A common problem with TR7's and 8's is that of a none functional "coolant low" warning light. While not essential to the proper operation of the car, it is one of the more useful "idiot" lights, especially given our Wedges propensity to over heat and leak coolant, if the coolant system is not dutifully maintained.

Location and function

The electric circuit is composed of three main parts: The "low coolant sensor" in the header tank which provides a signal to the "low coolant indicator". It is the "low coolant indicator" that turns the "coolant low" warning light on or off.

On the '8 the "low coolant indicator" is a tube about 1 inch or so in diameter and about 3 inches long. It's located up under the dash on the driver's side secured by a little spring clip. It's perhaps anti-intuitive, but the way to test it is to disconnect the sensor lead from the header tank, the light should come on. The light should also come on briefly when you first turn on the ignition. If that doesn't happen and the bulb is good, the "low coolant indicator" circuit is probably bad.

The "fix" is in

The schematic (size: 36.6KB) shows the layout of the circuit. On the working unit tested, a reading of 9.8V was taken across the 3.3K resistor when pin 3 of the 4011 IC was driven high. This translates to approx .3 mA. Although this is within the normal limits of this IC when driven @ 10V, it is to much when the chips power supply dips to 5V. This may explain why this gate is frequently found to be burned out. By following the modificationss outlined in the schematic, the pin 3 current will fall to less than .1 ma, well within the capabilities of the IC to handle at all power levels. As far as the other components go, The diodes are probably run of the mill signal diodes, any silicon signal diode should work, but they should not be bad. Likewise none of the capacitors or resistors should present any problems either.

The modifications shown are straight forward. Although the test unit was from a TR8, it is assumed that the unit in the TR7 is the same. Looking at the schematic, The 3.3K (base) resistor between pin 3 of the 4011 and Q1 (Lamp driver transister) needs to be replaced with one of 100K. Q1 (MPS A06) itself should also be replaced with an MPS A14.

If you replace the IC (4011), the lamp driver transistor and the base resistor, that's all you should have to do.

NOTE: Thanks to Paul Amaranth, for debugging the "low coolant" circuit and providing the schematic with modifications. Paul points out: "If you're in the US, I repair and upgrade the sensors (circuits)." but "If you're in the mood to try yourself, the schematic is available".

Coolant.gif by Jim TenCate.
Page formatting and circuit graphics by Mark Elbers.

Compiled by Tim Buja, 24 jan 96

Last updated: August 28, 1997