Newfieboy - Without looking at the vehicle, I would have to suggest that you start by checking your battery connections.  With the headlights and the engine controls being essentially isolated from one another, that's about the only possibility I can come up with right now.

I agree.  It’s got to be a vehicle wide voltage drop to kill the headlights and make the engine stutter.  Besides the battery, check for loose connections or wires getting pinched, maybe the engine torques over in its mounts at highway speed and aggravates the connection or wire.  Other possibility, if you have a single ribbed belt, are you running the AC?  Maybe with a worn belt, when the AC clutches in at highway speed, the belt slips and the alternator slows down for a few seconds.  My daughter had a squeal in her AC, but the belt LOOKED good. Got a new one and found the old one had stretched about 3 inches in length/circumference!

Blacksmith

Thanks for your advise. I checked the battery connections and alternator belt, both are in good condition and I am not using the AC this time of year. I was wondering if there could be something on the go with the computer. The only time I know that the headlights, the ignition system and the check engine light have anything in common is during the time when the engine is being cranked over during startup. Once the key switch returns to the run position, the engine can stall without affecting the headlights. I am not sure, but I think the computer shuts down the headlights during the startup to provide maximum cranking power to the starting motor.  

I reread your original post and your last post.  As I understand, this problem is when you're driving, not starting the van. Anytime the key is on and the engine is not running, the check engine light should be on and the headlights probably dim due to the starter draw, or the starter solenoid, not the computer, cuts the power to the headlights, not the computer.  Anyway, you need to check the connections at the starter, on a GM vehicle; most power comes from there. Also check your engine and chassis grounds, in 6 years there may be some corrosion buildup.  This sure sounds like an intermittent connection somewhere that is excited (vibrated) at highway speeds. Being extremely careful of moving parts, you might try wiggling all the positive and ground cables with the engine running - if the engine skips or stalls, you're holding the problem wire in your hand.  BUT wear short sleeves; be very careful of moving parts!


Blacksmith

Hi Guys

I found and repaired the problem. The power failed and would not come back. The engine would crank but would not start, there was no power going to the fuel pump. I checked for power at the fuse block and found that there was no power on either side of the ECMB fuse holder, this fuse supplies +12 power to the fuel pump through the fuel pump relay. It also supplies +12 power to the computer. I found that several other fuse holders in the fuse block were also dead, one of which feeds the headlights. Tracing the power back toward the battery I found a +12 volt power tap located on the firewall behind the battery. From this power tap there was a 4 prong plug with 4 fusible links suppling  " alwaws hot" power to various circuits. One of the pins on this plug had been overheated and was not making contact. This was the circuit feeding +12 power to a section of the under dash fuse block. Once I repaired the circuit the engine started and everything seems to be working fine.

Thanks for your help
NEWFIEBOY

Glad you fixed it.  As I stated, I felt that with your symptoms, it had to be somewhere in the vehicle's wiring.

Blacksmith