Below is a chart of the Judges. Abimelech's status is indeed questionable. I would join with those who would not include him as a "judge". Here is a quote from the "Believer's Bible Commentary" on Abimelech:
Judge |
Years in Power |
Scripture Reference |
Othniel |
40 |
Judges 1:11-13; 3:7-11 |
Ehud |
80 |
Judges 3:12-30 |
Shamgar |
? |
Judges 3:31;5:6 |
Deborah |
40 |
Judges 4-5 |
Gideon |
40 |
Judges 6-8 |
Abimelech |
3 |
Judges 8:33-9:57 |
Tola |
23 |
Judges 10:1-2 |
Jair |
22 |
Judges 10:3-5 |
Jephthah |
6 |
Judges 11:1-12:7 |
Ibzan |
7 |
Judges 12:8-10 |
Elon |
10 |
Judges 12:11-12 |
Abdon |
8 |
Judges 12:13-15 |
Samson |
40 |
Judges 13-16 |
Eli |
? |
1 Samuel 1:1-4:1 |
Samuel |
? |
1 Samuel 4:1-7:17 |