My son's 2008 Mazda Miata hard top convertible wouldn't start and found out the starter fuse was blown. So he put in a new fuse and it worked a while then blew the fuse then put in another new fuse and it worked for a while and then it blew also. He had a local mechanic shop check it out and they said the started was going out and pulling too many amps so it needed a new starter. So he had them put a new started on it yesterday for $500 and today the car did the same thing, wouldn't start because of a blown fuse. He called the shop but they acted like they weren't sure what to do next, any ideas why the starter fuse keeps blowing?

Bad starter or starter solenoid was my first guess. Next would be a short to ground somewhere along the power wire from the starter to the battery that doesn't get energized until you turn the key. If the shop is claiming it is drawing too many amps it might be a bad ground strap (unlikely but still worth checking). The problem will be that all the connections will look good unless you get super lucky and find something burnt. It's going to take someone who is good with a test meter to track down the electrical pathway between the fuse and the starter, but just know that Mazdas are notoriously difficult to work on, especially finding electrical faults.

Fully charge battery and then
Load test battery. If battery
Voltage is low amperage to
Turn starter will go up.

When was the last time you cleaned the battery terminals? I mean remove the cables and scrub off all of the crud which has built up on the battery terminals and the cable clamps. That crud causes extra current draw which could be the reason for the blown fuses.

__________________
Bill

What else is on that fuse? Fuel pump maybe??

Likely corrosion causing the amperage to skyrocket....Check cables, connections, and battery voltage. As the battery voltage decreases, the amperage will increase and cause all sorts of mayhem - blown fuses, melted cables, burned up starter. Best test of a battery is a good old fashioned carbon pile load tester, I still can't believe the pocket calculator style 'electronic battery testers' yet.

Good luck.

__________________
On the Oak Savannah

Battery corrosion, or a poor connection of any kind, will always LOWER current, and will not cause a fuse to blow.

A bad, or even missing ground strap will not cause a fuse to blow.

Been biting my tongue, here...

OP, check for chafed/melted insulation on cabling from fuse holder
to starter.

^^^ Same here. I know better than to dip my oar into discussions like this, but I could no longer restrain myself.

This has been told to me too many times, but I can't resist:

Your son has a short. Take it out and put in a LONG.

__________________
John 3:16
WAR EAGLE!

Use a skyhook and left-handed torque wrench.

"Measured once, cut twice...still too short!".

Rodents eating the insulation off the wiring?

Honestly, do not put another part into it without tracing the wiring in the complete circuit to locate the fault.

Mechanics who just keep throwing new parts at a problem rather than diagnosing the actual problem are not competent mechanics.

I don't know about fuses, but it did make the manual choke cable on a buddy's car get warm when the key was turned.

__________________
Release the Kraken

One of my hyper-interlock parked wonders did that and it DID turn out to be the starter. I was just glad to get the dam thing fixed.

__________________
"He was kinda funny lookin'"

Check the terminals in the fuse box. A burnt or corroded terminal will do that.

So saying it was unlikely didn't tip you off? If you are going to go through all the trouble of tracking down an electrical short on an older car like this, taking a few minutes to clean and re-secure the ground straps never hurts and only takes a few minutes.

On a different tack, some Google searching indicated that certain cars have both a starter relay and a starter fuse in the fusebox. If the fuse blows it can sometimes be attributed to a bad relay that is shorting to ground. If the Miata in question does have both then try changing both - it certainly can't hurt to try.