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MG TD TF 1500 - A4 fuse keeps blowing today

Well I thought I had the electric licked but I guess not. Everything worked great yesterday, including turn signals. Today I added some gas to tank, do a few cleanup things in wiring at dash, nothing major and nothing connected to A4. Now all of a sudden the fuse at A4 keeps blowing as soon as I turn ignition key on! As far as I can figure it none of the green wires at A4 should even be drawing any current at this time because:

Low fuel wire: taped off under dash because my indicator doesn't work.
Wiper motor wire: wire not even installed yet.
Brake switch wire: worked great yesterday when brake pedal pushed!
Turn signal switch wire: worked great yesterday!

I've changed NOTHING regarding any green wires so why is A4 blowing?

Help (again)!
Ed

efh Haskell

I should mention they are 30 amp fuses.
efh Haskell

More info: I removed all 4 green wires at A4 and added them back one at a time until I blew the fuse (again). It was #17, green, that goes to the flasher can itself that causes the fuse to blow! (Odd because I thought I tested this all yesterday after Thoralf in Norway solved my problem.) My flasher can's bracket is mounted directly on the firewall hence it would be grounded I guess. WSM figure N.23 does NOT show a ground needed there however. Maybe (not sure) yesterday I just had the can dangling from it's wires hence NOT grounded hence no blown fuse?? Should the can be somehow insulated from it's bracket since it is made of metal? If so, how to do?

Hopefully,
Ed
efh Haskell

Ed,

Today, see if you blow the fuse while the can is suspended. If you don't, and blow it as soon as you touch it to something, replace the can with one that doesn't need isolation to mount. Sure as shootin' the insulation will wear and you'll have problems again. I recommend the NAPA EL-13 flasher. It doesn't look stock, but it works well.

warmly,
dave
Dave Braun

Thanks Dave, good idea and I will try. However, I just noticed something looking N.23 diagram. Wire #66 (light green) sends current to a "direction indicator warning light" which, according to the diagram, is ITSELF grounded to the instrument panel! Guess what, I don't even own such a light - never have - but I've always had flashers and the very same flasher can that's now shoring out and has always been mounted to the firewall. Now I'm really confused??!! Should I just remove #66 from the "P" terminal on the can?
efh Haskell

Yet more...I just tried it with the can suspended and it STILL blows the fuse! Same can I've been using for years. It is about 1" long. Mared "Tridon F550 DOT" from Napa. I also have a new Napa "DOT 550 32CP-MAX6 15310" which is 1.5" long. My Napa didn't offer a "EL-13" Dave.
Still confused,
Ed
efh Haskell

Just tried the longer Napa 550 can and that one blows the fuse as well! So it's not the can per se I don't think. I also removed the light green #66 from the "P" terminal on the can just for "fun" since I don't own the indicator light. Well, the fuse didn't blow but it was twiching maybe like right before it blows so I aborted.

I also checked resistance in the green #17 wire from A4 to the can. It measured between .2 and .4 ohms, similar to other green wires so the wire itself seems intack I guess?

Maybe these cars weren't really mean to have turn signals. Another day totally wasted...

Depressed in CO,
Ed
efh Haskell

You have a dead short somewhere in the circuit...
Since you know which circuit blew the fuse, you will need to track down all connections to it...Somewhere, you probably have (-) going to ground, instead of (+).
Of course I am assuming you have Pos ground.
Look for a wire that should be "hot" going to ground, as in one of the bulb connections.
Edward
E.B. Wesson

Edward,
I've removed EVERYTHING else in the A4 fuse circuit! I removed all 4 green wires at A4 and added them back one at a time until I blew the fuse (again). It was #17, green, that goes to the flasher can itself that causes the fuse to blow! I can duplicate this over and over again.
I also just installed a brand new #17 wire to rule out a short in that wire. Still blows the fuse! Are you saying the short is in another part of the wiring system totally (i.e. headlights, panel lights, etc.)? But all that stuff still works! How could that be?

I think several beers will fix this. I'm fried and I now HATE electrics almost as much as I hate clear coat.

Happy father's day to all,
Ed

efh Haskell

Ed,

Read this note on flashers:
http://www.mgaguru.com/mgtech/electric/et104.htm

Then, look over this diagram:
http://www.mgaguru.com/mgtech/electric/circ_f2.htm

Which shows how they did it on the MGA 1500 which is indentical to the TD/TF except they took the power for the flasher and the power for the Turn Signal Switch from different locations... doesn't matter though because both locations come from fuse A4 eventually.

