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  • dietz_diggler
    started a topic ammeter bypass?

    ammeter bypass?

    Looking under the dash I see the ammeter is part of the circuit board. Its got a bad spot coming off it and my alternator isn't charging. I'm wanting to by pass it but not sure what wires I should be looking at. Any ideas?
    (76 doba)

  • 440roadrunner
    replied
    Yep the vehicles with an external shunt ammeter had much less current going through the bulkhead. If you've melted your ammeter connections, I'd say you have a wiring problem either out under the hood or in the bulkhead connector. "Something" came loose and "tried" to make the ammeter carry all the current.


    The big problem with external shunt ammeters, is that they were not very well engineered, either, mostly. Most Fords and some Chryslers were very "numb." That is, you could jump in the car, and turn on the lights, and you could not tell if the stupid needle actually moved, or not

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  • dietz_diggler
    replied
    Originally posted by DynoDave View Post
    This whole post was an interesting read.

    So are we saying that later Bs EQUIPPED WITH A SHUNT are not susceptible to having the bulkhead connector melt at the ammeter terminal (and risk burning the car to the ground), as we used to see in the earlier cars (and as seen in this MAD article)?

    I can certainly see where the design (including a shunt) would HAVE TO BE different once the late Bs made the jump to a flexible printed circuit connection for the guages, including the ammeter. There's no way you're flowing the whole current for the car through that paper thin copper strip on the printed circuit, and through a few small contact points on a copper clip to the threads on the guage.
    This makes sense. The printed circuit is melted/burned at the ammeter, and my alternator won't charge and I haven't been able to find the problem yet and thought, (rather hoped) this could be the problem. I'll have to keep looking then. One thing though, the trucks (ie. 1977) still had the ammeter melting problem. Apparently the late B's got an upgrade they didn't.
    Last edited by dietz_diggler; 12-11-2010, 08:30 AM.

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  • DynoDave
    replied
    Originally posted by 440roadrunner View Post
    OK here's the thing----that Mad diagram has nothing to do with your car. Your car uses a different approach to the ammeter, known as "external shunt". The meter in your dash is NOT an ammeter at all, but a more sensitive meter movement. It actually uses part of the in-harness wiring as the shunt. If you go back to the first reference I made, the diagram shows "shunt" in the engine bay area. This is not actually a shunt in so far as a separate part, but rather, the harness itself

    If someone could post a REAL diagram---I.E. the one out of the shop manual, I could help explain better how this works.

    I cropped a bit out of this part of the diagram:

    http://www.mymopar.com/downloads/1976/76CordobaA.JPG

    see the wire marked "shunt" on the left side? That is actually just part of the wiring, and is "tapped into" so to speak by the "ammeter" connection wires.




    To put it another way, your "ammeter" uses very small wiring, and the meter is actually just reading a portion of the current.

    And---to put it YET another way, you could cut the wires to your "ammeter" and the car would continue to operate completely normally--except that the meter would not indicate.

    The MAD diagram is referring to the older cars which use a "full current" ammeter and are easily recognized by having very large conductors --no10 or so-- rather than small no 18-16 or so as your car uses


    This whole post was an interesting read.

    So are we saying that later Bs EQUIPPED WITH A SHUNT are not susceptible to having the bulkhead connector melt at the ammeter terminal (and risk burning the car to the ground), as we used to see in the earlier cars (and as seen in this MAD article)?

    I can certainly see where the design (including a shunt) would HAVE TO BE different once the late Bs made the jump to a flexible printed circuit connection for the guages, including the ammeter. There's no way you're flowing the whole current for the car through that paper thin copper strip on the printed circuit, and through a few small contact points on a copper clip to the threads on the guage.

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  • 440roadrunner
    replied
    OK here's the thing----that Mad diagram has nothing to do with your car. Your car uses a different approach to the ammeter, known as "external shunt". The meter in your dash is NOT an ammeter at all, but a more sensitive meter movement. It actually uses part of the in-harness wiring as the shunt. If you go back to the first reference I made, the diagram shows "shunt" in the engine bay area. This is not actually a shunt in so far as a separate part, but rather, the harness itself

    If someone could post a REAL diagram---I.E. the one out of the shop manual, I could help explain better how this works.

    I cropped a bit out of this part of the diagram:

    http://www.mymopar.com/downloads/1976/76CordobaA.JPG

    see the wire marked "shunt" on the left side? That is actually just part of the wiring, and is "tapped into" so to speak by the "ammeter" connection wires.




    To put it another way, your "ammeter" uses very small wiring, and the meter is actually just reading a portion of the current.

