Hello All,

I've run into an issue with my 98 Blazer 4x4 (w/ 5.7L V8 Swap) with the three button 4x4. Recently the transmission does not shift properly if the 4x4 fuse or TCCM is plugged in (it doesn't seem to want to shift to second and rpms will bounce up and down when it should shift to second). With either of those unplugged the truck drives fine. I was thinking it was the TCCM and after replacing with two junkyard ones there's no change (tested my original and one of the junkyard ones on another 98 Blazer and that truck operates fine so the TCCMs are good). The 4 low light is always on and the computer reads the trucks in 2wd, while in park the 4x4 switch doesnt do anything, but it in neutral it will click under the dash and I can hear the transfercase clunk from 4 hi to low. The switch does send a signal to the TCCM when the buttons are pressed, that was tested with a power probe at the TCCM plug. When I've got time free I feel my next steps are tracing down wires unless anyone's got any ideas on other things to check that I've missed.

Thanks in advance for the help!

 

OK see it this makes any sense to you. I'm going to assume that you can read electrical schematics and have some experience in electrical diagnosis.

You say that Trans operates normally when 4WD fuse is pulled, but not when the fuse is installed. So we will start with that. Below is the schematics pertaining to the 4WD fuse on your vehicle. The first schematic is incomplete, and the front axle switch part is completed by the second. There is no direct connection of this circuit to the transmission, other than the PCM - so we must assume that something somewhere on this circuit is not allowing the PCM to see what it needs to, in order to shift correctly. To isolate the problem, unplug things on this circuit (one at a time) until the transmission shifts correctly.

Transfer case select switch at C2
TCCM (not actually shown in schematics)
Front axle switch
etc

Also you can check at disconnected wires to ensure that voltage is making it all the way to that point. Finally don't forget about checking for good grounds with a volt meter.

I know that is seems like you have already done this by disconnecting the TCCM. But have you monitored what goes on at the front axle switch with a voltmeter when you unplug the TCCM and plug it back in? Supposedly this is the same voltage the PCM is seeing on this circuit.

Give it a try and let us know what you find.

Best wishes for a successful repair!





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Excellent this will help a lot! I hope I can get time this week to dig around in the truck some more, will post an update on what I wind up finding.

 

Please do let us know. It's how we know if we gave good advice or not + it might help others. It's not like I have ever experienced this issue before. Learning experience for both of us.