Basically, the flasher is 'off' normally, and doesn't see power until it sees a load. It should not be grounded. My flasher can that I used for years also gave up the ghost when I reinstalled it on restoration. The current 550 cans may be normally 'on' which means the panel light will flash when the turn signals are off and vice versa. That's why I recommended the EL-13, it is normally 'off'.

Your problem my be in the relay box. It might be causeing a short to terminal 1 and with no load, it may short the fuse.

warmly,
dave
Dave Braun

Dave (et al),
I just removed almost all remaining electrical components from the dash plus the fuel pump. I also removed the metal cover on the relay box because it touches the "roll bar" behind the firewall. (Should this cause a short?? I have a solid state relay box by the way.)

I then hooked up a new wire from A4 after the fuse to flasher can "X" terminal (poor quality photo att but the green wire goes from A4 down to the can at "X"). No wires to P or L terminals on can. That's it! Nothing else has juice to it at this point except the starter motor.

I turned battery then ignition switch on and lo and behold the fuse does NOT blow! I confirm I have juice at X using a test light. I have no juice at L but the P terminal produces a very low glow from test light. I guess it's supposed to do this.

Now what? I guess I'll just add the L wire to it and see what happens...tomarrow. Sound like a plan?

Ed
PS: those are reflections on the paint not scratches!

efh Haskell

Ed - If the fuses in the picture are 30 amps, they are the wrong size. British fuses are rated at 17/35 amps, which means that they hold for 17 amps and blow at 35 amps - anywhere in between those two current levels, the fuse will blow after a time delay, which is very long at 18 amps and very short at 34 amps. American fuses have only one rating, the maximum current at which they will not blow - thus at 15 amps it will not blow. At 16 amps it will blow after a very long time delay up to 200% of rated value - 30 amps will cause the fuse to blow almost instantly. If the fuses in your fuse holder are American fuses, they should be 15 amps. Cheers - Dave
David DuBois

Hello Ed,
in your first post you mention:"Today I added some gas to tank, do a few cleanup things in wiring at dash"
If these are really the only two issues that changed your luckyness sooo dramatically, than I would start here.
Adding fuel means that the tanksensor will do its switching. I would disconnect the wire from the tanksensor and see if it makes a difference.
Clean up things?? There is a lot of wires and hot points around behind the dash and I would take a very close look there to see if maybe one wire got loose from its contactscrew or maybe in some way a false connection can be made there.
And I am sure that after a while you will find it and solve it and enjoy the beers again. Good luck, Huib
Huib Bruijstens

David, the fuses in the photo are currently 25 amp. Normally I run 25 amps in A2 and 30 in A4 but I burned up all my 30's doing this test so I'm using some 25's in both because that's all I can find locally today. The specs I've used for these choices are based on many old posts in archives like the one from Don Harmer below. Search for "fuse sizes" and you will find it:
.
.
=========== quote from archives =============
British 50 amp = American AGC 30 A3 - A4 (Horns)
British 35 amp = American AGC 25 A1 - A2 (Ignition Switched)
British 30 amp = American AGC 20
British 25 amp = American AGC 15
British 20 = AGC 10
10 = AGC 7.5
5 = AGC 3
etc.

British fuses are rated at their "Blow Point" while American are rated at their carrying capacity.
========= end of quote ===============

Why do you say 15 amps s/b used?

Huib, thanks for the input. I've already tried all those things however...sigh.

Ed


efh Haskell

Sunday: I start to add back stuff and the fuse still is NOT blowing! Using a test light I even see it blinks at #1 on relay box like it should! I then snap the metal cover back on the relay box and try again. Immediately the fuse starts to flicker & tick so I abort. How can the metal box be shorting the circuit out?????
efh Haskell

So, If I am understanding you, the cover is acting as short to ground....
Look very carefully at tne base of the regulator, and at the top inside of the cover....
Is it possible that one of the regulator relay parts , are touching the housing?
If so, there might be a tell-tale soot mark, where it has arced...
Also check to see, if one of the wire tabs (connections), is touching the base of the cover, somehow.
Good luck
Edward
E.B. Wesson

Edward, by "regulator" I assume you mean the signal relay box, right? That's the box I'm talking about, not the voltage regulator. I see no arching in it and it has no moving parts inside. It appears to be solid state. Just 2 little plug in modules inside the box. It's been fine forever, I just moved it from front of firewall to "correct" position inside. Wish I hadn't bothered! Impossible to access now. Who knows....
efh Haskell

Hopefully this picture will get me moving. As you can see the metal cover touches ALL 8 terminals on this relay box. That's how it's built. So all 8 contacts are always connected via the cover? How the @#$#@# can that possibly work? With this cover off I get no shorts. As soon as I put it back on I immediatly blow fuse A4. HUH????

