Hello all,

I am having an issue with my 4 wheel drive not wanting to engage. When I try to shift into 4hi or 4 low it wont engage the front differential. I can see that the actuator is trying to work but it cant pull the cable all the way. I can manually engage it but depressing the actuator the rest of the way but that is not an option when on the trails. I have replaced the actuator, actuator cable, and the transfer case vacuum switch. That only leaves vacuum. I have followed the lines and replaced everything that I can reach but the vacuum lines that run down the firewall to the bell housing are attached with some kind of bracket that I cant get to. They then run down the passenger side of the transmission and then over the top of it to the transfer case. I need to know where the 3 lines on top of the vacuum switch go to. I know that one is for vent, one is for the actuator and the other is the lead vacuum line for the motor. Being that this is going to be a monster to get done, I want to make sure that I am not missing something. Is there anything that you guys can think of that may be the problem before I start trying to replace the vacuum lines?

Also is there any way that I can check the vacuum to see if it is the problem?

Thank you very much for your time and help with this and have a great day.

 

Before tackling the job you're considering, try getting a MityVac or something similar, and testing each line. The lines very well may be in need of replacing, but at least you'll know before jumping in. I would start with the line that goes from the transfer case vacuum switch to the actuator, and test from the switch side of the line. This picture shows which line goes where - it is oriented so you are looking into the rubber cap thing that fits onto the t-case switch.

Another thing to consider when you're checking for vacuum is how well the rubber cap thing fits onto the transfer case vacuum switch. I bought an aftermarket switch one time, and the plastic prongs weren't as wide (diameter) as the original, and the cap didn't fit tight enough to seal the vacuum. So when you test the lines for vacuum, test at both ends of the line.

Hope that helps...let us know how it goes.

 

Thank you very much for the tips. The picture will help out a lot. I went out and got a vacuum tester but I am curious as to what amount of vacuum should be read at the actuator.

I am guessing that I will have to cut the vacuum lines in order to replace them because of the brackets that are holding it right at the bell housing but whatever it takes to get it taken care of. I am one step away from running the cable directly to the cab and doing it like that. Thanks again for the help.

 

I tested the vacuum as soon as I got off work and am reading 13lbs at the actuator. I have no idea what it is supposed to be and the only thing I have to compare it to is the main vacuum line that runs to the master cylinder which reads in at roughly 22lbs. I am not sure if it is supposed to be at the same pressure or not but logic would dictate that it is a vacuum line and not the actuator which was replaced a month ago. But it is entirely possible that I got a bum piece. I can manually depress it the rest of the way and the truck will go into 4 wheel drive but as soon as I let go of it, the front differential disengages. So if any of you have some insight, please let me know.

Also, I know that there are a lot of threads out there regarding the manual cable that you can install in the cab to take the vacuum and actuator out of the equation. Could anyone direct me to a good one? Thank you all for the help and have a good night.

 

I'm not sure what level of vacuum it's supposed to be at...I do my tests on how well the lines hold vacuum, not how much vacuum is there. So I usually just let the MityVac apply vacuum to a line (at no particular level of vacuum), then wait a couple minutes and see if the vacuum reading stays about the same on the gauge. I'm not a physics expert, but the difference in your numbers would suggest there's some significant vacuum leaking somewhere, and that's why you have to "help" the actuator.

 

Before I bought my second gen s10 4x4 blazer I did some researching for common/expected issues and the 4x4 engaging was one of them. I guess from what I read the vacuum line that works the actuator is underneath the battery. I watched a YouTube video that will help you diagnose 4x4 issues. Just search 4x4 blazer issues on YouTube.

 

Problem solved..... After replacing all the vacuum lines, actuator cable and the transfer case vacuum switch...... the problem was a pinhole in the 1 1/2 month old actuator..... All of the parts that were replaced worked but were definitely original equipment so, it wasn't a bad thing that they were replaced. It never occurred to me that it would be the brand new actuator. I visually checked it but couldn't see the bottom of the diaphragm where the pinhole was. The vacuum lines were on the verge so at least I know that they are good to go for years to come. Like most things that you take care of yourself, you learn from mistakes and I now have a MUCH better understanding of how the 4 wheel drive system works. Thank you all for the help.