98 blazer 4x4 issue at high speed

[blazerman47]

Got a 98 4x4 150k. While quick acelleration will cause a whiney sound. know that sounls like a ratcheting sound. I found I have to stop and put it 4hi drive 10 feet and switch back to 2hi and goes away, jump on the acellarator and it will do it again. It comes and goes. It low speed it is loud. Once in awhile it will make a bang in the frt end after doing a start and stop routine. Help

[old_master]

Classic symptom of the vacuum switch on the transfer case leaking vacuum past the seals. The front axle actuator is located under the battery tray. It is vacuum operated and pulls a cable to engage the right front axle shaft with the front differential. The vacuum switch is located on the driver side of the transfer case, just above, and slightly inboard of the front output shaft There are 3 vacuum lines connected to the switch: One comes from manifold vacuum, from a Tee that shares vacuum with the HVAC system. One line runs up a few inches and Tees into the transfer case vent pipe. From the tee the line continues up alongside the transmission dipstick tube and has a small black plastic filter in the end. The third line runs to the axle actuator. When you're in 2WD, if the switch leaks vacuum past the internal seal, it will slowly supply vacuum to the axle actuator. The actuator starts to pull the cable and engage the axle, this is when you hear the "ratchet" sound.... the clutch gears on the inner and outer axle grind while trying to mesh. When the system works properly, vacuum is switched rapidly and the clutch engages the axles rapidly. Very similar to a manual transmission shifting into reverse, do it slow... grind, do it quick, no grind. When you shift to 4WD and back to 2WD, you're venting the vacuum on the axle actuator, that's why the noise stops. Once enough vacuum leaks past the seal, it starts to pull the cable again. If you shift into 4WD, the noise will stop. Long description, short, cheap fix. Most auto parts stores carry the switch in stock. Dorman makes it: http://www.oreillyauto.com/site/c/de...eyword=600-500

They also make the exact same switch and market it in their HELP! line of parts:
http://www.oreillyauto.com/site/c/de...&keyword=49315

The vacuum line connector has all 3 lines in one common, keyed, connector so it only goes on one way, (can't mess it up). To replace the switch, use a 15/16" deep socket, 3/8" drive, and a ratchet to break it loose, then unscrew it with your hand. About 15 minutes if you haven't done it before When you have the old switch out, you'll probably find the spring loaded ball binds in the switch. In the image below, you can see the switch:



Check each vacuum line for transmission fluid. If you find some, the line must be cleaned out using brake cleaner, carb cleaner etc etc. Then check fluid levels in the transmission and transfer case.

[blazerman47]

Thanks. I do hear air or a vacuum making a sucking sound. Even after I sut off the truck you hear the vacuum leaking for a second or two.

[old_master]

If the vacuum switch on the transfer case is leaking, you wouldn't hear it. What you're hearing might be a vacuum leak in the power brake booster though. Plug the vacuum line to the booster, idle the engine for a few seconds then shut it off. If you don't hear the leak when you shut it off, the booster is leaking.

[blazerman47]

I will try and check it this weekend. If not next weekend. I will be touch either way. Thanks again. I have a few other things that give my headaches. Not starting on really cold morning. It also idles rough when the engine is warm. Rpm drops to 600. Some autozone guy told me it was a master eng filter. Som name he calls tthis part that is screwed into the manfold. He told to clean it out every 5k.

[old_master]

The AZ guy is on drugs, see if he'll sell you some No such thing as a master engine filter. He might be referring to the EGR valve, who knows. IF that's the problem, there's a replacement gasket for the EGR valve that has a screen built into it that's designed to break up the carbon chunks so they don't interfere with the EGR valve operation. Best to start a new thread for the other problems though.