Fixing my Volvo 740 temp gauge....

James.... Thanks for your ideas on the battery isolator issue. Will see if the alternator has an adjustable regulator or not.
As for the inop temp gauge....well, I thought it was going to be a bad sensor, but I replaced that, and it did not fix it. There is not a loose conection at either the sender or the instrument cluster. The gauge does come up as the engine heats, but it stop at exactly 12 o'cock and never goes higher. I have the instrument cluster loose right now--- I am going to check if the pointer is just mechanically hanging at that place or not. If not, DOes anyone know what value of resistance is supposed to make the gauge go to full-scale and zero? It has three wires, one is for the voltage regulator input (regulated DC) in the instrument cluster, and the other two go to the thermistor. The haynes diagram shows neither end of the thermistor grounded. I'd like to get the original gauge working again, and remove my jury-rigged Autozone water temp gauge. regards
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geronimo wrote:

What happens if you short the plug to the sender? These gauges are compensated so that they sit at 12:00 when the engine is within the normal operating range and rise above that into the red only once the coolant temperature is excessive. This was done because people don't understand that it is normal for coolant temperature to fluctuate, and at least in the 240 series the compensation board can be removed and bypassed, I don't know if this applies to the 700 and 900 series cars.
Keep in mind that there are two temperature sensors, one for the gauge and one for the ECU.
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You may be right about it being compensated. I found a table of temp vs. resistance in the Haynes manual, but it may be for earlier models, as one side of sender is shown grounded, while mine has neither side grounded. I think the Haynes is only for up to 88 models. It sure does not read anything like that table. I disconnected the sender and connected a trim pot to it. At 62 ohms, it reads full scale. At 92 ohms, it reads zero. BUT....if you keep increasing the ohms, then the reading starts climbing again. A short across the sender terminals will max out the gauge. So maybe there is a center resistance there at mid-scale?
I know I changed the correct sender, as the color coding of the wires to it matches the wiring diagram in the Haynes manual, and of course, the trim pot substitution confirmed it.
I know that there is a voltage regulator (what voltage?) supplying both the fuel level and eng. temp gauges, and the fuel level gauge doesn't work...but in the case of the fuel level gauge, it's because the fuel tank sender is bad/near open.
Since I got it I don't think I have ever seen it read other than mid-scale once warmed up...but my brother thought it was bad (it was originally his), because he said that one day it seriously overheated...the windshield got all steamed up....but still showed only mid-scale reading. I would like to check that voltage regulator powering the temp gauge, but I think it is on the inst. cluster, and the Haynes manual gives no info on checking it.
Looks like I will have to take the sender out and dunk in boiling water or heat with soldering gun...that's a major job, as the intake manifold has to be taken loose to get that sender out.
Do you think I could safely put a propane torch turned down low on it with it in place? Geronimo
On Wed, 29 Oct 2008 11:26:00 -0700, James Sweet

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