    And---to put it YET another way, you could cut the wires to your "ammeter" and the car would continue to operate completely normally--except that the meter would not indicate.

    The MAD diagram is referring to the older cars which use a "full current" ammeter and are easily recognized by having very large conductors --no10 or so-- rather than small no 18-16 or so as your car uses
    Last edited by 440roadrunner; 12-10-2010, 04:06 PM.

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  • dietz_diggler
    replied
    Originally posted by 440roadrunner View Post
    Red--hot to start relay ammeter?

    Black--to fuse link ammeter?
    Thats some great information 440roadrunner. heres the diagrams I'll be using from the same site O-D posted. this is the way its wired and the next is the way it should be re-wired.





    One other question I have concerns the 2nd diagram. Where the red and black come out of the "firewall bulkhead connector" they appear to come together to the new 16 gauge fusible link. I'm thinking the red should go to this and the black to ground? Am I wrong? They are using black boxes for splice symbols, so it could be all three are spliced together. I'm not sure really, and if so, then I don't quite understand.
    Last edited by dietz_diggler; 12-10-2010, 03:13 PM.

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  • 440roadrunner
    replied
    Some of the traces on the board MAY be underneath. There's only so many places the dash cluster connections can go----- in round numbers you have

    a ground

    high beam indicator

    left turn indicator

    right turn indicator

    dash lamps

    switched 12V to instrument regulator/ and to fuel/ temp/ oil gauges

    sender to fuel gauge

    sender to temp ga

    sender to oil light/ ga

    ammeter 1

    ammeter 2

    ==============================================
    So on the top row of the 12 way dash connector you have:

    Dark Blu--to fuel sender

    Violet--to temp sender

    Dk green--to speed control?

    Red--hot to start relay ammeter?

    Black--to fuse link ammeter?

    Bottom row of 12 way connector:

    Gray--to oil pressure sender

    Dk Blu--appears to be switched 12V from ign switch

    Dk blu/Red--says SB interlock, must be lamp/ buzzer

    Gray--not sure seems to be fused hot 12V

    Black--seems to be brake warning lamp

    Smaller connector: (6 way)

    Top

    Orange--must be dash lamps through dash dimmer

    Tan--left turn indicators

    Bottom:

    Red--high beam ind.

    Black--ground

    Lt green--rt turn indicator
    Last edited by 440roadrunner; 12-10-2010, 03:00 AM.

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  • dietz_diggler
    replied
    Originally posted by Old_Demon View Post
    Thats the upgrade I want to do, but the back of the gauges have no wires, including the ammeter, its part of the circuit board. I mean theres posts coming out of the back but no wires all the posts have copper strips going over them but not connecting them or running to them. I'll look at the diagrams provided and try to figure what wires in the boards plug in, to jump. Anyone done this on a late B ?

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  • 440roadrunner
    replied
    Originally posted by Old_Demon View Post
    You sure his '76 doesn't have an "external shunt" ammeter?

    According to these sometimes poor diagrams:

    http://www.mymopar.com/downloads/1976/76CordobaA.JPG

    http://www.mymopar.com/downloads/1976/76CordobaB.JPG

    These indicate "around the bend" that he has an external shunt ammeter

    On schematic B, there is no separate ammeter shown

    On schematic A, if you find the black at the top of the start relay, follow it to the left and back down, you'll see it's labeled "shunt." This "shunt" is actually just part of the wiring harness, I.E. the ammeter---a sensitive instrument rather than the old "full current" type, is now sensing the voltage drop across a section of the wiring harness.

    If you were to take your dash cluster, and follow the ammeter traces off to the plug in connector, you should be able to identify their wire colors, and ALSO notice that these wires are very small--along the lines of #18-16 or so.

    From the given diagram (I don't have a shop manual for these years) it's difficult to identify just where the ammeter circuit ties in. It USED to tie in via an "in harness" shunt. It is possible that this "in harness" shunt has become loose and allowed the ammeter to attempt to become the "main path" for the current, thus causing the mentioned damage.

    The areas I would examine would be:

    Carefully remove all the bulkhead plug sections, and look for corroded, rusty, overheated connectors and wiring.

    Identify the wire colors of the ammeter---even if you have to find a "real" Mopar diagram--and untape the harness to inspect the in-harness splices

    (Over at Moparts a member JUST NOW found a "no charge" similar problem---the alternator was popping light bulbs [he said 30 bucks worth] because an in-harness splice in the dash harness had failed.)

    I've found at least 3 of these over the years, "back in the day."

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  • Old_Demon
    replied
    Catalog

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