My granite has the AMI-370 mack motor, intermittently the high stage goes out, and then the low stage only works with the switch in the high position, and low does nothing. It usually works fine for the first 25 miles or so then it starts to act up, always as I start over a big hill here in town. I will down shift on high, without trouble and as I start down the hill, the high stage quits and only low functions normally, but with the switch in the high position, almost as though a loose wire wiggles with the grade of the hill or something. Hopefully this isnt too confusing!!! Thanks!
The best thing you can do is a complete visual inspect, start on the outside and work your way under the covers, check in the dash behind the switches too, you are right it, definately sounds electrical.
Thanks for the input, I was considering looking into it but I'm a little reluctant with all of the electronics. It would be great if it was just in the switch or something simple.
I was talking to another driver today who drives a granite, he says there is an oil pressure line that controls the engine brake, and there is a small valve in that line that gets gummed up. The reason for high cutting out and only low working is that the engine brake is not getting the required oil pressure to function properly. It seems plausible to me.
I have installed an update kit from mack on a couple 350 macks, it was an external oil pressure line from the oil filter housing to the jake stands, seems some of the engines dont get enough pressure and volume to the engine brakes after they are good and hot. Not sure if this update kit pertains to your engine model, a call to your mack dealer would tell you that.
Thanks! I still havent had time to get it looked at!
Took it in the other day, the problem was a bad solenoid on the front head, works like a dream now!!