Have a 2004 Discovery with about 100000 miles on it. Run great but up untill recently. Has a SES light on and after some code reading and investigations determined that the #2 fuse was blown. Replaced the fuse and everything was perfect. All the codes cleared and no pending code too.

Five minutes later the SES light came back on.... #2 blown!!! Looked at a wining diagram and have disconnected the 02, map and canister sensors individually to no other outcome than pop.

Inspected the major thicj wire at the firewall and the "T" where the nest flows down towards the O2 sensors nothing. Also the wires on the DS laying on the valve cover, they are also perfect.

On wierd thing when running if the fuse blows then the engine would keep on going. Pull the fuse and it would stall out. Now the issue is getting worse, when the key is in the run position then the fuse is intact, as soon as the starter is engaged then the fuse pops. Could the issue be at the cam sensor area??

Any ideas??? I'm going crazzy!!!!

 

Do not replace the fuse or you will cost yourself some big bucks.
How mechanic are you?

 

So what is causing all the grief?? Is there a chafed wire going to ground or is the issue more serious. Mechanicially, not scared of very much. Have access to scanners and Mitchell if need be. Looked at the wiring scematics and traced most of the functions of #2 with no luck. The problem is getting worse too. Truck sounds like it is missing or a plug has been washed out.

 

Looking over your shoulder at the several pages of engine management diagrams, that fuse runs:

Air injection valve
Purge valve
CMP sensor
EVAP cannister vent
4 O2 sensors
Mass air flow sensor

Truck may run without some sensors disabled, as ECU has a plan "B" for many sensors, but not all.

IMHO one method would be to unplug the air injection valve, purge valve, and EVAP cannister vent. The RAVE will show you where they are, and the wiring diagram has connector and pin numbers. The electrical library has color photos of the connectors in their location.

The truck wants to activate the SAI system as soon as it does a cold start, so that may be a clue. A fried and stuck SAI valve could make new sounds.

Do not replace with a larger fuse, because you do not want a possible ground to start melting a wiring harness or catch fire.

 

Basic electrical principals. A short to ground before the load will always blow the fuse.

 

I have unplugged all 4 o2's, the mass air flo sensor and purge control valve before the post and still no change. Turn the key and pop.....


Unplugged the SAI pump and fuse is still intact. Going to take the truck for a drive and see what happens. Is it normal for the SAI to go bad??

 

Not the pump.. Fuse blew again. Running out of wire to look at, everything looks perfect. The fuse will blow as if it is a dead ground issue. (black inside) But I cant seem to see any visual scars or issues in the wiring.

 

SAI pump is on fuse LINK 2 (bolt in) , not fuse 2. Are we on same page?

I forgot to mention that leak detection pump is on there (if you have that style of evap control).

Of course there are always the plugs on bottom of underhood fusebox.

Pix of various connectors from RAVE attached

c0880, C0152, C1339, C0133

 

Unplugging the loads before you start it wont change the outcome. You have a short BEFORE the wiring gets to the sensors. You need to trace and seperate the harness to find it. You are looking for a wire rubbing against chases ground.

 

So I unpluged the wires comming from the fuse box under the hood going to the engine side of things. Did a continuty test to ground and found all wires good. This eliminates the #6 & #7 wiring circuts of fuse #2 . So Air injection, cam shaft, purge control and Mass air flow are all good.

Some things I can't find even with the pics. One the EVAP canister vent not sure if this vehice is equiped with it and the other is the leak detection pump which is tied into the 02 sensors.

Any idea on where the splices on circut 21 can be found. There are two one for the 4 o2 sensors and the other is the leak dection pump.