VOLVO 740 - 1986 Temperature gauge stops working sometimes?

efinet · Post by **efinet** » 19 Nov 2004, 20:35

I have a 740 - 1986. The yellow wire to the sensor seems to be ok.
I moved it forward and backward. When the temperature reaches a certain level (middle) , then the meter drops to 0. This is the right lower meter on the dashboard.
How can I measure the sensor/meter? Am I looking at the right sensor?
Where is this sensor located?

Thanks, Etienne

LaRy · Post by **LaRy** » 20 Nov 2004, 07:24

It seems that you are looking at the right sensor. Loosen the yellow wire from the sensor, now the temperature should be at minimum value. Connect the loose wire for a short moment to ground. Now the temperature meter should show maximum value in the red "hot" area. Note that you should just ground the wire for maximum 2 seconds, otherwise the meter could be damaged. The resistans at normal motor working temperature 200F is 87 ohms +/- 15. If you loosen the sensor and test the resistance when boiling it 212F, the value should be 67 ohms +/- 11.
If you want to check the meter more accurate, you should connect a 68 ohms resistance between ground and the yellow wire, the meter should then show 75% of maximum value, just a little below the boiling indication.
It is simple to change the sensor, the water will just poor out very slowly. In Sweden the price for an origin sensor is 40 USD.

efinet · Post by **efinet** » 20 Nov 2004, 20:27

I checked it out today. the meter works properly until it's about at 33% of its reading. Then it sudenly drops to 0. Defective metr???
When I short the wire to ground, then I see movement.

LaRy · Post by **LaRy** » 21 Nov 2004, 05:28

If the meter is going fast to maximum right position when shortening the wire to ground ( just for a second or two) the meter is working and it is most probably the sensor. The more professional way to check the meter would be to get some 68 Ohms resistans connected between the wire and ground. If the meter then is showing 75% of maximum value, the meter is definitely just OK.

efinet · Post by **efinet** » 21 Nov 2004, 19:27

I put about 62 Ohm to the ground. The meter does not move at all!
How is the wiring to this meter? Does it go directly to the meter on the front panell? Maybe the +12V is dropping on the other side of the meter?
Is there any wiring diagram available?

LaRy · Post by **LaRy** » 22 Nov 2004, 10:02

This sensor is passing a couple of connections before arriving to the instrument console. It is yellow-white all the way. In the console the temp.meter, speedometer, rpm.meter and fuel meter are all getting the same feeding from a voltage stabilisator unit. So if the other things are working, the feeding is good. But anyway, it could be a good idea to take the complete console out. It is just to loosen 2 screews on the lower edge corners at the front. The are covered by small plastic units. Take the console out 4-5 inches, loosen the connections in the back, make some notes what and where they are connected, otherwise you will be in trouble later. In the back of the console, you will find a green plastic sheet with copper lines inside for the electrical distribution. This sheet is covering the whole back. The connection to different units are made by some sort of clamp, simple and unreliable. It could be a good idea to check this up and clean the connection point for the temp.meter by some sandpaper. The temp meter is just feed by the stabilised voltage and the ground is coming from the sensor. Noting more according to the circuit diagram.

It seems like you have some electrical knowledge. I would have used an Ohm-meter checking the ohm at the sensor compared too ground. The when loosening the console I would have checked that the ohm is equal when messauring at connection pont to the console(still white-yellow wire ) .

PS. Check also the grounding of the whole engine! It is rather common that the ground connection for the whole engine is broken or bad, and then you get problems like this. A simple test is to start the engine and check the voltage between the engine and the fuselage of the car, it should of course be 0 volt = good grounding.

efinet · Post by **efinet** » 22 Nov 2004, 14:06

I took the console apart. Checked everything. (Loosened the wiring and I re-tight them)
Put it all together. The results:
The meter now works fine. However when the engine heats up and the meter gets above 50% of it's scale, then the meter suddenly drops to 0!!

When the car is cold, and warms up again, the meter is normal... but we get the same effect when it reaches a certain temperature.

So maybe it's the sensor???

LaRy · Post by **LaRy** » 22 Nov 2004, 14:31

Good job that you managed to take the console out and reinstall it. I think it sounds like a bad sensor. If you have a multimeter, loosen the sensor and put it on the stove and start heat it. Messaure the resistance while heating it up until boiling point. The resistanse should start at something like 1000 ohms an drop to appr. 67 ohm at boiling. You could of corse do the same messaurment with the sensor still installed, start with cool engine and the resistance should be more than 1000 ohm's. When the engine is at working temperature, it should be appr 90 ohm's. Follow the reading continously and compare if the sensor suddely jump to a very high value at the same time as the temp. meter drops. If it does, change the sensor!

740ecuador · Post by **740ecuador** » 27 Aug 2016, 16:20

Hello all, great forum, I have an 1989 740 GL sedan. The temperature gauge is dead, it never works, whether the engine is hot or cold. I checked the ohms on the sender and there was none. I replaced it with a new sensor reading aobut 1500 ohms at about 30 C. The gauge still didn't move, when I ground it for half a second, it went all the way to the hot end on the dashboard. I am unsure of what to check next. Any help would be highly appreciated.