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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2006Solenoid resistance too low -- damage car's computer?
A Toyota factory original fuel shutoff solenoid for a carburetor measures 90 ohms, but the aftermarket ones I've checked measured 30-35 ohms. Can this low resistance damage the car's computer by overloading the transistor that drives it? I can add a driving relay between the computer and solenoid for a lot less than what a factory solenoid would cost. <<I can add a driving relay between the computer and solenoid for a lot less than what a factory solenoid would cost. >> Or a transistor. Possibly a switching transistor. Or maybe the aftermarket relay has a diode across the coil to save it from spikes or to elimate interference to the original transistor/circuit. Bob AZ <<I can add a driving relay between the computer and solenoid for a lot less than what a factory solenoid would cost. >> Or a transistor. Possibly a switching transistor. Or maybe the aftermarket relay has a diode across the coil to save it from spikes or to elimate interference to the original transistor/circuit. Bob AZ > A Toyota factory original fuel shutoff solenoid for a carburetor > measures 90 ohms, but the aftermarket ones I've checked measured 30-35 > ohms. Can this low resistance damage the car's computer by overloading > the transistor that drives it? I can add a driving relay between the > computer and solenoid for a lot less than what a factory solenoid would > cost. JUST BUY A NEW CAR. > A Toyota factory original fuel shutoff solenoid for a carburetor > measures 90 ohms, but the aftermarket ones I've checked measured 30-35 > ohms. Can this low resistance damage the car's computer by overloading > the transistor that drives it? I can add a driving relay between the > computer and solenoid for a lot less than what a factory solenoid would > cost. The simplest and most robust fix would be to put a 60 ohm resistor in series with the solenoid coil and see if you get enough current from the computer drivers to activate it reliably. You might not, (and then again you just might.) Some solenoids work over a fair range of voltage and current. If that doesn't work, I would use the factory computer as a driver for an outrigger relay. As another poster mentions, you could also do it with a transistor. > A Toyota factory original fuel shutoff solenoid for a carburetor > measures 90 ohms, but the aftermarket ones I've checked measured 30-35 > ohms. Can this low resistance damage the car's computer by overloading > the transistor that drives it? I can add a driving relay between the > computer and solenoid for a lot less than what a factory solenoid would > cost. Your CCM (Carburetor Control Module) (just wrote that cause it sounds funny) is a hearty unit, capable of much abuse. Secondly, I suspect you measured the resistance of the OEM part that has failed... that resistance value would be fairly meaningless. My apologies if you got that measurement from a new unit. 30-35 ohms is a fairly common spec for Toyota solenoids, I wouldn't be worried. Toyota MDT in MO > > A Toyota factory original fuel shutoff solenoid for a carburetor > > measures 90 ohms, but the aftermarket ones I've checked measured 30-35 [quoted text clipped - 9 lines] > > Toyota MDT in MO I checked the secondary (1-wire) fuel cut solenoid, and it measured the same 90 ohms. I was warned that coils can fool digital meters because they measure ohms by sending out pulses, but the resistance read the same with an analog meter. The replacement solenoid is a Neihoff (good? bad?) and doesn't seem to be built as well as the original.. Its threads were rough (I lapped them to keep aluminum bits from shaving off and getting into the carb), and the wires were insulated with common 105 Celcius vinyl that I'm sure will become rock-hard and crack in a year. I sleeved each one with high-temp heatshrink to reduce the chance of shorts from this. > I checked the secondary (1-wire) fuel cut solenoid, and it measured the > same 90 ohms. I was warned that coils can fool digital meters because > they measure ohms by sending out pulses, but the resistance read the > same with an analog meter. Resistance checking a coil with no current passing through it will yield meaningless results in some cases. It won't tell you if the coil only fails under a load. The part about "sending out pulses" is generic and IMO wrong. The DVOM passes a very small amount of current through it's leads and into the tested component and determines an ohm value based on known current and voltage. > The replacement solenoid is a Neihoff (good? bad?) and doesn't seem to > be built as well as the original.. Its threads were rough (I lapped > them to keep aluminum bits from shaving off and getting into the > carb), and the wires were insulated with common 105 Celcius vinyl that > I'm sure will become rock-hard and crack in a year. I sleeved each one > with high-temp heatshrink to reduce the chance of shorts from this. Neihoff is decent in general. Typical aftermarket shop stuff that in most cases would exceed Autozone products. I don't think you have anything to worry about with using it. I've always tested and reused the original solenoids on Toyotas. I've never found a bad one during a rebuild. The heatshrink was a good idea. Toyota MDT in MO > > The replacement solenoid is a Neihoff (good? bad?) and doesn't seem to > > be built as well as the original.. > Neihoff is decent in general. Typical aftermarket shop stuff that in > most cases would exceed Autozone products. I don't think you have > anything to worry about with using it. I've always tested and reused > the original solenoids on Toyotas. I've never found a bad one during a > rebuild. Thanks for the reassuring information about Neihoff. The original Toyota solenoid leaked badly and didn't become better when suberged its tip in various substances and pulsed it on and off in an effort to clean it. I found the bad solonoid only because I took apart the carb to fix a no-idle problem caused by rubber bits shed from the fuel hose, and that occurred because several years ago I had installed an aftermarket carb rebuild kit whose float valve lacked the fuel inlet filter screen found on the factory float valve. Why did I install that kit? Because I had a rolling idle and was told that turning the mixture screw 1/8 - 1/4 turn would fix it, but the screw was hidden by a plug that required removing the carb to drill it out. So I thought, as long as I have the carb off the car, I may as well do some preventative maintenance. Not doing that preventative maintenance would have prevented the no-idle problem. :( > A Toyota factory original fuel shutoff solenoid for a carburetor > measures 90 ohms, but the aftermarket ones I've checked measured 30-35 > ohms. Can this low resistance damage the car's computer by overloading > the transistor that drives it? I can add a driving relay between the > computer and solenoid for a lot less than what a factory solenoid would > cost. If its controlled by a transistor then I wouldn't worry as most of those will have current limiting and heat sensing built in nowadays. If its a fuel pump I would be more concerned as sometimes these are PWMed, but a shut off solenoid I don't think I would worry about.  SignatureThank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 30 ohms across 12 volts will draw about 0.4 amps. Your typical transistor, when fully on, drops about 0.4 volts. So the transistor will be dissipating about 0.16 of a watt, not enough to cause any major heating, unless it's a micropower signal transistor, which would be a ridiculous design choice. I wouldnt worry about it.
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