5A "RUN" fuse blows instantly..dead short...

mikeceli

Registered User
Even w/ 10A fuse, "run" curcuit blows instantly.

HVAC blower, R def, traction assist, firm ride light, fog lamps all inop . Must have relays, all that can't run on 5Amps.

Where should I begin looking? I do have FSM and EVTM BOOKS.
 
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Go to each device that is listed as being fed by that fuse. Unplug all of them and check to see if the fuse blows again. If not, plug things in one at a time until the fuse blows and you'll find the shorted device. If the fuse still blows then make sure you have everything unplugged and didn't miss anything. If the fuse continues to blow you'll need to use the EVTM manual to locate and trace the associated wiring. Look for overheated or blistered connectors or a chafed wire. This could be a real bear to find as some of these things are in hard to get to places.
 
I'm working on it now. It seems the "run" curcuit dosent power devices, except , I THINK, relays and / or/ control curcuits. When I remove the EATC from the car, the HVAC blower runs on high, even w/ the run fuse out.

I am looking in the FSM trying to really UNDERSTAND the RUN curcuit.

If anyone understands it, pipe in please!
 
Check the single lead coming from the coil pack and going to the ignition condenser. This wire cannot touch anything and if a capacitor goes bad it will cause a short so if the wire isn't pinched or touching anywhere then unplug it to see if the condenser is bad. The wire is a single wiring coming out of the coil pack harness. Lots of times the connector insulator falls off and people think it's just a ground so they don't worry about re-insulating it.
 
Check the single lead coming from the coil pack and going to the ignition condenser. This wire cannot touch anything and if a capacitor goes bad it will cause a short so if the wire isn't pinched or touching anywhere then unplug it to see if the condenser is bad. The wire is a single wiring coming out of the coil pack harness. Lots of times the connector insulator falls off and people think it's just a ground so they don't worry about re-insulating it.


Would the engine start and run(no codes) , w/ above problem?

I'm gonna look now. Wire looks OK, disconnected condensor. Still blows RUN fuse.

So far, I have disconnected

shift indicator light and OD switch
coolant level sensor
brake fluid level sensor
ETAC
console cover and it's switches
1 relay in trunk (ride control)
coil condensor

AGHHA! needle in a haystack!
 
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UPDATE short in 5A RUN curcuit is in air bag system, but where?

UPDATE

When I disconnect the gray multiple connector, from the supplemental restraint system controller (blue box on passenger side kick panel) the short goes away. The other connector (black) has NO effect on the short).

What shorts out in air bag systems? Clock spring?
 
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Go to each device that is listed as being fed by that fuse. Unplug all of them and check to see if the fuse blows again. If not, plug things in one at a time until the fuse blows and you'll find the shorted device. If the fuse still blows then make sure you have everything unplugged and didn't miss anything. If the fuse continues to blow you'll need to use the EVTM manual to locate and trace the associated wiring. Look for overheated or blistered connectors or a chafed wire. This could be a real bear to find as some of these things are in hard to get to places.

This is your best bet in finding the problem like Mike said above... And if the Airbag/SRS system is shorted you will have an Airbag lamp flashing a code indicating so. Dont go messing with that system if you have no airbag light on...It has an Airbag Diagnostic Monitor for a reason...
 
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The SRS airbag light remains on, with the engine running. Can I scan for codes, with my Actron CP9190 scanner(I'm away from it and cannot read the instructions or FSM).

I know SRS is nothing to "monkey" with!
 
It should flash a code when started then remain on. That flash code is what you need to go by. I Could be a crash sensor, safing sensor, clock spring, driver or passenger module (Bags themselves), or the the Airbag Diagnostic Monitor has blown its thermal fuse. That later is usually the case....You will need to retrieve the code and use the Ford Shop Manual to go from there, with the pinpoint test. Otherwise its a guessing game.
 
Unplug the Airbag Diagnostic monitor connectors (2 of them, one black, one gray)and replace your run fuse and see what happens( Im betting it will still blow the run fuse). Blue Module about 7"x4" located in the passenger side dash/kick panel area. Dont quote me on the location its been a while since I messed with a MN-12 Airbag issue....Meaning it could be on the driverside...If you have an EVTM look it up in the back of it on the Item location charts before you go searching....If it does still blow I suspect you have other issues and just happen to have an Airbag system fault as well. Unless it is shorted to B+....
 
