Okay, I have a 1997 GMC jimmy with the 4.3. Ive had problems in the past with the 4 wheel drive, and ive gotten them fixed before. This one is puzzling me though. When I click the 4HI button, the light just blinks for about a quarter of a second and doesn’t actually enable 4 wheel drive, nor does the light stay lit. I’ve replaced the 4 wheel drive actuator, which is located directly under the battery, and all of the vacuum lines seem intact, although there is a possibility that there is still a leak in some of them. When I press it, I don’t hear any noise to indicate that anything is trying to engage or move underneath the vehicle. I just replaced the selector switch, and now the 2HI light does infact stay on, which it should, but it still doesn’t fix my issue. Can anybody possibly give me a suggestion or a solution to this please?
How is the 4wd engagement configured on that vehicle? On my truck I move a lever which through physical linkages, pushes on stuff inside the transfer case. It sounds like on your your truck you push a button, which then causes current to flow in a circuit … and then … … if you explain how it is supposed to work you might get some more ideas here. Otherwise you’ll have to wait until a diy’er who owns a similar 4WD vehicle posts. .
It uses a 3 button selector switch, with 2HI, 4HI, and 4LO buttons. As for your previous question though, the 4 wheel drive is mainly controlled and activated by vacuum on that vehicle. I’m not entirely sure the entire process, but I know it has a 4 wheel drive actuator located directly underneath the battery with a vacuum line running to that. It also has a transfer case motor right near the driveshaft with an electrical connector to physically move it into position for the 4 wheel drive, but its all mainly done by vacuum. I am wondering though if it could be the ground that could possibly not be making good contact? I might check that
Similar but different. While my vehicle does have a 4 wheel drive actuator, it doesn’t work in that same way. On my vehicle, it’s located underneath the battery, and I don’t think the hubs have those weird tooth things on them. I’ll do some research though
Thanks for the explanation. Some locking differentials work in a similar fashion, a vacuum signal puts them in lock mode, and then the two rear wheels won’t slip w/respect to each other. My 4x4 truck on the other hand uses a limited slip differential, no vacuum control needed b/c it doesn’t support full lock mode. The full lock mode is used by aggressive 4-wheelers for steep hills, rock crawling in Moab, etc.
Ok, back to your problem … hmmm … well, it makes sense to do a couple of measurements.
Voltage at the activation electrical solenoid at the xfer case. Probe between the activation signal pin and the ground pin (or solenoid case) as close to the solenoid as practical. Leave everything connected, may require some invention to figure out how to do that. If you have a ground problem, it will show up by probing the ground nearby to the solenoid. You’ll have to look up what the voltage level is supposed to be, probably battery voltage. So look for something in the 12-13 volt range.
Vacuum level where the vacuum line runs into the xfer case. T-in your gauge at that point. Again you’ll have to look up the spec, but probably the full intake manifold vacuum, which will typically be in the 17-22 inch-Hg range.
Note that there must be more controls than this discussion is considering b/c it has to support 3 modes.
I’m guessing that the solenoid is just a vacuum switch, allowing the vacuum to pass through when selected, and it’s the vacuum what powers the “motor”; i.e. it’s a vacuum motor, not an electrical motor (remember I have no experience w/this configuration, just an educated guess). Let’s do the math. A vacuum of 20 inch-Hg is about -10 psi. (If you don’t believe me, type ‘convert 20 inch hg to psi’ into Google.) So that amount of vacuum can push on something with a force of 10 pounds per square inches of the area that it is pushing on. Mechanical levers can increase the force as required.
Those are for the locking hub mechanism. Not every 4WD configuration uses those. Mine does though. I have something very similar in appearance in the front hubs on my truck, but mine are manually activated. If I want to use 4WD mode, I have to get out of the truck and turn a little lever in the center of each wheel, from “free” to “lock”. That configuration is called “free running hubs”, b/c in 2WD mode the wheel just spins on the spindle, no xfer case involvement, better for mpg. When the hubs are locked the drive axle from the front differential (which runs through the center of the spindle) powers the front wheel instead, and those teeth are how it does it.
Generally if you have full time 4WD, the hubs are always locked. If you have part time 4WD, then you have free-running hubs & a method is required to lock the hubs in 4WD mode. Full time 4WD doesn’t require the need to lock the hubs, but requires a fore/aft differential in the xfer case.
I’d like to help, but I’m not familiar with the system. The 4wd system on the GM full size trucks of that era doesn’t operate on vacuum at all. Does it have any sort of actuator on the front axle? Those were problematic on the full size trucks, but they’re electric.
Connect a vacuum gauge to the vacuum switch and axle actuator to see if there is a vacuum supply present.
The problem could also be with the transfer case actuator, fault codes from the control module would help.
Its got the transfer case motor. The only actuator I know of on that vehicle is the 4wd actuator powered by vacuum, and its located directly under the battery tray. Its been replaced already. Im going to check the ground for the transfer case motor and see if thats making good contact, and that very well could be the problem.