I have a '97 Blazer 4WD that has developed an odd problem. The 4HI and 4LO lights come on when pressed but the front axle does not engage. No trouble codes, no blinking light - but the axle doesn't engage.

- 4WD fuse in fuse block is OK.
- All the diagnostics in the manual for testing the dash lights pass. (The lights all respond as per the manual when buttons are pressed.)
- I can hear and feel the electrical servo in the front axle engaging and disengaging when the key is on and the buttons 2HI and 4HI are alternately pressed.
- However, when I drive the car, the front axle does not actually engage, either in 4HI or 4LO.

I'm working my through the factory service manual to find a fix, but I thought I'd ask here in case someone has encountered this specific set of symptoms before. Any ideas on where to look FIRST?

I'm increasingly thinking this is not an electrical or servo problem, but a mechanical problem inside the front axle. If so, it won't show up in electrical diagnostics.

Thanks!

 

I'll be damned. I found it.

On the underside of the hood on the driver's side, there's a vacuum reservoir shaped like a black plastic ball. I remembered that the manual referred to its functioning with a couple of systems.

The vacuum hose going to it is short and it tends to pull off. I happened to see that it was off again, said to myself "it can't be that simple, can it?" and re-connected it. Test drive followed...

Problem solved. Front axle now engages!! The fix will be 50 cents worth of new vacuum hose.

I posted this follow-up in case someone else runs into the same problem.

 

Good work pilgrim. The follow up is always the best part.

 

I have the exact problem but on a 99 blazer. I went under the hood but could not find any canister. Could you be more specific about the location? Thanks so much for your help.

Jerry

 

It's attached to the hood, driver's side by the hinge.

 

Under the Brake Pressure Modulator Valve ^^^ Like Rammer said

 

On the newer ODBII vehicles 96+ the vacuum "ball" is located underneath the battery box and the vacuum actuator should be next to it too.