Hello All,

Working a 1990 K5 Blazer... ECM B fuse blows when installed. I've checked other posts and found similar problems, but none worked for me. I found another post that stated the ECM B fuse should be 20 amps, however all other info I have states 10 amps. Don't want to fry my ECM...

I have checked the Fuel Pump Relay, the fuel pump, the Fuel Pump Oil press switch, all were disconnected and the fuse still goes.

Any insight will be greatly appreciated.

 

You have to be careful with some of the information relating to searches for 'ECM B' on here as it can cross over to the s-series trucks which won't be applicable to your fullsize truck. This is where the 20A rating is coming from.

As far as your problem is concerned, inspect the harness at the back of the engine near the distributor & fuel pump oil pressure switch. Damage to the orange wire in this harness would cause the fuse to blow regardless of whether the switch was good, bad, plugged in, or unplugged.

The ECM B fuse also runs to the ECM so it goes through the big bulkhead pass-thru that the main wiring harness goes through to the ECM. It is very possible that the ECM can have an internal short that is causing the issue. To check this, the connectors on the ECM should be removed and the fuse installed. If the fuse does not blow with the ECM disconnected, then you may have to source a replacement ECM. If that makes no difference, then you may want to go over the wiring between the fuse box and all those items mentioned with a fine toothed comb, removing the wire wrap & really looking at the orange wire and all junctions involved. I would concentrate on the underhood wiring first as these are the most likely places for heat, weather, and other damage events to occur.

I have also seen where people have used that bulkhead as a 'convenient' means of passing their own wiring into the cab and damaged the factory wiring in the process often resulting in corrosion and/or shorts long after they passed the vehicle to someone else. The damage is localized to the firewall area where the rubber booted seal is located right at the bulkhead.

Good luck!

 

Thanks... The 20 amp fuse post was found by my son, it was for a Suburban.

Looks like we have isolated it to the ECM..

Thanks again..

 

Where are you located? I have one from my '88 K5, if that will work.

 

At the moment, Upstate NY, then heading to NM

 

Before labeling the ECM as bad, I would observe the fuse as you move the ECM wiring around with the ECM still disconnected. If the fuse survives, then it is very likely that it was the cause for the fuse blowing. If the fuse blows, then the problem is in the harness to the ECM and may not actually be in the ECM.

 

Good point.. I'll check that out this morning..

 

Never mind. I'm all the way in California.