This article is part of a 6-part series on automotive chassis electrical systems. Part 5 covers how relays and solenoids use small currents to control high-current circuits and mechanical actuators, along with the operation of cooling fans, horns, wiper motors, and rear defrosters. Knowing why a relay clicks but the horn does not sound, or why a cooling fan only runs when the engine reaches a specific temperature, separates guesswork from diagnostic logic.
Relay, Solenoid, and Thermal Switch Operation
The chassis utilizes electromagnetic and thermal properties to manage high-current loads and temperature-sensitive operations. A relay uses a small amount of current to energize a magnetic winding, which is a coil, and this pulls a set of contact points closed. This allows a low-power switch to safely control a high-current circuit, reducing voltage drop and minimizing the weight of high-gauge wiring required at the dashboard. Similar to relays, solenoids use electromagnetism to create mechanical movement. When energized, the magnetic field moves a plunger or control rod to perform mechanical work such as latch release or door locking. Cooling fan circuits utilize temperature-sensitive switches that react to coolant heat levels. As coolant temperature reaches a threshold, for example 194°F or 90°C, the internal switch closes to complete the ground circuit for the fan relay. Multiple switches or dual-winding relays allow for staged fan speeds based on fluctuating thermal loads, which acts as a fail-safe.
Cooling Fan Circuit and Power Lock Solenoid Logic
Sub-systems are cross-linked to ensure environmental and security parameters are met. In the electric cooling fan circuit, coolant temperature switches and A/C pressure switches provide input. These inputs trigger the Radiator Fan Relays No. 1, No. 2, and No. 3 to energize the radiator and condenser fan motors. The circuit is typically wired through the Ignition Switch (IG) and Engine Main Relay to ensure fans only operate when the engine is running or in a specific cooling state, which is an interlock. For power door lock solenoids, the solenoids feature two separate windings. Energizing one winding moves the control rod to Lock, and energizing the other moves it to Unlock. This is dual-directional logic. For trunk release solenoids, a single-position solenoid retracts a latch when energized by a dashboard switch or keyless entry module. A spring provides the return force to the closed position. This is single-directional logic.
Local Shop Note:
This brings back a story I picked up from a technician out on Tompkins St in Cortland, N.Y. We were at an Automotive Seminars, Inc automotive electronics seminar, and he was telling me about a pickup that came in with a complaint that the cooling fans wouldn’t run — but only sometimes. Other times they worked fine. Customer said the engine would start to overheat in traffic, but if you turned the AC on, the fans would kick in and cool it down.
He checked the coolant temperature sensor — resistance matched the temperature. Checked the fan relay — clicked when he jumped it. So he knew the relay and fan motor were good. He started tracing the ground circuit for the fan relay coil. The relay was triggered by the Engine Control Module (ECM), which completed the ground path based on the coolant temperature sensor input. He used a test light on the relay control wire — it was pulsing on and off erratically as the engine warmed up. That told him the ECM was getting inconsistent temperature data.
He back-probed the coolant temperature sensor and found the signal wire had intermittent continuity — it would drop out when the harness was wiggled near the thermostat housing. He peeled back the loom and found the wire insulation had chafed through on a sharp bracket, exposing the copper. The wire was making intermittent contact with the bracket, causing the sensor signal to spike and drop — confusing the ECM. Sometimes the ECM thought the engine was cold and wouldn’t trigger the fan relay. Other times it thought the engine was hot and would trigger it. The AC override circuit still worked because it bypassed the temperature logic.
He repaired the chafed wire with a proper splice, heat-shrink tubing, and a protective sleeve, then rerouted the harness away from the sharp edge. The cooling fans worked consistently after that.
If there’s one thing to remember from that story, it’s that a relay is only as good as its trigger signal. If the ECM doesn’t get reliable temperature data, the fan won’t run, even if the relay and fan are good. Always verify the integrity of the sensor wiring — intermittent electrical faults are often physical damage, not component failures. And remember, the AC override bypasses the temperature logic, so don’t let a working AC fan fool you into thinking the engine fan circuit is fine.
