An offering of some repetition and hopefully some light.
Accurate reading of coolant temperature depends on the proper functioning of the variable-resistance sensor, good coolant flow past the sensor, clean and low-resistance connections in the wiring between sensor and gauge and a good, constant +5VDC as generated within the gas gauge. The +5VDC is generated by a rather crude system of points, bimetal strips, heating coils and who knows what? Any significant variance of the +5VDC power supply will cause the position of the temperature gauge needle to be affected. More Voltage = higher reading. This Voltage can be read at the top stud on the back of the fuel gauge. It will have an AC component so an analog meter may give a better reading. I used a small temperature indicator to read the quickly-cooling water temperature.
The sensor can also be tested by measuring its resistance while it is immersed in very hot water. I’m sorry, I don’t have any records of the calibration curve for the sensor. I did test a new one by heating water in a Styrofoam cup to boiling, then plunging the tip of the sensor into the water, suspended by alligator clips to the body and output post and connected to an Ohm-meter.
I’ve read about the area becoming vapor locked. Drilling a small hole in the thermostat flapper cured that.
The wiring between the sensor and the temperature gauge passes through the bulkhead connector (the violet wire). Corrosion and loss of tension on the brass connectors will add resistance to the circuit Disconnecting the 8-wire connector on the underhood side and cleaning all the brass is always a good idea.
Others have recommended using an IR temperature gauge to check the actual temperature near the water pump and this is a good idea. There are apparently a lot of junk temperature guns out there so, as always, purchasing and using a quality instrument always makes sense.
If a ram car, there is an unboosted master cylinder on the firewall. It is physically nearly the same MC as one would find on any ’64 Chrysler (and other Mopars). The only difference is there is not supposed to be any so-called “check valve” in the outlet nose of the device. The outlet check valve is present on non-power brake cars and non-ram power brake cars and is there to hold a small residual hydraulic pressure on the system to reduce any tendency to ingest air and moisture at the wheel cylinders. If working properly, the check valve will allow most hydraulic pressure back into the MC reservoir. It seems possible this little check valve or variable orifice could begin malfunctioning with higher than normal underhood temperature and/or just failure of this tiny, unsophisticated and critical element.
On ram cars, the little MC without the check valve drives a vacuum-boosted “slave” MC located in the LH wheel well and under the battery rack. The slave MC is supposed to have the little check valve in its outlet. Emphasizing, the improper presence of a check valve in the outlet of the firewall-mounted MC will result in brake shoes being actuated and staying activated after the brake pedal is released. Trivia: The boosted and non-boosted MC’s on the firewall are mounted to and require different heavy steel reinforcing plates that are attached to the firewall.
Others have noted that the various synthetic flex hoses in the brake system can swell and choke off fluid flow. This may be due to heat, age or interaction with differing hydraulic brake fluid types. Any question on the quality of the hydraulic fluid in the system should be answered by a complete purge with new hydraulic fluid and bleeding each line to each wheel. This is not a bad idea, anyway. If gobs of rusty gunk come out, there may be issues in the wheel cylinders.
As another possibility, I’m thinking that moisture in the hydraulic system may be converted to steam under certain conditions. That could create pressure in the system. That’s a reach.
Finally, proper backing-plate hardware and springs, a little dab of proper lube between plate and shoe, cleanliness and proper squareness between the lining base and attachment plate are all carefully specified in the Service Manual.
Best wishes for Fathers’ Day to all that qualify and to Sam with his temperature and brake-drag problems. Let us know what you find.
Hi if if fluid in lines expands it simply backs into master cylinder on non ram j/k drum brakes --even with a residual pressure valve , it cannot hold the brakes to drum ? is there a residual pressure valve in these non ram stock ? Also issue of hydrovac on ram cars . If a master "for a 300"-- the common one --w residual pressure is used as the master drive to hydrovac it will actuate the hydrovac by itself leading to locking . Probably caused by expansion in connecting line. That might be happening if this is a ram car. I believe with hydrovac you have to have a special no residual valve master or remove it. Same thing happens on non ram when you take foot off brakes. It runs back in, non ram. If it locks up on touching that has to be a booster engagement problem, as there is simply no way to generate enough psi to lock brakes unless booster is active. Over active .
Posted by: "Rich Barber" <c300@xxxxxxx>
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