Note that the cover is also touching the roll bar. That's how it fits. How could anyone build a device that allows raw voltage to touch ground!!?? Even if I moved the box somewhere else it would seem the cover still would short out all 8 contacts?

I've gotta be missing the obvious here. I apologize for wasting so much time/space with this post but I'm totally at a loss and our very short driving season will be gone before I can blink. Chances of snow tomarrow! ANY help would be greatly appreciated, as always.

Ed


efh Haskell

Ed, Seems to be a logical explanation to this problem. If that's the cover that came with the relay when new, then I'd say the fellow who designed the fastening system for the cover must have had his head in the sand. There's no way it should touch any of the terminals. Not knowing how it's secured, I can't offer a suggestion, but there must be a way to move it up a little and secure it away from touching any terminals. Is that the original cover, or possibly off of another relay? PJ
Paul J

Here is an old photo of this box in action before restoration began. As you can see the metal cover clearly touches the terminals! Yet this thing has worked for years and years and never blown a fuse! How is that possible??? I should mention the cover is "keyed". It's impossible to install it upside down. Someone please help! Below is another photo of the inside. Maybe that will somehow help.


efh Haskell

inside...

efh Haskell

Ed, mine has a thin strip of micarta coming out of the base plate just enough to ride under the cover and insulate. The cover is also notched so as to provide some clearance.
Mike

Mike Hart (52 TD 16378)

Picture of the cover. Mike

Mike Hart (52 TD 16378)

Ed,

Per my last post "Your problem my (may- sic) be in the relay box. It might be causing a short to terminal 1 and with no load, it may short the fuse."

In an original relay, the cover should rest on the phenolic sheet that the relay is assembled on. The contacts should emerge from beneath the sheet.

Here is a picture of how the Moss Relay is supposed to look with its cover. Notice how deeply the contacts are located away from the cover relief.

Install some wood insulators OVER the contact terminals so the cover does not short out the contacts. I'm thinking popsicle stick strips fashioned to provide some insulation and then glued in place. OR trim up the metal of the cover so it can't touch the contact terminals.


warmly,
dave

Dave Braun

Ed
That is your problem...You are shorting all of those circuits to ground, and to each other, with that cover...You are missing an insulator....You can make one out of plastic, and/or cut the cover so it cannot touch the tabs.
Edward
E.B. Wesson

Ed,

In the pictures you just posted I don't think in your original installation the terminals were shorting to the cover... they were close, but not touching

Dave,
from 900 miles away, peering at a low rez photo...
Dave Braun

Hi Ed,

PJ and others are dead right - the cover shouldn't touch the terminals! Although the lettering is indistinct, it appears to be a Lucas original - their name isn't always on the component. I can make out DB 10 & 33117 all of which is correct but the build date is hard to read. On the original there is a gap twixt cover and terminals, as well as insulation -paxoline I think. It appears someone in the past has made and supplied a new base and not thought too much about the problems of using the old cover. The cover snaps on as you know - shouldn't be difficult to move it up slightly with four very small corner wedges, or an insert on top, to keep it there.

Hope this helps,

Cheers,

John
J C Mitchell

Okay guys, thanks, this has gotta be the problem! I'm going to cut the relief area higher on the cover. Easiest solution as we speak.

Is it okay for the cover to touch the roll bar or not? (Hopefully my final question!) Dave, your's looks like it's touching on your web site, no? It s/b ok if the cover it 100% insulated from the tabs.

Ed

efh Haskell

Ed,

My cover just clears the cowl stiffener bar. If the cover touches the bar, not a problem, as you say if it isn't touching the contact terminals.

warmly,
dave
Dave Braun

Cover has been filed down 1/8", installed, and fuse did NOT blow!

Thank you all very much especially on father's day for your time!!!!! (Cindy thanks you too!)

What's next (sound familiar)?
Ed
efh Haskell

Ed - "Why do you say 15 amps s/b used?"

I am not saying to use a slow blow fuse. I am talking about either a 3AG fast acting fuse or a ACG fast acting fuse.

"British fuses are rated at their "Blow Point" while American are rated at their carrying capacity."

