Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
With the circuit board out and top surface up, set the multimeter to ohms.
You'll see the 4 LEDS in the rev counter forming an arc at the bottom right. Each LED has two legs and they're all aligned the same way. When I refer to the bottom leg, that's the leg closer to the bottom edge of the board with the two connectors and vice versa.
The 4 LEDs, starting with the one closest to the green connector are the battery, oil, CODE and engine fault lights.
1. Put one probe on the bottom pin of the oil w/l and the other probe on the bottom pin of the engine fault w/l or CODE light ( doesn't matter which ). You should see a resistance of about 1000 Ohms ( 1K ).
If you get 1k, it shows that the anode of the LED is getting it's +12v supply. The reason for the 1K resistance is that the current limit resistors are between the +12v supply and the LED's anode ( contrary to the way it's shown in the workshop manual! ). You get 1K because you're measuring 2 parallel pairs of resistors each with a value of 1K ( so each parallel pair equates to 500 Ohms resistance and there are two pairs between the probes so you get a total resistance of 1K ). Anyway, 1K measured is good.
2. Measure the resistance between the two legs of the oil w/l. You should get a resistance of 240 Ohms. This is because each LED has a 240R resistor in parallel with the LED. This is a delayed switch on feature.
3. Touch one MM probe to pin 10 of the green connector ( bottom row of pins in the green connector, second from right ) and the other probe to the upper pin of the oil w/l. You should get little or no resistance. This means the cathode of the LED is connected to pin 10 of the green connector ( good ).
If all those tests are good, the circuit board is good. That leaves two areas for the fault -
a) failed LED
b) wiring problem
With the instrument pack out of the car, you can check for a wiring fault easily.
With your multimeter, measure for resistance between pin 10 of the green plug in the car and ground. Here you're testing the wiring and the oil pressure switch. If both are good, you should see a resistance of zero Ohms ( or near enough! ). With the engine off, the oil pressure switch should be switching the LED to earth.
If all those tests are OK, it may be the LED that is at fault. Unfortunately, owing to that 240R resistor, it's a bit difficult testing the LED in circuit. I'll have to think of a way of testing that.