Relay help

Discussion in 'Technical' started by 7D2 Grabber, Dec 27, 2016.

  1. 7D2 Grabber

    7D2 Grabber Member

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    I'm not too familiar with relays so I need some help. I'm trying to setup an MSD box and electric fan with switched 12v. I have regular 5 pin Bosch type relays setup in line. I believe I have them set up properly but I'm not very good with electrical. I have constant 12V to the #30, MSD box to #87, ground to #85 and switched 12V to #86. I'm getting 12V to the switched post going out but only 10v to when the switched 12v from the ignition. The box does not like this for some reason and will not fire.

    I can clarify if needed, i know it reads convoluted. Thanks.
     
  2. bmcdaniel

    bmcdaniel Senile Member

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    I don't understand this sentence, "I'm getting 12V to the switched post going out but only 10v to when the switched 12v from the ignition".
    Usually an MSD box has two large gauge wires the go directly to the battery + & - and a smaller gauge wire that goes to the ignition switch. No relay is needed.
     
  3. 7D2 Grabber

    7D2 Grabber Member

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    I'm getting 12v from the ignition when the key is on the "on" position. As soon as I connect it to the relay I lose several volts from the same wire but I get 12v from the switched outlet post on the relay.
     
  4. 7D2 Grabber

    7D2 Grabber Member

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    I ended up having a bad starter solenoid on the apron. The "i" post had continuous 12V instead of switched. I swapped out the solenoid with an old one and the I post was back to switched. I was having issues with current being delivered after the Key was turned off so I was trying to add a relay to the MSD box since I couldn't find another switched 12V source.
     
  5. Bryant

    Bryant forgot more than learned

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    Dont use the I post on the starter relay to trigger anything. It comes from the ignition hot wire that went to the original coil which then came from the resistor wire from the ignition switch. It designed to bypass the resistor wire while cranking.
     
  6. 7D2 Grabber

    7D2 Grabber Member

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    So the I post is not a 12v switched source? I'm not using it to power my dist as I have an MSD dist and box and also a mini starter. There's not many switched 12v sources available, where do you normally draw from?
     
  7. Krazy Comet

    Krazy Comet Tom

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    It is but isn't... It's the feed from resistance wire in harness to ign coil... With load of coil, voltage will be 5-8v, and is jumped by solenoid to provide full cranking battery voltage to coil while starting... With aftermarket electronic ign systems the I terminal isn't used(in fact deleted from newer factory system)... If there is no load on the circuit, voltage will be same as battery and should be able to supply more than enough current to trigger a relay... I'm doing so in my Comet, battery voltage(engine off) at relay coil falls from 12.6 to 12.2v with relay energised...
     
  8. jtbshaw

    jtbshaw Member

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    I'm going to attempt to raise this from the dead....

    I'm in the same situation. I need a 12v switched lead to run a electric fuel pump, efan, and coil on my HEI distributor. I was told there is only one true 12v switched wire in the car and it was off of the igntion harness. Found that, tapped into that, ran a wire to engine compartment still tested 12v switched, fired the car up and it died shortly. Checked the voltage on the new wire ran, and it is 5-8v switched. Not firing my relay up for my pump. I don't understand why it was 12v then it changed.

    Car is a 71 Grabber, 302, C4 auto with column shift.

    Someone please help.
     
  9. bmcdaniel

    bmcdaniel Senile Member

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    You evidently have the coil resistance wire Krazy mentions above, not a "normal" wire to 12V. I found the wire you're asking about on my '70 right at the ignition switch connector. Don't remember the color now. I use it to turn on relays for my fan and ignition. There are schematic diagrams around here that will show what wire you need to use.
     
  10. Krazy Comet

    Krazy Comet Tom

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    The relay must be fed constant 12v from battery to pin 30... The original coil feed may be used to trigger(but nothing else) the relay via pin 86... Wire it exactly as the first poster describes, it should work...
     
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  11. jtbshaw

    jtbshaw Member

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    The one I found is light blue. As I mentioned, the puzzling part is once its loaded, along with every other wire I have found that has a + voltage switch, drops to 5-8v when loaded with a draw. I see why it's been over 35 years since I've worked on these older cars.
     
  12. jtbshaw

    jtbshaw Member

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    This is exactly how I have the relays wired. The problem I'm having is the wire I was told and found off the ignition harness drops like all the others wires I've found to 5-8v when loaded. Is there a way around this?

    One other thing may be worth mentioning: the car was swapped over to electronic ignition by someone before we bought the car. There are many unconnected wires underneath the hood. Some are are switched, but most are not. Those that are also only have 5-8v. It doesn't appear that the harness has been cut due to the loose wiring I have found tied off in places.
     
  13. jtbshaw

    jtbshaw Member

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    All I need is a trigger wire to run three relays. Every wire that I find switched drops to 5-8v when loaded. Am I just looking for something that doesn't exist? Is there no way to make this work with three relays?
     
  14. Krazy Comet

    Krazy Comet Tom

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    Is you battery charged?? May seem like the obvious but I was working on signal lights for a friend that I knew were correct, turned out his battery was discharged... He'd started the car a couple hours prior...

    How much of a voltage drop are you getting with only one relay connected?? Check with only pins 85 & 86 connected, pin 30 & 87 disconnected...
     
  15. jtbshaw

    jtbshaw Member

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    Yes, battery is fully charged and showing 12v or slightly better. Charging system is working properly and pumping 13.4v when running. Battery is grounded to the block, which is grounded to the chassis in two separate places. All new wiring harness off the alternator (new also), new battery cables, new voltage regulator, new solenoid, all grounding cables are new and 2guage. Even added an extra 10guage ground wire directly to the battery from the ground post on the alternator (which didn't change anything, one way or another).

    I'll go check the voltage drop on a single relay here shortly and report back my findings.
     

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