Ignition switches keep failing.

Discussion in 'The Truck Stop' started by Jwilson2, Jan 13, 2020.


  1. Jwilson2

    Jwilson2 Farmall Cub

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    I have a 1978 Loadstar 1700 in excellent condition but the ignition switch keeps failing in Start mode. This is the 4th switch in a year. It works for a month or two then fails to engage the starter. Each time it fails the switch is found to be bad. I finally opened the last switch up and the only thing I noticed was the contact had minor pitting and some carbon deposits. Hard to believe that too much current could disable these contacts and not burn a wire up. But the ohmmeter showed no continuity when in start position though it did when it was new. The internal contacts are heavy duty. All accessories work and truck can be started with a remote switch connected between battery and solenoid. No sign of melting or wear on switch or wiring. Bought one switch from Autozone, and two from NAPA. They also tested the switch to verify it failed. All wiring in cab looks like new. No shorts found in the wiring. The solenoid was replaced after switch 2 failed so it isn't that. All the ignition switch does on this age truck is connect 12v to the solenoid signal terminal when key is turned to start. There are no safety switches. Wires inside the engine bay are in a harness, so no chance for shorting. There are no signs of shorting such as smell, hot wires, blown fuses, and it immediately stars by inserting a new switch. I cant think of what it could be. I ran a separate push button switch to the dash, ran it to the battery, directly to the solenoid and installed a 30 amp fuse and that has worked fine but not long enough to be 100% confident. Anyone else seen a problem like that? The Start line is only engaged a few seconds during startup, so the switch really fails quickly when you consider it fails after 20 or so starts.
     
    Last edited: Jan 13, 2020
  2. Dana Strong

    Dana Strong Lives in an IH Dealership

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    I would think a 10 amp fuse would be enough, although temporarily putting an ammeter in the circuit would give a definite answer of what the current actually is. In any case, I see no reason a good secondary switch would be a problem, assuming it's properly wired so that it only gets power when the key is in Run position.
    The only reason I could think of so many failures is if dirt or oil were somehow getting into the switch, something you would have known by now.
     
  3. winchested

    winchested High Wheeler

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    Have you checked all the wiring to make sure your contacts are clean at the starter solenoid etc.?
     
  4. Jwilson2

    Jwilson2 Farmall Cub

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  5. Jwilson2

    Jwilson2 Farmall Cub

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    The switches are very clean and inside was very clean. The way these switches work it is hard to believe thar the minor soot and pitting could interfere but apparently it does. You are probably right on amperage. I meant to say 20A fuse. I specd a few out and they said 20A with possible 30A inrush so I got a 20A switch, 12g wire, and went that route. For the original switch to last 40 years and then the next 3 failing in 1 year I was sure I would find an easy culprit. Here is a photo of the switch contacts. The center contact is the solenoid terminal. The minor soot and very minor pitting was on the center contact of the plate that slides over these contacts as the key turns. Gut instinct would say it was a defective switch, but having the old one fail and then getting three defective ones from two suppliers says otherwise. I doubt the push button could fail as fast due to me being able to apply pressure to the contacts by force but I hate mysteries like these.
     
    Last edited: Jan 13, 2020
  6. Jwilson2

    Jwilson2 Farmall Cub

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    Yes
     
  7. winchested

    winchested High Wheeler

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    I would think to cause the pitting you're seeing you've got high resistance somewhere in the circuit causing heat and pitting thus killing your switches.
     
  8. Dana Strong

    Dana Strong Lives in an IH Dealership

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    High resistance elsewhere in the circuit should cause low current flow, thus possibly not enough field buildup for the solenoid to work, but not arcing at the switch. Any heat would be at the high resistance point, not the switch.
    But... Suppose too little of the field in the solenoid is being converted to movement; perhaps somehow a momentary open is developing there during activation and that is backfeeding higher voltage to the switch in a similar fashion to how an old "ignitor" circuit works in early stationary engines. That system has a large coil is in series with the battery and points; when points close and current flows, current through the coil develops a strong field in it. The points are internal to the cylinder, and when they open, breaking the circuit and allowing the field to collapse (inducing a backcurrent with high voltage), an arc is developed across those points, firing the mixture.
    If that could be occurring here, the problem would be with the solenoid. They are fairly easily removed, so replacing it with a known good one might be worth the trouble to test the theory; if it solved the problem, dissecting the bad solenoid might lead to finding the specific cause of the problem.

    Another way to stop such a problem would be to put a capacitor in parallel with the switch contacts, but I have no idea how to choose the size or type needed for the job. Others here would, though. Can a switch that's been opened to see or clean the contacts be reasonably-well reassembled and used again? That might be a good candidate for such a capacitor test. If it lasted a long time with the capacitor, problem solved.

    BTW, Jwilson2; I think there should be an edit (and a Delete) button on the left, under your posts, so you should be able to modify all your posts, adding or deleting something, if you later decide to. [Or, does that feature depend on how new you are/how many posts you have?]l For example, your link in Post 5 doesn't work for me. You might also want to go to your Personal Details (which shows when hovering the cursor over your name on the top right) and adding your location so it shows in the box to the left of your posts. A member here might find you are a reasonably-close neighbor!
     
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  9. Jwilson2

    Jwilson2 Farmall Cub

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    Yes, the solenoid was the first suspect after the second switch failure. I put a brand new one on since it was fairly cheap. I agree, high resistance means low current but high localized heat. I found no evidence of localized heat like at the back of the switch. And a full blown short should cause widespread heat either blowing a fuse or melting the wire. That didn't occur either. It seems like a collapsing field could do it but I ruled that out with the new solenoid. After looking at it again I noticed the battery cable on the solenoid was very close to the signal line. That seemed to be the natural position for that cable due to its size and difficulty bending. So it may have been equally close on the other solenoid as well. I moved it just in case the starter coil collapsing was causing it to arc across over to the signal line and maybe that was causing the switch to arc as it was being disengaged. That is the only thing I can see that might cause it. I will report back on what I find out. A blocking diode on the signal line might protect it though it drops your voltage .6V to the solenoid which might hurt you in a low voltage situation. Thanks for the replies. My guess is the push button switch will not fail so easily.
     

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