It's been a while i notice these gauges response in my Lj80. Fuel never pass 3/4 and coolant temp never move. As i read both are resistive type response, which should easily be checked.
Is the gauges broken or wiring broken or sensor broken?
Fuel meter responds, means no wiring issue, fuel sensor may have issue or the gauge display it wrong.
Coolant temp not responding, can be wiirng issue, sensor issue or gauge issue.
First I check coolant sensor , comparing to other (car/new sensor). My thermistor read 400 ohm when cool and 140ohm when hot (car runs for sometime). The other thermistor read 400 when cold. Ok no issue on sensor. How to check cable and gauges. As the gauge made from bimetal type,it should have coils to heat it up.it means it is "Ok" to short the sensor side just to check if it respond. So i connect the sensor cable to ground. And after few seconds, i notice that coolant temp needle move on the gauge. OK, i have no wiring issue and the gauge respond. But what is wrong?
Comparing the coolant temp vs fuel meter gauge, both have no sensor issue (at least it give reading), it has no wiring issue, but both may have gauges issue.
I remove the speedometer from console (4screws), remove cables (socket) and odometer cable, i need to remove plastic cover of steering wheel to be able to get the speedometer out.
Thereare 6 screw on the back, remove them, now i got the back plate of the speedometer out. All thing inside are mounted there.
After checking/guestimating the connection, i inject 12v and ground to it. And measure some points inside.
Basically, it has 3 bimetals. 1 acting as regulator (on the right side of fuel meter), 1 acting as the display of fuel ( on the left of fuel meter), and one acting as display of coolant temp (on the coolant temp gauge).
Regulator works by having a single coil from the bimetal point to ground. The bimetal itself will be connected to side metal giving 12v. When it connects, current flows, bimetal (regulator) gets hot and bend, as it bend 12v disconnected. This way it act like pulse width modulation, sending burst of 12v pulses on the bimetal (regulator) itself.
When voltage raise up, the bimetal become hot faster, means faster to disconnect, this way it regulate the output to stay on the target value. This happen vice versa.
Checking the regulator output with analog meter, i got 4 volts. It is connected to 2 gauge bimetals (shared). On the back of the pcb there is written 7V on the regulator out line. Now i understand that the issue of my gauges is due to less voltage on regulator output. With less voltage, the 2nd and 3rd bimetal will get less heat = less bending = less value on display.
How to increase the regulator output?
1. Cut the regulator coil (disable it), get a LM317 voltage regulator and a potentiometer, set the output to 7V. This is the modern way. But i may add some soldering/components on the back of speedometer.
2. Bend the metal pin inside(where 12v connected to bimetal (regulator). This will make bimetal need more bending to disconnect, means it will give longer time of 12v connected means output will have bigger 12v duty cycle (higher output). After several bend and check (with analog voltmeter) it give me 7V and time to test. I connect 120ohm resistor between coolant temp input to ground and it works now. Gauge move halfway.
Time to reasembly everything.. Or should i reset the odometer as well.. Maybe later .
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