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Thread: Overdrive Cuts Out when...

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    Overdrive Cuts Out when...

    ...I use my turn signal, or turn on my driving lights. My embedded voltmeter reads lower than I think it should while driving, but I cannot attest to its accuracy. Might this be a voltage regulator issue? I'm running a generator on a 100-4.

    Thanks in advance for thoughts...

    John

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    Re: Overdrive Cuts Out when...

    Is this a recent occurrence, or was overdrive working reliably before? Have you checked transmission fluid level and condition?

    It could be that the overdrive solenoid is failing. The solenoid initially takes a power-draw to energize, but then a minimum draw while engaged. Try a simple test in the garage without the engine running. Turn ignition "on", shift trans into 3 or 4 gear, then flip the overdrive switch to "on". You should hear a loud "click" as the solenoid engages on the transmission. You could then turn on any accessory to try and reproduce the cut-out issue. If the solenoid disengages, then you should hear another, yet softer, "click". Hooking up a test light to the electrics should help isolate an electrical problem. Check if the generator is charging between 13 to 14.5V engine running 1500 to 2000 RPMs. Check battery's reading at rest.

    Then check the solenoid for proper mechanical adjustment - the adjustment is easy but complicated because the transmission tunnel may need to be removed to gain access. See shop manual or try a forum search for further details and instruction. Good luck, GONZO

    GONZO

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    Jedi Warrior Bob McElwee's Avatar
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    Re: Overdrive Cuts Out when...

    Be careful if you have a Pertronix ignition of leaving the key on w/o engine running.
    Bob McElwee
    '62 AH Tri-Carb(sold)
    Miss BT

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    Re: Overdrive Cuts Out when...

    Thanks, Gonzo. I'll try your suggestion tomorrow.

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    Re: Overdrive Cuts Out when...

    Bob,

    Why? Out of curiousity, why the caution regarding Pertronix? (Which I do have, BTW...)

    Thanks,

    John

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    Jedi Warrior Bob McElwee's Avatar
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    Re: Overdrive Cuts Out when...

    Supposedly it will fry the unit. Several threads on it, one within the last week or so. I had a Crane unit in two different cars and never had a problem with either.
    Bob McElwee
    '62 AH Tri-Carb(sold)
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    Yoda Michael Oritt's Avatar
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    Re: Overdrive Cuts Out when...

    This may be a longshot but since there is an inverse relationship between voltage and amperage (amperage draw goes up as system voltage goes down and visa versa) perhaps the load of the turn-signal or driving lights is reducing your voltage and thereby increasing the amperage draw of the overdrive hold-closed relay to beyond its capacity, causing the OD to drop out.

    In any case you should address the voltage issue as it is fundamental to all segments of your car's electrical system.
    Last edited by Michael Oritt; 09-08-2019 at 12:52 PM.

    Best--Michael Oritt
    1954 Austin-Healey 100 Le Mans
    1958 Elva Courier (FOR SALE)
    1959 Elva MK IV Sports Racer
    1961 Ginetta G4

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    Re: Overdrive Cuts Out when...

    re: "... amperage draw goes up as system voltage goes down and visa versa ..."

    I think you're thinking of the Power Equation: Power (usually, in watts) = Voltage X Resistance, in which case yes, (available) power is reduced when voltage is reduced. In a simple DC circuit, Ohm's Law applies: E = I X R (E is voltage, and I is current in amps; used for historical reasons). Solving for I gives:

    I = E/R

    So, a smaller E produces a smaller I (current and resistance are SWAGS for the OD circuit): 1Amp current = 12V/12 Ohms; if voltage drops to 10, then about 0.8 amps is available (for a fixed resistance). The OD solenoid is an inductive load, but once impedance builds it becomes, in effect, a resistance circuit (hence why less current is needed to hold the plunger after retraction, and why a slow blow fuse is used in this circuit). The load on a car is parallel DC, and resistance decreases as load is added (the sum of the loads, inverted IIRC), so voltage drops and your lights dim because you are reducing resistance, more current is drawn and, since our batteries are 'constant current' supplies less voltage is available (the regulator will compensate to a point, but we know our generators are, uh, 'challenged'). If the solenoid gets less current due to less voltage, it will drop the plunger and deactivate the OD (but if your OD throttle switch is adjusted properly this won't happen until you open the throttles with a blip or downshift).

    Voltage is pressure, if your water pressure is reduced you get less water (current) out of your faucet or hose. Here's a cool calculator (enter 2 values, solves for the other two):

    http://www.sengpielaudio.com/calculator-ohm.htm

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    Yoda Michael Oritt's Avatar
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    Re: Overdrive Cuts Out when...

    Yeah, that--I think....

    Best--Michael Oritt
    1954 Austin-Healey 100 Le Mans
    1958 Elva Courier (FOR SALE)
    1959 Elva MK IV Sports Racer
    1961 Ginetta G4

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    Re: Overdrive Cuts Out when...

    Think of your household water supply: You have a fixed-size main to the city supply--or well, pump and tank in the country--that can supply X number of gallons per minute at a given pressure. Turn on one faucet and you get the full pressure and volume (limited by the size of the pipe and the faucet), but if you open all faucets in your house and garden the flow of water can be reduced to a trickle. Same thing happens with the circuits in your car; turn on all the lights and your OD solenoid gets less current (water), and the plunger drops.

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    Yoda Michael Oritt's Avatar
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    Re: Overdrive Cuts Out when...

    Bob--

    I am not sure whether you feel that lack of current/electrical energy might be the source of the problem. The reason I said "long shot" is that I have a hard time envisioning how the relatively small current draw of a turn signal could lower the power enough to cause the OD rly to kick out.

    Best--Michael Oritt
    1954 Austin-Healey 100 Le Mans
    1958 Elva Courier (FOR SALE)
    1959 Elva MK IV Sports Racer
    1961 Ginetta G4

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    Re: Overdrive Cuts Out when...

    I'm thinking a marginal OD solenoid that "lets go" when the voltage drops. Could also be an issue with the OD relay, though when mine failed the OD dropped out randomly.

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