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Thanks guys. I located the Airbag (SRS) module, pass kick panel. With it connected, there is NO codes flashing because the RUN fuse blows, as soon as ignition is turned on.

When I disconnect the GRAY connector, RUN fuse does NOT blow, SRS light glows STEADY.

I just finished a 12 hour night shift, off to bed.
 
Short in 5 amp "RUN" curcuit solved!

PROBLEM SOLVED! I hate treads that OP's don't advise outcome!

I removed the SRS module and examined it. I saw a burned capicator. I took it to my radio guy (Daves Radio, Santa Rosa, CA).

I have had lots of radios/decks/amps, from my cars, fixed by Dave over the years. He soldered in a new capacitor (free) .

I re-installed the module. SRS self tests good, short in "RUN" curcuit gone!!
 
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same problem

My 5a fuse kept popping so I disconnected the control module and took it to an electronic expert . He thinks that it is a problem is with the MOV cap and not a burnt fuse. As seen in the picture attached. Does anyone know what the part number is is on that Mov cap as it isn't legible anymore. Thanks.


I can't find the solution anywhere are the net.
 

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My 5a fuse kept popping so I disconnected the control module and took it to an electronic expert . He thinks that it is a problem is with the MOV cap and not a burnt fuse. As seen in the picture attached. Does anyone know what the part number is is on that Mov cap as it isn't legible anymore. Thanks.


I can't find the solution anywhere are the net.

Found a very detailed write-up on capacitor replacement for that type of computer. However, the author focused on the electrolytic caps.

http://blog.kf7lze.net/2012/08/13/1991-mazda-miata-air-bag-diagnostic-computer-repair-cars/

And a little more discussion:
http://forum.miata.net/vb/showthread.php?t=472516

I also found out that the thing labeled MOV1 is not a capacitor per se ... it's a "metal oxide varistor", and it serves a function of transient overvoltage protection.

Here is a less detailed write-up by someone with a Ford truck that actually had a failed varistor. He tells you how to test if it is bad. He also gives a general recommendation of what to buy as a replacement, but nothing exact.

http://www.ford-trucks.com/forums/602583-airbag-diagnostic-control-module.html

I am not sure if you can even measure the proper value from a good varistor. I am not too knowledgeable about this type of component. Are there any markings at all on the varistor in your module, even faded ones?

One thing I do know is that I would advise replacing all the electrolytic caps in there while you have it out. They are all pretty much time bombs.

I would also suggest inspecting the thermal fuse and mounting pads (if they exist on your version of the board) as suggested on the electronics blog post about the Miata.

Edit:
I also found this page, which explains a little better what a MOV does, and gives a few examples of a typical circuit.
http://www.industrial-electronics.c..._Surge_Protection_for_Rectifier_Circuits.html
 
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Burnt out MOV varistor

Thank you very much for your reply. It is one of the reasons why I'm still driving this car is because of help I receive from people such as yourself.

I haven't done anything as yet after taking the module out of the car . The part number of the Mov has a TDK part number on it which isn't very legible .

I'll followup as soon as i find the fix .

Thanks

John
 
I'll be very surprised if the MOV1 is the problem, I'd put my money on the transistor/voltage regulator (with the heat sinc) or a diode on the board ---the MOV is used more to account for voltage differences in the cars charging system.....Dan
 
I'll be very surprised if the MOV1 is the problem, I'd put my money on the transistor/voltage regulator (with the heat sinc) or a diode on the board ---the MOV is used more to account for voltage differences in the cars charging system.....Dan

As I said, I don't know much about varistors, but it's clear that something in that module is a dead short or close to it. So I am thinking it will be easy enough to check as long as you're ready to take the plunge and start desoldering leads from the board.
 
Since everyone likes a happy ending , it was the Mov - varistor. It cost under $4 .My electronics guy put it in and so far so good.

Thanks for your help guys.

( now to find out why the car stereo LCD has gone blank)
 

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