MCI M3 or M4 electric shift solenoid problems

[Classicrider]

Hi all.
I have a 1961 MCI, that I beleive is an M4.. not sure though. It has a 4 speed crash box with an electric solenoid for low and reverse. This thing works fine at first when the engine is cold , but after its warm...and idling slow...  hint... it won't shift to low or reverse unless I speed the engine a bit. My thought is the voltage drops at low idle and is not working the solenoid.  Now I am thinking... always have lots of air..even at low idle, would an air shift like a high low range on a fuller 10 speed shift this thing into low and reverse.  or better still can this old box be replaced with a 9 speed or 10 speed from a semi...or are there buss's that use this type off gear box.. after 35 years driving semis, I would like more gear range for this old 6v71

[fe2_o3]

  Just a quick question, if I may. Does your MCI have a generator or an alternator? I have a 4104 with the old generator and won't kick up the voltage till about 1100 rpm. Was told an alternator would raise voltage much sooner. I have a similar, but intermittent, problem with the reverse solenoid. I'm wondering if switching to alternator would be better than rebuilding the solenoid or adding a relay to give it direct voltage from the batteries...Cable

[bevans6]

Later versions of that box had only reverse needing the solenoid, first and second were on the same shift rail, and the solenoid moved the shifter over to activate the reverse shift rail.  anyway, those solenoids usually have two coils, a pull-in coil that does the heavy moving, and a hold-in coil that keeps it while the shifter is moved into gear.  Maybe one or other coil just has dirty contacts or something like that.  Also, there is usually a reverse relay that switches the current to the solenoid, it may be getting old and tired.  On my MCI it is located in the rear electrical bay.  At the least you could measure voltages there and make sure it is switching reliably.  This goes with my philosophy of troubleshoot the things that are easy to get at first - the reverse solenoid is hard to  get at...  

Brian

[Classicrider]

  Just a quick question, if I may. Does your MCI have a generator or an alternator? I have a 4104 with the old generator and won't kick up the voltage till about 1100 rpm. Was told an alternator would raise voltage much sooner. I have a similar, but intermittent, problem with the reverse solenoid. I'm wondering if switching to alternator would be better than rebuilding the solenoid or adding a relay to give it direct voltage from the batteries...Cable

Good question, you know I have not looked.. As soon as the sun comes up I will "pop the hood and have a look see"..That may very well be the problem. Its not an issue on the level ground, but trying to shift and work the throttle and the brakes on a steep hill... yuck

[Classicrider]

Later versions of that box had only reverse needing the solenoid, first and second were on the same shift rail, and the solenoid moved the shifter over to activate the reverse shift rail.  anyway, those solenoids usually have two coils, a pull-in coil that does the heavy moving, and a hold-in coil that keeps it while the shifter is moved into gear.  Maybe one or other coil just has dirty contacts or something like that.  Also, there is usually a reverse relay that switches the current to the solenoid, it may be getting old and tired.  On my MCI it is located in the rear electrical bay.  At the least you could measure voltages there and make sure it is switching reliably.  This goes with my philosophy of troubleshoot the things that are easy to get at first - the reverse solenoid is hard to  get at...  

Brian

Good  points, will get out the meter..