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No…. it's not about getting wet, it's actually getting "heat soaked" for all you rodders out there! You know what I am talking about…read on!
The term "heat soak" is applied to engines that start just fine and dandy when cold or cool, but subsequently are operated or driven and as a result build heat in the engine compartment and surrounding engine parts. This is especially true in and around the exhaust manifolds, headers and engine blocks or in the slipstream of the radiator exhaust. The acute abundance of heat actually raises the ambient temperatures of the surrounding areas including that of the starter, solenoid and wires thru conduction from the actual block, radiation from the exhaust/headers and simple air convection/conduction/movement as well. This is an inherent problem by design.... or lack thereof and a multi-faceted problem to explain. This malady usually accompanies the hotter summer weather for the most part but can be multi-seasonal.
First, a physics factoid. Copper looses its ability to conduct electrons as its temperature rises. Current is the sum effect of electrons moving through a typical copper wire. When the wire heats up for example by a heat source, in our case manifolds, headers and blocks, it causes the electrons to become chaotic and unorganized than when cool. Therefore, current flow is greatly reduced by this random chaos affect of the electrons. This means that the copper wire now literally has a higher resistance to current flow. Most metals or alloys thereof increase their electrical resistivity by about 0.005° per degree C. So, if the resistivity of a copper wire at 25° C is 1 ohm, then at 100° C (212° F) it will be 1+(100 X .005) = 1.5! This means if you heat your wires to 100* C, then its resistance increases 1.5 times! The current is thus inversely proportional and then drops by 1.5 times.
The result is big energy losses in long wires and is typified in these coil designed (literally hundreds of feet of copper wire in windings) apparatus such as starter solenoids and old starter motors. The resistance rises dramatically with the addition of heat and acts literally like a built-in resistor to limit the correct operating current and therefore wattage (or ability to do work) of the device whether it be the solenoid that goes click…click or the starter that barely grinds over or both that work real slow or not at all.
Put on top of that, the feed wiring from the battery (the major carrier of large amounts of current to the starter) is also tremendously affected by the heat by raising its resistance somewhat as well. This is an exacerbating issue with small/undersized starter feeder wire where the resistance is high anyway to large starter currents.
Focusing on the solenoid, frequently waste heat will get the solenoid conductors/coils so hot and dysfunctional that the solenoid will not fully engage the starter, if at all. This is indicated by the infamous clickity-clickity or buzzing sound that mimics a loud buzzer! This is why in some cases moving the solenoid to a cooler location is a preventative measure for alleviating some aspects of "heat soak", but does NOT guarantee it "heatsoak" proof. I have seen Ford solenoids put on many applications which moves them to a remote and less heated location making for a better engagement situation of the starter. Sometimes this works. But, when things got really hot at the starter plus the feeder cables become so resistive that heatsoak continued to be an overall problem.
Another thing to throw in is the condition of the "start" wire that comes from the ignition switch via the neutral safety device to energize the solenoid. This wire can become heat degraded and annealed at the connections and induce further resistance to the solenoid coil circuit causing a dead start circuit (this is where you turn the key and hear dead silence).Also there are other mitigating nuisances. In summer weather, hot ambient temperatures reduce a battery's cranking capacity. Most engines will operate at about 180-210 degrees Fahrenheit and in traffic the temperature under the hood rises quickly as does the parts contained therein. Additional heat load comes from the air conditioner and the automatic transmission. Even when the engine is shut off, all components, including the battery and starter, continue to heat up for some time before they begin to eventually cool. During this "heat soak", engine components expand; increasing friction on the moving parts and making them harder work for the starting system!
The TOTAL effect of all this resistive mess is a dramatically reduced current flow and energy to accomplish work, which in turn means the starter is limited in current/wattage and therefore either turns very slow or not at all and that's only if the solenoid is working at this point! Some of these old time starters take almost 200+amps typically to achieve the work necessary to successfully turn over your engine. Some of the old starters are not made for high compression engines with lower torque capability. Chevrolet makes a high-torque starter for high performance applications, however they are as susceptible as any to "heatsoak".Where does all this heat come from?
The heat that creates "heatsoak" comes from many sources. Of course, given that the engine is producing all this heat as a byproduct of combustion in the block, exhaust and coolant jacket. Thus, the real culprit heat comes from radiant heat from the exhaust manifold or headers (headers are worst because they run hotter than cast iron manifolds) and block, direct conduction of heat from the block to the starter/solenoid mounting frame, and lastly from heat-saturated air within the compartment. Another physics fact. Heat is like a fluid when conducted directly through conductive materials. This occurs because there is a temperature differential (really a gradient) between the engines metal that is in close contact with the coolant and that which is farther away. So, heat always flows from the hottest points to the coolest points. When the engine is off and there is not coolant flow through the cooling system, the cooling effect is low in the areas surrounding the coolant, which removes this heat during normal operation. This allows heat from these substantially hotter areas to flow to areas that are usually much cooler when the engine is running and coolant is absorbing the waste heat as designed. Sometimes this also results in fuel percolation, evaporation, coolant boiling and our beloved subject, heatsoak of the solenoid and starter apparatus. So, in essence, heat comes from everywhere!What steps can I take to eliminate heatsoak?
Here is a list of things to minimize and even eliminate the likelihood of heatsoak.
Happy Summer Cruising!