• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

240 Coolant Temp Gauge - moves but not very far


Active member
Dec 1, 2021
1983 B21FT. Coolant Temp Gauge doesn't rise properly as far as I can tell. Coolant level is between markings on the bottle.

Keyed off the gauge will sit on the lower end, similar to fuel gauge. Keyed on, and after driving around for a while, it only advanced maybe 1/8th of total range, I assume ideal temp would sit somewhere around 50%.

Does it sound like the movement seen is purely a response to vehicle being keyed up, and that the oomph from the actual sensor doesn't come in (check sensor / wire to gauge), or that the gauge itself may have an issue and the sensor may be fine?

If someone has a bit of initial guidance as to which part to look at first (and any resistances / GreenBook numbers to pull from ozvolvo) that'd be helpful.
Here are the resistance figures for testing the sensor. A 100 ohm resistor installed in place of the sensor (one end to ground, the other to the yellow gauge wire) should make the gauge read in the middle if it's working properly.

Ok. Looking at my wiring diagram, I'm guessing the yellow wire from the sensor to the cluster is part of a bundle so it's not as simple as unplugging a single spade on cluster and trying a 100W resistor to ground there.

Looks like easiest will be to attempt to reach under and unplug the yellow wire from the sensor itself. Relative location on cylinder head block (http://turbobricks.org/forums/showpost.php?p=5204988&postcount=2)

I assume to remove it if it is dead I will need to drain coolant at least below a certain level. What's the best way of doing that / what size socket do I need for the sensor itself?
Connect the 100 ohm resistor at the sensor end of the wire.

If you leave the cooling system sealed it won't leak very much when you remove the sensor as long as the system isn't under pressure. If you have a replacement sensor to swap in then you don't need to drain the system. If the sensor is going to be out for a while then you may want to drain the system first.

Use a 17mm deep socket with a swivel and extension to remove the sensor. A shallow socket may not engage the hex properly because the terminal sticks out too far.
You might also have some resistance in the wiring. My 242 also has this issue. Had it with its original engine, and with the current engine. The gauge did go up when the head gasket popped on the original engine and properly pegged itself as the coolant level dropped. When I installed the original engine into my 245, after replacing the head gasket, the gauge in that car read normally. Didn't read low. Read around 8:30 when fully warm, since I was using an 82 deg C t-stat, instead of a 92 deg C or 87 deg C t-stat. The 242's gauge usually sits around 8:00 when fully warm. Might hit 8:30 if I bomb up a hill in boost on most days, 8:45 if the ambient temperature is close to 100 deg F.
finally had nice enough weather to take a look. It appears impossible to reach the connector without removing stuff. I have a B21FT - anyone know the least intrusive method to gain access? I can see a bit of the yellow wire but no good way of touching it.
If I can't fit my hand in there. a long needle nose plier should reach it. Grab the wire firmly on the plastic cover and you can pull the wire off.
managed to work some magic (easier if i had shorter forearms) and got the connector off, cleaned stuff up, checked stuff, no major resistance errors so I'll just live with current situation until intake needs to come off.
With the sensor testing good appears that you need a new thermostat. Running the engine cold all the time is not good for it or your fuel mileage.
With the sensor testing good appears that you need a new thermostat. Running the engine cold all the time is not good for it or your fuel mileage.
Is there a way to test the thermostat independently? I didn't check everything on the coolant gauge circuit so can't necessarily say there's not still a problem on that side of stuff.
Sure, take it out and boil it in a pot of water. You clearly described a thermostat that is not closing correctly. Why not change it anyway? It is a $10 item at any autoparts store.
I think I recalled incorrectly what I had tested but I just found my notes. Ambient temp was about 60F. I disconnected the yellow wire and measured from that to ground - I got 170 Ohms (this is through the wire, through the gauge and whatever else is in that circuit). I also measured 1160 Ohms to ground on the sensor itself (mind you, kind of hard to get a probe on there). I put a 100 Ohm (multimeter verified) resistor on the yellow wire and through a switch to ground, in KeyII engine off the dial got most of the way to level.

So if I extrapolate the 3 points from hiperfauto's image then I get in the range of 800-2k ohms at 60F (exp vs power) which aligns with 1160 being somewhere in that gray area. It does truly sound like the thermostat needs to be looked at.

So, green book I have says drain coolant using the draincock on the engine, the drawing shows it under the exhaust manifold right? Without removing the thermostat, how do I know when I'm below that level? I assume if it stops draining at the draincock it's low enough but that seems quite crude.

Then remove, boil test, compare to spec type, replace if needed, new gasket, reassemble.
If you don't mind spilling a couple cups of coolant over the side of the engine, you can pinch off the upper hose with a pair of those pinch-off plier things (or some suitably padded and adjusted vice-grips) then just unbolt the thermostat housing. Otherwise I think you're just stuck guessing how much is above the thermostat in normal operation and draining about that much out of the draincock.
The choice between 87C and 92C - any useful factors to take into account? It's going to stay stock B21FT, and live at 4k+ ft elevation for a while.
I have been told it's not a massive difference either way. Lower temp gives you slightly more headroom if you think it'll spike a lot. Higher temp supposedly is a little more efficient.

The coolant system is supposed to hold pressure (a good, leak-free system will hiss a little when you open the cap hot) so I don't think elevation should change anything.
The choice between 87C and 92C - any useful factors to take into account? It's going to stay stock B21FT, and live at 4k+ ft elevation for a while.
My 1983 B21FT car also reads lower. I was thinking about cleaning the sensor sometime and giving that a try. I go hotter on the Thermostat personally. I think running a hotter engine is more efficient. I have a 92C in my 1993 in Arizona at the moment. Coolant system is working properly but will be switching to 20w50 once the temps reach the 100's again.
I believe the 87c is the oem version that should put the guage dead center but I would choose the 92c as long as ambient temp kept the guage well within the normal range and I didnt live in Texas or AZ. Myself, being a little anal on car maintenance, would replace a 30 year old sensor also, another $10.
I've been using the 82C thermostats for many years and like how it reduces the heat stress on engine compartment items. I also have adequate heat in the winter but if you want the super hot heat then use the stock 87C one.
well, followed Khrrck's advice and clamped the radiator hose, removed the thermostat, tested it and found it opened at ~70C. Have a new one arriving at my local autoparts store tomorrow.
Looks like prior owners had put in a 71C Calorstat (decided to try to read all the markings vs just directly testing). Replacement is a Murray 91C. Owner records indicate thermostat replaced in 1985 and 1990, possibly after too. No idea if the 71C was intentional. I'll at least verify that temp does get to the middle point on the gauge. Thanks to the radiator upper hose clamp, one of the easier repairs to do (although the drip paths do want a fairly wide mouthed catch basin)