* Low Differential Fluid: This is the most common and easiest to fix. Low fluid level allows the gears to grind against each other. When you engage 4HI, the front driveshaft is engaged and the fluid is better distributed within the differential, reducing the grinding. Check your differential fluid level immediately. If low, refill it with the correct type and amount specified in your owner's manual.
* Worn or Damaged Differential Gears: If the fluid level is okay, the grinding could be due to worn gears within the differential. The engagement of 4HI might slightly alter the load distribution or meshing of the gears, temporarily masking the noise. This is a more serious problem requiring a differential rebuild or replacement.
* Damaged Differential Bearings: Similar to worn gears, damaged bearings can cause grinding. The load distribution shift in 4HI might alleviate the stress on the damaged bearing, reducing the noise. This also requires professional repair.
* Front Axle U-Joint(s): A worn or damaged U-joint in the front driveshaft could cause a grinding noise, especially under load. Engaging 4HI might redistribute the load, lessening the noise from the damaged U-joint. This needs inspection and likely replacement.
* Binding in the Front Axle: Something might be binding in the front axle assembly—a sticking caliper, a problem with the CV axles, etc.—that is relieved by the engagement of 4WD.
Why 4HI makes the noise go away (or lessen):
The most likely reason is that when you engage 4HI, the front driveshaft is connected, and the load on the front differential is distributed differently. This might:
* Lubricate the gears better: The increased rotational speed and fluid movement from the driveshaft help distribute and spread the lubricating fluid, reducing friction.
* Shift the load: The load is now being shared between the front and rear axles, reducing the stress on the potentially damaged components in the front differential.
* Slightly change the gear mesh: The engagement of 4HI might cause a slight shift in the position of the gears, resulting in a less pronounced grind.
What to do:
1. Check the differential fluid level immediately. This is the easiest and cheapest check. If low, refill with the correct type (usually 75W-90 or 75W-140 gear oil for a 97 Blazer) and amount.
2. Inspect the front U-joints and CV axles for damage or wear. Look for cracks, play, or binding.
3. If the problem persists, take your Blazer to a mechanic specializing in four-wheel-drive systems. They'll have the tools and expertise to properly diagnose and repair the problem. Ignoring this could lead to more extensive and expensive damage.
Don't continue driving with the grinding noise. It's likely indicative of damage that will worsen if left unattended.
