Fixing my Volvo 740 temp gauge....

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.

 

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

 

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