Resistance of instrument gauges

[Eurasiaoverland]

Background: 1989 60 Series, 12V. Bought the vehicle more than 4 years ago then stripped it down for a rebuild almost immediately. Can't confidently remember which gauges worked, but think fuel was good, temperature was not reading, oil and voltage I guess were OK. Vehicle going back together but having electrics connected is a few months off.

Referring to 'Landcruiser Heavy Duty 1984-90' FSM (covering 60 and 70 Series diesel and petrol) the spec for the resistance of fuel level, water temperature and oil pressure gauges is recorded as 55Ω.

When I check my three gauges, they are all (separately) 25-26Ω. I can't really believe that all three of my gauges are not just defective, but have strangely lost about half of their resistance. Could it be that the quoted 55Ω is for a 24V system?

Thanks

 

[Hippmitten]

Im looking for same issue. May be someone reply to us ..

 

[mcmlxxxv]

If they are 24v it should be tagged on the housing , like this one is tagged 12v

 

[Eurasiaoverland]

If they are 24v it should be tagged on the housing , like this one is tagged 12v View attachment 4038594

There's no confusion over the voltage of my cluster (12V); I am wondering if the resistance values in the FSM are for a 24V version since all my gauge resistaces are half what the manual suggests they should be.

 

[mcmlxxxv]

ho ! ok !

I have both type at home , I will try to check that for you tonight if nobody answer first.

 

[Eurasiaoverland]

ho ! ok !

I have both type at home , I will try to check that for you tonight if nobody answer first.

Thanks, that would be very useful to know

 

[Eurasiaoverland]

Let me explain my reasoning for suspecting it's a 24V specification in the FSM.

Using the power law, P = IV and Ohm's Law, V = IR, we can see that:

-If the 24V gauge were to be designed to have the same power as the 12V gauge, then the power law tells us that if the voltage doubles, the current halves. Then Ohms law tells us in this scenario that with double the votlage, for constant power, the resistance must quadruple in the 24V gauge. However, as the gauge is just an indicator and is not doing any useful mechanical work as such, I don't think the power rating is important (as it would be in the case of the starter motor for example).

-If the 24V gauge were to be designed to draw the same current as the 12V gauge, then Ohm's Law tells us that to double the voltage, for constant current, the resistance must double. I think it is more likely that the current rating is kept the same in both gauges (as this would relate to the wire thickness required).

It need not be the case that either the current or the power are held the same between components, but as mentioned I find it strange that all my gauges have basically half the specified 55 Ω resistance. If they were defective, I would expect them either to be burned out with no internal circuit, or an open short. Plus, I'm pretty sure at least one of them was working correctly.

 

[mcmlxxxv]

On the 24v I get 25ohm on the fuel level gauge and same thing on the 12v fuel gauge .

24v

12v

There is the big ceramic resistor on the 24v gauge tho as you can see on the 2nd picture , that might just be the only difference.

 

[Hippmitten]

Most valuable informations

 

[Eurasiaoverland]

Thanks very much for checking that. Interesting that they are the same. Another example of misinformation in the FSM. I think I can conclude that all my gauges are fine.