Dear Kent,
At 05:58 PM 4/23/2012, Kent or Jackie Ashton wrote:
As I recall, the blinking light has no relation to coolant temperature or resistor selection. When the light blinks, it only means that the two prongs in the coolant reservoir cap are no longer submerged in coolant or the same circuit has otherwise lost continuity.
Actually, without undue modesty I can say that I am an expert on that blinking light, and your Samba search will turn up a good deal of material I wrote. Its primary purpose is an overheat warning.
The temp gauge has only three terminals, +10v, ground (for the blinker circuit) and the sender terminal. The sender is a resistor with negative temp. coefficient, i.e. the resistance gets less as its temperature rises. The gauge employs a heating coil to cause a bimetallic element to bend in proportion to the amount of current passing through the gauge (thus moving the needle through a lever movement), which rises as the sender resistance drops.
At a sender resistance of 265 ohms the gauge should read at the lower set of calibration marks (see Bentley 90.21), and at resistance of 35 ohms it should read at the threshold of the overheat zone and the light should blink just as it says on 90.21.* Note that adjusting the calibration affects only the needle; the blinker threshold is set by components on the blinker board inside the gauge.
*The rule for converting settings on the VW1301 tester to ohms was given to me by Mike S and proved in practice in my working with gauge calibrations: tester setting / two, plus ten equals ohms. This works for temp gauge calibration and also fuel gauge calibration on the previous page of Bentley.
The low-coolant function VW achieved by cleverly tricking the gauge: The low-coolant controller output terminal is connected to the gauge sender terminal in parallel with the actual sender, and when the controller triggers it supplies that terminal with a low enough resistance to ground to trigger the overheat blinker. On the older controllers this is done continuously, so that the gauge needle also rises to the overheat zone. On newer ones (cube-shaped instead of tall) it's done in short pulses, often enough to keep the light blinking but not enough to materially affect the needle. This is also a clever trick, because the blinker circuit is designed to always blink for 2-3 seconds any time it's triggered (see Vanagon owner's manual under Coolant Temperature Gauge, subsection Warning Light - page 37 in the '87 manual). This is done as a power-on bulb test, but VW used this characteristic to advantage in designing the low-coolant warning circuit.
Yours,
David
ps - for those interested, the zero and span calibrators are a couple of toothed levers accessible through holes in the back of the gauge. I always forget which is which and have to look: the zero one moves the needle pivot and the span one moves the end of the flat tension spring. They interact somewhat. Start with the zero, and remember to give the gauge a couple minutes to stabilize temperature and a couple of taps to make sure the needle isn't lagging each time you change the calibration resistors. Two or three times around should suffice. Gauge input voltage is ten volts; you can get it from the panel if you don't have another way. Quarter-watt 5% resistors are sufficient for the calibrators; I suggest 10R + 10R + 15R for the high and 100R + 100R + 68R for the low. The calibration points are shown in Bentley, but also printed on the cupped edge of the gauge face where you can sight along the needle to match them.
