Fires up in the START position only if lights are on?

[guhfluh]

I've been having trouble getting my truck started in the morning. It didn't seem like it would fire up unless it was warm or had been running for just a few minutes.

I went to start it again tonight after it sat for a while and had more troubles. BUT, this time I got it to start up for a few seconds RIGHT at the moment I let the key off. I kept trying it and letting the key off and got it to start up and run fine. I let it run for a minute to warm up and smooth out a hair, then tried to restart it, but kept the key in the start position and it didn't want to start. It would only sputter once maybe, but most of the time it wouldn't fire at all with the key in the start position, but as soon as I let the key go, it would fire right up. THEN, because it was dark and I needed to see what I was doing, I turned the parking lights on and kept trying BUT, IT FIRED UP EVERY TIME I TURNED IT TO THE START POSITION!!! as soon as I hit the key. To make sure I wasn't dreaming, I turned the lights back off and tried again, and sure enough the truck didn't want to fire in the start position while cranking, but when I let off the key will it fire up! I must have tried it both ways 10 times just to make sure, as well as with the headlights on just to see, because my truck turns the front parking lights off when the headlights are on.

So, what the heck?!?! The coil and ignition module get 12v while cranking from the key. What the heck do the lights have to do with anything? All I can think of is checking my engine to body ground again and maybe adding another one or one from the battery to body as well. The truck originally only has a battery to block ground at the front below the alt and block to body ground in the back on the driver side by the oil pressure sensor. The battery cables are all new as well as the battery and the starter spins like crazy. I remade the block to body ground a while back out of thicker guage wire, solid copper ends, crimped and soldered the connectors.

Ideas?

[tac]

It does sound like a bad ground cable connection but the headlight thing is a little confusing. I would recheck the voltage at the ignition module while cranking. Make sure the meter is connected to the block and not the battery when making this measurement. If you connect the ground of the meter to the battery it will not pick up the drop in a bad ground cable and give you a false reading of 12V.

[guhfluh]

I've been trying to figure how the bad ground could cause it, but I don't know how. When I tested for voltage at the coil and module, I was testing to the body at the hood hinge, so that "should" mean that the block to body ground is good, but I just don't know. I'm going to try checking to the block as well as from the back of switch.

[tac]

That's true if there is not an "extra ground" added from the battery to the frame/chassis. I would clip you meter red to + on the coil and the black to the distributor case.
I found this on a car that had a bad ground cable and would drop about 2V across it when cranking. The car was old and would crank at 10V but the electronic ignition would not work at or below 10V(GM). But if I went to run position at just the right place during cranking it would fire and run. This was because the voltage would jump up when the starter load was removed. Replaced the ground cable cured it.

Good luck.

[guhfluh]

Factory never had a battery to chassis ground as far as I know, and I don't have one now, but I will probably be adding one. As far as I know there was only the battery to block ground and then a block to firewall ground.

When cranking, I never saw the coil and ignition module voltage drop below 10 v and it was normally 11-12v to the hood hinge which is why I'm confused. I'm going to test again though and also check with the lights on and try to find out what's different.

[guhfluh]

Well, I believe I found the culprit. My tachometer. It has been reading way high for whatever reason, but I never thought about it having anything to do with this because it runs fine and has great spark when running. Why I makes a difference if the lights are on, I have no clue still.

At first I thought I found the culprit with the engine to firewall ground being poor at the block because it had block paint on it still and the bolt felt loose, so I sanded off the paint and eyelet and made sure it was tight, but that made no difference in how it started. I'm still going to add some more grounds though. I'm going to add one 2ga. from the battery to radiator core support and probably another 2ga. from the block ground point to the frame. I have plenty of 2ga. cable left over and a couple eyelets, so I only have to pick up a couple more eyelets to accomplish that and I'll feel much better at least.

I do have a new tachometer already that I got under warranty to replace the bad one, I just hadn't installed yet, so I'm going to also retest with the new tach connected up just to make sure. It's a Sun retro tach that I bought new because i thought it would be reliable and it wasn't cheap either. Go figure.

[tac]

Maybe you hooked the tach power to the lights. My dad hooked a CB to the courtesy lights once. He stopped 2 or 3 times to check them, but it was always working when crawled under the dash (Door open).