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Why is previously successfully used Window Relay setup now blowing fuses in Convertible Top setup?

Discussion in 'Automotive Electronics' started by John Salla, Apr 28, 2015.

  1. John Salla

    John Salla New Member

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    To those of you who help me with my window relay setup, I hope that you will please help me here, and to those of you who did not, I also ask that you please help me here. To recap, I used relays to operate the windows in my 1987 Jaguar XJS. MikeML did a diagram of how the relays should be setup and used. That was a winner setup. So I decided to use it again for my convertible top. My convertible top has a button like a window that makes it go up and down. I am attaching a diagram of the relay setup for the top. The top works when I press the button but after about two seconds it blows a 40 amp fuse. One of the reason relays are used is to reduce the amount of amperage draw by the load. I have read that a convertible top motor can draw up to 100 amps. I do not know how many amps my motor draws. But I started out with a 20 amp fuse and worked my way up to a 40 amp fuse and that 40 amp fuse is still blowing. Remember, the relays reduce the amp draw by the top motor. I do not know how one figures by how much. Now, I can keep putting higher fuses in the fuse holder, but before I did that, I wanted to touch base with people in the know. I knew my windows would draw no more than 15 amps. This top motor is an unknown. I don't want to one day be driving down the street and have a fire in my trunk. (The top motor is in the trunk.) This car has already suffered two fires. I nursed it back from the first one, but the second one destroyed its 12 cylinder engine and was a very costly event. So, please help me out here. Thanks so much.
    John
     

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  2. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    If the hydraulic motor windings have insuffient ESR, the plastic coated magnet wire may have melted and shorted.

    It s essential that you measure this resistance on the disconnected motor leads using a current limited source. Get any headlamp and connect to each motor wire and using a 10A DMM measure current 1~2A when connected in series to motor and then change to voltage in parallel at same conncts with respect chassis ground in fullscale then small scale. After,ensure plug disconnected supplies 12.5V to either pin with respect to chassis ground when selected. No need to run engine.

    If the stall current is 100A then expect resistance to be 0.14 Ohms and average impedance if motor is not seized in hydraulics or electrical motor coil to be 1 Ohm under typical load or 12A.

    Obviously it is seized by one or the other or some hidden latch .

    If the voltage is reversed for direction on each direction, this must be a floating return or common wire... Which makes more sense using a DPDT relay.
     
    Last edited: Apr 28, 2015
  3. shortbus=

    shortbus= Well-Known Member

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  4. dave

    Dave New Member

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  5. rfranzk

    rfranzk Member

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    Hi John,

    Based on the schematic for the convertible top the window setup will not work because it reverses the power and ground connections at the motor to achieve the reverse which will cause a dead short. The vehicle schematic shows no such polarity reversal which makes me think the motor has two windings or the power changes hydraulic valves or circuits to achieve the up and down. The pump does not reverse directions from what I can see. This looks like a very simple set up if the schematic you showed is it. To start with see if you can apply power and ground to either the yellow wire and chassis or orange wire and chassis and see if the pump operates. If it does the original setup should work without much effort.
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  7. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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  8. shortbus=

    shortbus= Well-Known Member

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    Another thing to consider. Are you using the bosch style relays to actually provide the motor its power? Don't know about Jaguar, but the older convertibles I've worked on used "tin can" style relays to switch the motor current. Don't think a bosch type has enough capacity. This is a link to what I mean, the type of relay used. Even though they call it a 'solenoid' it is a relay to the actual starter solenoid.
    http://www.amazon.com/Ford-SW3-Starter-Solenoid-Relay/dp/B004PKC1YO
     
  9. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Realy , ratings for Motors must be 8x DC rated resistive currents so if it uses 12A with a 20A fuse, you need a Relay rated for 100A

    Sometimes high torque motors need 10x rating. as it depends on L/R ratio and magnetics.
     
  10. shortbus=

    shortbus= Well-Known Member

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    Again don't know about Jaguar, but old Ford and GM cars had a motor the size of a engines starter motor to run the hydraulic pump.
     
  11. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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  12. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Chances are high that the insulation has melted and there is a short in the motor.

    Unless you can measure the resistance of the motor, or you cannot dissassemble the motor, we cannot help.

    The voltage drop of a measured known current is all you need to do.

    A 10 A source can be made with a 12 Ohm load from 12V. this will generate 120W of heat and could be anything like several parallel strands of heater wire from a toaster or flood lamps or anything which 12 Ohms and can run hot safely at 120W.
    Alternatively, you can measure current in a short duration with a homemade ammeter, or measure the resistance of the 40A fuse, but why bother thy will not be very stable as they get hot.

    If you dont have a 200A Ammeter , you can make one with flat brass or copper wide strip for low inductance and 1mΩ into a 200mV scale on a $10 DMM

    You can use a 5" length with clips on 3.0" span of AWG16 wire to get 1 mΩ. then use twisted phone wire going to the meter banana sockets at RIGHT ANGLEs to the high current to reduce EMI coupling.

    Then make a brief contact to the unfused line no more than 1/10th of a second to avoid further damage to motor windings.

    it would help to have a fast meter or one with peak hold or a scope with short leads <1" to the twisted pair.

    If you get more than 100mV or 100A then 12/100A = 1.2mOhm then the motor is toast with shorted windings and may be rewound by any motor repair shop.

    If you have jumper cables to make the contact briefly, that may work well.
     
    Last edited: May 16, 2015

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