Why do I keep blowing a 20A fuse marked ECM-I?

Please confirm which fuse is blowing. Is it the cluster fuse, the ignition fuse or both? From what you wrote more than likely the harness out of the fuse box is shorted somewhere. With everything plugged in, does the engine crank, start and run? You can look around the engine, behind the gauge cluster and around the steering column. But if find nothing then you may want to consider either investing in a Power Probe short finder #PPECT3000 $123. This kit only has the short finder. The master kit #PPKIT03 $250 (is what I have). The newer master kit #PPKIT03S is $194. I recommend this last kit if you have the money and think you will use it in the future. Its great for 12 volt circuits, not low voltage circuit boards. It does not do everything. Other wise you may want to sent it in for diagnostics. Write back if there is anything else.
Good Luck.

Something that fuse powers is shorted to ground . Need to look at wiring diagrams , service repair manual , power distribution diagrams , disconnect everything it powers - one circuit at a time till fuse doesn't blow any more . Then find the short , videos on youtube showing how . EGN A ???? IGN A fuse . ENG 1 or 2
Underhood Fuse-Relay Center Fuse/Circuit Breaker
Rating
Description
ABS Fuse M4-N4
60A
Electronic Brake Control Module
A/C Fuse K11-L12
10A
A/C Compressor Clutch Relay
AUX A Fuse P7-R7
30A
Upfitter Provisions
AUX B Fuse P8-R8
30A
Upfitter Provisions
BATT Fuse M7-N7
50A
Instrument Panel Fuse Block
BLOWER Fuse M3-N3
60A
Blower Motor, IP Fuse Block
ECM-B Fuse F11-G12
20A
Fuel Pump Relay, VCM, PCM, Fuel Pump and Engine Oil Pressure Gauge Sensor
ECM-1 Fuse H7-J8
20A
Ignition Control Module, Ignition Coil, Camshaft Position Sensor, VCM, Fuel Injectors, Crankshaft Position Sensor
ENG-I Fuse L8-K7
20A
Heated Oxygen Sensors, Mass Air Flow Sensor, Exhaust Gas Recirculation Valve Solenoid, Evaporative Emission Cannister Purge Valve, VCM, Secondary Air Injection Relay, WATER IN FUEL Indicator Sensor, Fuel Heater, Glowplug Relay, Wastegate Solenoid, Crankshaft Position Sensor
HORN Fuse H11-J12
20A
Horn Relay, Underhood Lamp(s)
IGN Fuse F5-G6 (Diesel Only)
20A
PCM, Fuel Solenoid Driver, Engine Shutoff Solenoid
IGN-A Fuse M6-N6
40A
Starter Relay, Ignition Switch
IGN-B Fuse M5-N5
50A
Ignition Switch
IGN-E Fuse F7-G8
10A
A/C Compressor Clutch Relay
LIGHTING Fuse M8-N8
40A
Instrument Panel Fuse Block, Headlamp Switch
LH-HDLP Fuse F1-G2 (Export Only)
10A
Left Hand Low Beam Headlamp
LH-HIBM Fuse F3-G4 (Export Only)
10A
Left Hand High Beam Headlamp
RH-HDLP Fuse H1-J2 (Export Only)
10A
Right Hand Low Beam Headlamp
RH-HIBM Fuse H3-J4
10A
Right Hand High Beam Headlamp
RR BLOWER Fuse K5-L6
30A
Auxiliary Blower Motor Relays
ABS Relay (Expot Only)
--
ABS Warning Indicator Lamp
A/C Relay
--
A/C Compressor Clutch Coil
A. I. R. Relay
--
Air Ijection Reaction Pump Clutch Coil
Fuel Pump Relay
--
Fuel Pump
Horn Relay
--
Horns
Starter Relay
--
Starter Solenoid

Disconnect the cigar lighter and see if the short goes away, I will bet it does.

Sorry to say it could also be a faulty ECM. The ecm uses circuit boards to provide grounds to a bunch of components and can short out internally.
Most of those components can fail and cause the same problem by causing a short "thru" the ECM. Things like injectors, O2 sensors, and so on.
I doubt you will find the problem without a book and some test equipment.

97 only had one fuse panel in the dash on the drivers side. There is/was a junction block for power that had/has fusible links on the driver side fender too.  a) 20a pwr accy
b) 30a pwr wdo
1) 20a stop/haz
2) 20a horn/dm
3) 20a tl/ctsy
4) 20a guages
5) 20a Eng-1
6) 25a htr/ac
7)25a pwr aux
8) 30a rr defog
9) 20a ecm/batt
10) 20a emc/ign
11) 20a radio
12) not used
13) 10a rdo/batt
14) 10a illum
15) 10a drl
16) 15a turn/back up
17) 25a wiper
18) 20a brake
19) 20a 4wd
20) 10a crank
21) 20a fog
22) 10a air bag
23) 10a trans
24) 10a prndl

The A/C relay is in the glove box behind a panel that indicate relay. There are three relays in there, the middle relay is the fuel pump and one of the out side relay is for the A/C compressor and the other is for the third brake light. Just can not remember which out side relay is the A/C so just swap the out side relays with one another. Good luck and hope this helps. Keep me posted. 

if you keep blowing a fuse there's a good chance it's not a faulty part but a short in the main harness most likely under the dash where all the connectors tend to get brittle over time and short out constantly. also check the ecm connection there is a 60 pin type.

your fuelpump could be causing the fuse to blow as it may be seizing up

1996 GMC 1500 Pickup.
Replaced Coil, Dist cap/Rotor, Ign Mod & pick-up.
All showed to be bad with OHM meter.

20A ECM-B fuse keeps blowing out and I'm afraid to burn these new parts up....

ANY IDEAS OUT THERE???

Before replacing fuses try disconnecting neg battery lead first. If still blows look loose wire under motor