What would the role of a thermistor be in the power door lock circuit of a car? It's in series with the door lock "motor", which might be a solenoid or maybe a rotating motor.
Is it possible that it is meant to lower or turn off power to the mechanism if the circuit is left closed for some reason and the solenoid would otherwise overheat?
The symbol it uses for a thermistor is a resistor symbol in an elongated circle, like a high school running track around the football field. Is that the usual symbol for a thermistor.
After 40 years of resistance, just bought a foreign car, a Toyota, and the shop manual is not nearly as clear as the ones I've had for Ford, GM, and Chrysler. Part of that is because the car is 5 years newer than the last one and so cars are more complicated, and some is because parts of it are in bad English, and other parts in incomprehensible English, but parts of it just don't include as much information.
FTR, the symbol in the Toyota manual looks a lot like the symbol for a light-dependent resistor at the URL below. Except the one in the Toyota manual has more rounded corners.
It might be a positive temperature coefficient thermistor, such as a "polyfuse". These have a very sharp PTC "knee", and can be used as self-resetting overload switches/fuses.
At a guess, this one might be selected/timed so that it would remain cool (closed) if the door lock is operated for a second or so, but would heat up enough over several seconds to "open" and disconnect the solenoid/motor thus protecting the motor from overheating or stripping a gear if somebody holds the door-lock button down continuously.
It could also protect the power circuitry from damage in the case of a short circuit at the motor/solenoid itself.
That would be my guess.
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Dave Platt AE6EO
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Yep, the shop manuals for Toyotas are almost idiot's guides. They give specific tests, but no clue as to how the system operates. I think Toyota wants you to pay for a course to find that information out. I've seen pieces of Toyota training publications and they are more generic, but do define how various systems function.
The Chevrolet guides I've used (2000) has a lot more design type info in their manual. A similar year Toyota is just different,
PTC thermisters are regularly used for battery pack protection and solenoid protection. They are way more reliable for this application.
Well, at the beginning of the the Toyota Electrical manaul, are two pages with 36 electrical symbols and what they mean, and for this circuit, the diagram uses a symbol which it says at the start of the manual represents a thermistor, but it's not the symbol everyone else uses.
Plus they use the same symbol** for the intake air temp sensor, the engine coolant sensor, the Air Conditioning room temp sensor, and the AC ambient temp sensor. . Might those really be thermistors too?
**a zigzag resistor symbol inside a circle that has been elongated in one direction, like a model train track cricle with extra straight track in opposite sides. .
I haven't taken apart the door yet, so I don't know what is actually there.
My plan was to add to the passenger door-unlock output of the door/burglar alarm Eloectronic Control Unit a wire to a relay, which relay would unlock the trunk. What's connected to that output now is the door lock/unlock motor (solenoid?) and this "thermistor", in series, according to the diagram, and I wanted to understand what is there now and what it does, before I start fiddling with the circuit. The other end of the door lock circuit goes to ground and so will the other end of my relay circuit.
I'm reticent because with the previous car and the previous alarm, that I installed from scratch, everything was fine, except to open the trunk, I had to hold down the third button for several seconds. Because my turnk lid didn't move when unlatched, I thought it would work better for me if I connected the trunk relay to the both-door unlock output of the alarm I installed, in parallel with the door-unlock relay. . However that appeared to screw everything up, so that many of the alarm functions no longer worked. I must have put too much drain on that output, even though I just doubled the drain (two standard automotive relays instead of one) I shoulld I guess have put the second relay in series with the first one, so that when it powered the door-unlocks, it also powered the trunk relay.
I'm trying not to make the same sort of mistake here.
A properly chosen thermistor will heat up and limit current if the motor is stalled. It will also provide more a bit more current on cold days, when you need more torque because of stiff grease in the mechanism.
Car companies are not in the electronics business, so they use what they feel that mechanics can understand.
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It sounds like they are using a generic symbol to represent a thermo sensitive device. It does not mean it would be a breaker or a thermistor. It just means that there is a thermo sensitive device there.
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