Relay and Solenoid Mounting Locations
The placement and connection of solenoids and relays are dictated by accessibility and environmental protection. Relays are often grouped in junction blocks or power distribution centers to protect them from moisture and vibration while simplifying the wiring harness. These are centralized relay centers. Door and trunk solenoids are mounted internally within the body cavities. Disassembly requires removing interior trim panels to access the mechanical linkage between the solenoid plunger and the latch mechanism. This is solenoid integration.
Coolant Temperature and Pressure Switch Specifications
For coolant temperature switching points, Switch No. 1 closes at approximately 194°F (90°C) and opens at 181°F (83°C). Switch No. 2 is typically used for high-speed or secondary fan activation, for example at 203°F or 95°C. For pressure switch thresholds, the ON pressure is 221 lb/in² (15.5 kg/cm²), and the OFF pressure is 177 lb/in² (12.5 kg/cm²). Relay contact integrity requires that high-current contacts remain free of carbon buildup, which is pitting, to prevent excessive resistance and heat.
Horns, Wiper Motors, and Defrosters
Horn Diaphragm Oscillation and Defroster Grid Heating
Chassis components utilize electromagnetic oscillation and electrical resistance to perform mechanical and thermal work. The horn operates via an electromagnet that rapidly attracts and releases a metal armature. This movement flexes a diaphragm at high frequency, creating pulsations in the air column that are magnified through a projector, which is a spiral trumpet, to produce audible sound. This is horn diaphragm oscillation. Rear window defrosters utilize a grid of fine, calibrated wire applied to the glass. As current flows through the grid, the material’s inherent resistance generates heat, which is transferred to the glass surface to melt ice or dissipate moisture. This is resistive grid heating. In many defroster systems, as the grid temperature rises, the electrical resistance increases, which naturally opposes current flow to keep the temperature constant. This is self-limiting thermal logic.
Horn Relay Circuit and Wiper Motor Park Function
High-amperage components are isolated from control switches to prevent voltage drop and switch failure. In the horn relay circuit, the horn button completes a low-current ground circuit for the horn relay coil. Once energized, the relay contacts close, providing a direct, heavy-gauge path from the battery to the horn. This ensures maximum current for sound volume while protecting the steering column wiring. This is the load path. Windshield wiper motors are high-torque DC motors designed to overcome the friction of dry glass or the weight of ice. They are often spring-loaded to return the blades to the park position automatically when the switch is turned off, ensuring the blades do not obstruct the driver’s field of vision. This is the wiper motor interlock.
Door Lock Actuator, Wiper Linkage, and Defroster Repair
Access to chassis-mounted motors and solenoids requires systematic removal of protective interior or exterior trim. Door lock actuators, which are solenoids or motors, are mounted within the door cavity. Linkage rods must be disconnected from the latch assembly before the actuator can be removed from its mounting bracket. The wiper motor is typically mounted to the firewall or within the cowl. The drive arm must be synchronized with the linkage, also called the transmission, to ensure the sweep pattern remains within the windshield boundaries. For defroster grid repair, because the grid is a series of fine lines, a single break in one line will disable that specific heating path. Repair involves applying conductive epoxy to bridge the gap in the circuit.
Horn Armature, Wiper Motor Ratings, Heater Grid Resistance, and Remote Module
The vibrator or armature is the moving component in a horn that makes and breaks the electrical contact points. The wiper motor is a high-torque, permanent magnet DC motor. Modern vehicles may employ up to ten small DC motors for various chassis functions such as sunroofs, power seats, and windows. The heater grid is a series of parallel resistive paths; total circuit resistance must be maintained to ensure even heating across the glass surface. The key fob or remote module interfaces with the power lock circuit to trigger solenoids via an electronic control module rather than a physical key cylinder.
The key takeaway from Part 5 is that relays and solenoids allow low-current switches to control high-current loads and mechanical movement, while temperature and pressure switches manage cooling fan operation based on fixed threshold values. Proceed to Part 6.