I've been having trouble with my 4WD all winter. Until recently, it did not work at all - the lights all blink on when starting the truck, then the 2HI light only. Pushing 4HI or 4LO did nothing. No blinks, no noises, no 4WD.

A few days ago, 4HI worked about 3 or 4 times. Then it stopped working again.

Yesterday and today, 4LO works - the 4HI and 4LO lights blink, and then either both stay on for a while and the 4HI goes off after about 30 seconds, or the 4HI goes off as soon as they stop blinking. It IS in 4WD.

While driving in 4LO, it is in granny gear, but when I get going faster (20-25 MPH), it kicks into a higher gear.

We have another 6" of snow coming on Tuesday, then more expected this weekend. I haven't been able to get up on ramps due to constant snow - the ramps slide on my garage floor.

I don't understand why 4LO works and not 4HI.

Any ideas?

Thanks.

 

See this thread https://blazerforum.com/forum/tech-a...ill-help-7180/

 

I replaced the vacuum lines that I could easily reach from under the hood - and it works now. None of the hoses had obvious holes or tears. The smaller ones had cracks and the 7/32" hose coming off of the intake manifold was spongy.

I took the battery holder out and verified that the vacuum actuator works now.

It works now! I have 4WD again, just in time for another 6-8" of snow.

Thanks for the help, rockp2

 

One thing I am not clear on. How did the front axle engage in 4LO when it wouldn't in 4HI because of a vacuum leak?

Thanks.

 

Switching to 4HI is a "Mode Shift", switching to 4LO is a "Range Shift". While your transfer case shifted to the 4LO range, the fork to engage the front axle was still not operating. You just went into a low mode.

The easy way I describe it is the old 4WD that required you to physically lock the hubs at the front wheels. So I picture it as...you can sit in the cab of your 1978 F150 (with a "Straight 6") and put the truck into 4LO and the truck moves slow and has all that torque at the rear wheels...but the front wheels aren't helping at all and still spinning freely until you get out and lock the hubs.

Your vacuum actuator (along with a couple other parts) basically takes the places of you. No vacuum = you didn't get out and lock the hubs = no 4WD = just a slow moving truck with a lot of torque at the rear wheels. Make sense?

 

Yes, but it sure felt like it was in 4WD.

 

I had a few good days.

Now, when in neutral, pushing 4HI causes the 4LO and 4HI lights to blink, then back to 2WD. Pressing 4LO causes both 4HI and 4LO lights to blink, and both stay on. It is not in 4WD.

Pressing either 4LO or 4HI in anything but neutral gets nothing. No blinking lights or anything else.

I am really frustrated with it now.

Is 4HI really nothing different from 2WD except the vacuum actuator pulls the cable that engages the front axle?