This is true if you look at the second number in the British rating (in the case of the fuses used in the A1 - A2 slot of the TD a 17/35 amp rating) the first number is the carrying capacity and the second number is the blow rating. This means that the proper British fuse will carry 17 amps and blow (nearly instantaneously) at 35 amps. To determine the carry/blow rating of an American fuse, one has to go to the spec sheets for the 3AG or AGC fast acting fuse. If you look at the spec sheets that I have attached to this post, you will see, in the Electrical Characteristics box, that the rated current is in fact the carry capacity and the blow point is rated at 135% and 200%. For a 25 amp 3AG or AGC fuse, the blow point and time at 135% is 33.75 amps is 1 hour and at 200% the blow point and time is 50 amps at 10 seconds (for a 3AG fuse) and (2 minutes for a AGC fuse). For a 30 amp 3AG or AGC fuse, the blow point and time at 135% is 40.5 amps at 1 hour and at 200% the blow point and time is 60 amps at 10 seconds (for a 3AG fuse) and 2 minutes for the AGC fues). Finally, for a 15 amp 3AG or AGC fues, the blow point and time at 135% is 20.25 amps at 1 hour and at 200% is 30 amps at 10 seconds (for a 3AG fuse) and 2 minutes (for a AGC fuse). For an American fuse substitute for a British 17/35 fuse substitute, either a 15 or 20 amp fuse should be used - 15 is a bit on the low side and 20 is a bit on the high side. A 25 amp fuse is, in my opinion definitely too high and my preference is to go to the low side with a 15 amp 3AG or an AGC fuse (although a 20 amp fuse would probably be a fairly safe choice. Cheers Dave






David DuBois

Intruiging to see how the final "criminal" escaped from analysis right in the beginning. What can we learn with respect to failure analysis from this event?
greetings, Huib
Huib Bruijstens

Huib,

***disclaimer*** I do not consider myself a sharp electrical troublehooter, so I could easily sympathize with Ed's predicament. But I have wired about four of these cars and I've seen a lot of things that have given me a perspective on two fuse MGs.

When Ed started having trouble, I first assumed a dead short to ground with the metal case of his flasher. When that proved not to be the case, I poured over the wiring diagram and decided that the only other likley location to short out the system was terminal 1 on the relay box.

So what can we learn? It is difficult to trouble shoot something over the internet. Ed's failure mode was placing the cover on the relay box which he also never consider important enough to report. I can't blame him, in the past he could place the cover on his relay box and nothing bad would happen. But it is indicative of being presented with failure modes and not being able to sort out the trivial from the consequential actions as evidenced by Ed as he covered many other trivial actions in his post(s).

In hindsight it might even have been easier to trouble shoot from a distance without being intimately involved because it prevented distraction by trivial changes (although I have to admit that half filling the fuel tank caught my eye... but then I rejected the possibility of a failure of the sender because the sender is a GROUND SIDE devise only.) In aviation we see a lot of accident reports where the problem was obvious to the armchair pilot but not to the pilot who rode his left seat all the way to disaster. I remember in my instrument flight training with being presented with so many issues and variables that my mind would shut down or fixate on the wrong issue.

Summing up, what we learned is to put down the wrench or ohm meter and back away from the problem and to approach it fresh after a day of mental perculation. Also, posting a few questions on a friendly forum usually helps. Congratulations to Ed for finding his conseqential issue when he found the cover shorting the terminals. Notice by this time though, he had lost a lot of confidence in his troublehooting. But in the end you could easily say that Ed solved his own problem. And for that I salute him.

warmly,
dave
Dave Braun

I'll tell you what I learned from this! When trying to find a short don't assume it has anything to do with the "wires" per se! In this case I removed all wires and added them back till I shorted. However, it was NOT the wire causing the short. It was something that was inadvertently touching that wire - the metal box covering the relays, which I have never had delt with at all! How did that happen? I don't have a frigin' clue and I guess I don't care because it's now fixed.

I may have "solved my own problem" Dave, but without you guys to air ideas with I would still be hacking away out there! Thanks once again. Headin' out to finally screw my new dash panel in place and move on.

Ed
efh Haskell

Wow...this is the exact problem I am having on my 59 mga. Reassembling after sitting in garage for 25 years...this blown fuse fiasco forced me down Ed's slow path to madness. Narrowed it down to the same green wire.Replacement flasher unit did not solve so have just left disconnected. No turn signals when not connected but no blown fuse either. Will have to try pulling relay box cover.

Either way this thread was enlightening.
cg
ccg calvin

cg, before you pull the cover look real close at those contacts to see if they are touching the cover, especially #1 because this is where the power comes in! I shaved 1/8" off the cover on a grinding wheel.

Glad you enjoyed the thread,
Ed
efh Haskell

This thread was discussed between 18/06/2011 and 24/06/2011

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