I an blowing the main run circuit fuse 15 amp just as soon as the engine starts and drops to the run circuit

Hello. It seems there is a 'short' circuit in either the 'run' circuit of the ignition switch and/or in the fuse box. Positive polarity (+) is touching negative polarity (-) somewhere for to 'blow' the fuse; maybe in the device that is blowing fuses. Good luck and keep me informed; if you want to.

You have a short to ground on that circuit . Find a wiring diagram an see what all the main 15 amp. fuse powers up . Disconnect all components an isolate the circuits till the fuse stops blowing . Could help but you don't say what make , model an year vehicle you have.

Fuse blowing indicates some circuit is drawing too much current. Figure out what that fuse supplies and investigate that circuit.

Well u will need an amp meter that will check out the current thru fuse with nothing on " multi function" besides the motor running and see if it solves your problem and tell us your current reading .

15 amp is a fair current draw and for it to blow after a few minutes means that there is a short in that circuit that is allowing a drain to earth that just exceeds the 15 amp. It is in the ignition part of the system and may be a bad connection ( water ) or a problem appearing in the ECM or relay . Run fault codes to find the area of the fault. Because it is taking time to blow the fuse, it will be difficult to find. Check all wiring for rub marks .

Possible causes:
- Fuse is under rated.
- Short circuit in wiring.
- Fuse No. 15 is overloaded.
- Positive wire is shorted to ground.
- Check all the loads one by one which are put on this fuse.

It will help if you can identify the 40 Amp fuse that is being toasted when the key is turned.

I looked briefly at a fusebox diagram and found 3 circuits protected by a 40 Amp fuse. The Lamp circuit protects several circuits which have smaller rated fuses on each separate wire. The 40 Amp protection seems to be a safeguard if all the smaller circuits are active in total. Turning off the automatic headlights, should reduce power drain.

The next circuit is the Rear window defroster. Can easily pull 40 Amps, but once the 40 Amp fuse breaks, it should have no effect on the engine.

The last circuit, on the cars with Trailer Tow or Police package has a
40 Amp fuse on the Battery charge Relay. This is a remote possibility. But the engine should be able to run without an Alternator for several minutes using Battery power.

Have them check the fuel pressure. The Inertia switch could have tripped while the car was being repaired. It does not explain the wiring short, but can explain the no start condition.

Again, if the same 40 Amp circuit is overloading, identify the circuit with the fuse box diagram.

50amp only clue and you dont have the book, the FSM.
nor access to alldata.com?
if you did youd, not be asking right?
my guess is that fuse is in the mains box under hood..
and is a real fuse and not a circuit breaker found elsewhere....????
old car, and can be anything this old.
sounds like fuse #33 in the main box blew. and feeds the white wire main.
USA car? ABS opted? A/C opted. sure.
top cause, shorted alternator, so take off that big wire from alternator
and see if fuse blows now. (do so with battery neg lug removed
the remove alt big wire and tape it all up, so it can touch metal.
then put back battery Neg lug, and last the fuse.

the whtte feed goes vast places and runs whole sections of
the cab fuse boxes and power windows.

if the Alt is good.
then you must start dropping all white wire loads.
if it still blows, the harness is shorted.
I personally just uses a temporary circuit breaker in place of the
50 amp to find the short and an ampclamp DC ammeter.
fast.
login to alldata.com and follow the big white wire.

Get a wiring diagram & see what is on that fused
circuit & work that circuit. Leave the fuse out at
first & make sure only that fuse & circuit is involved

When a car gets over 15 years old, then & only
then, do you start to have fuse issues as wiring,
connections,relays,switches & devices using electrical
current, start to get loose,corroded,& develop resistance

I would remove & clean every ground wire on the car you
can find

Both ends of the battery cables as well

Do voltage drops afterwords on cables

Check for alternator diode leakage or what
is called a/c ripple

Check for current draw by putting an amp meter
in series with either battery cable & wait for things
to time out,make sure you leave the doors closed
& allow at least 10 to 15 min.

You shouldn't have over 50 to 70 milli-amps

Just the clock & computer memory & some keep
alive items

Don't keep replacing the fuse,that is not a fix

If it's fuse 36, it should have a 15 amp fuse.
Do not confuse this fuse with the radio backup fuse, which is 7.5 amps but runs a lot of other circuits.
Put in a 15 amp fuse.
If it burns, the lighter may have a short.
See diagrams at link below.
http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=4576