.)

I cant remember now, but either one or both quit when I pulled the auto shutdown relay.

I didn't take the time to tell if all 3 were operating every time, but for sure the fuel pump relay was kicking in and out, roughly 6 times per revolution.

This doesn't make sense. These are electro-mechanical relays. I think I recall seeing a life cycle on larger 120V octals less than 1,000,000 cycles.

So: 1,000,000 cycle life / 2,500 RPM / 6 = 67 minutes.
That relay should last about an hour driving down the road. Is this normal? I guess the voltage is pulsing so fast the coil never disengages the contacts at normal operation? @1,000 RPM that's something like 1 pulse each 0.01 second. Just seemed a bit odd.

I could also hear injectors firing and could smell the gas before I pulled the fuel pump relay.


Any thoughts before I swap coil off the 360 and pull a valve cover to check the valve timing?

Thanks for all the feedback so far!

Mike

 

the relays are clicking because the signal is too slow and the PCM shuts them off as a safety measure (shuts the ASD relay off to kill spark and fuel if the engine stops running).

The coil might be failing under repeated load. That's entirely possible. But you've pretty well ruled that out.

 

The other thought I've had is the starter...its cranking so slow, wonder if that could cause failure to start? I'm going to verify timing looks right by valve operation & if so get the starter rebuilt. Even if it's not the cause it's not far from needing it, provided rockers are rocking like they should. There's no internal engine drag when turning with a wrench & plugs out.

 

You can try putting a spark plug directly in the coil wire and bypassing the distributor. It's worth a shot.

My 3.9 has started with some really slow cranking- slow enough i thought the voltage was too low to fire the injectors. Takes about 11-11.5 to fire them.

 

Figured it out!

I had the starter rebuilt by a local shop. Put it on and it fired right up. I didn't let it run b/c I had the belt and fan off. I put everything back on the motor and it wouldn't start and was dragging again, not as slow as before the starter rebuild, but noticeably slower.

I pulled the belt off the tensioner and it started.

Turns out the compressor is locked solid. A bit embarrassing I didn't check the accessories earlier, and apparently I never tried to crank it without the belt on. It's also apparent the 3.9 needs to spin fairly quick to hit.

Thanks for all the help troubleshooting!

 

compressor locked solid sounds wrong though unless it always had the A/C turned on or the clutch was also locked up?

If its now ok so be it

 

My 3.9 has started at some awfully slow speeds, but getting to the 500RPM or whatever cutoff is for the ASD relay could be a challenge.

I've heard of the bearing for the AC compressor clutch seizing, causing the system to overpressure and seize the whole compressor.

 

If it was locked up a very loud squealing belt would be very obvious I should think.

Another ooops story I suppose