The Allison in my 2001 GMC set the "MIL" warning lamp following the coldest overnight low we experienced last winter. The MIL would come right back after clearing the codes - I cleared them many times - they always came right back. The fluid level and color/condition were great.

The solution came as a result of what we always recommend as a first step when troubleshooting most transmission problems... a full service. Changing both fluid filters, along with what fluid drained as a result of changing the filters, solved the MIL problem. A series of trouble codes had been set. I'll have a short tech page soon that talks about the codes and the trans service.