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Wedge TR7 sender replaced...but still sweeps to hot

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sammyb

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I replaced the temp sender, but the needle slowly moves up to the hot peg once the engine turns over. I don't think it's a short, since that would probably make the gauge peg without the engine running.

So what do you guys think -- faulty gauge? I don't think there's anything else in between, right?

And yes, I've felt the hoses, the bottom is ice cold...the top eventually begins to heat up, but well after the gauge has pegged hot.

Although, could it just be an air pocket or something?
 
I seriously doubt it's an air pocket. If it was, it would probably be showing/maintaining a cold temp instead of a hot one.

How long does it take for the gauge to sweep from hot to cold? Also, does it only do it after the engine is running, or does it do it with the ignition simply switched on? If it's within a few seconds, or does it with the ignition on but car not running, then it still could be a short somewhere. I'm not sure the exact process, but you should be able to test the sending unit and the gauge socket using a multimeter. If the sending unit is OK, then the current sent to the guage should gradually change as the temp climbs. Test at the sending unit and see if this happens. If that checks out, then pull the gauge and check it there (after the car cools off). Someone could probably give you a better test procedure description than this.
 
current sent to the guage should gradually change
Some manufacturers do this, but almost all will change the voltage signal. Voltage into the sensor, which changes resistance, and then a varying amount returned to the gauge. Just, as the GM instructor told me, like your old girlfriend was; the hotter it gets, the lower the resitance. So, when the engine is cold, or when there is high resistance in the form of corrosion or loose connections, the gauge will read cooler than it should.
Go to someplace like Radio Shack and get a variable resistor of about 0-110k ohms. Connect it to the wires at the sender in the engine bay, turn on the key and sweep the resistor from one side to the next. The gauge should move full sweep. If so, the sender is bad. If not, then repeat the test right at the gauge in order to prove the wires.
Good luck. (Best I remember though, there is a printed circuit sheet on back of the cluster for the TR7 - just be careful to not rip anything, and carefully follow the tracings.)
 
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Go to someplace like Radio Shack and get a variable resistor of about 0-110k ohms....

[/ QUOTE ]

Actually Sam, you probably already have one: the heater fan control on the TR3A. That gizmo is a pretty robust variable resistor.
 
Hello SammyB,
the simple check is to simply disconnect the sender cable and short it to earth, the instrument should read full scale. (This also applies to the fuel gauge.)
A lot of instruments are heavily damped and take time to come to their correct reading, I would turn the ignition on and wait (Don't start the engine). I suspect that in your case you will see the temperature gauge climb to 'hot'. Disconnect the sender cable at the sender and wait a few seconds to see if there is a change, it should gradually drop to 'cold'.
If it does not then:-, the sender cable has a short
the gauge is faulty, or wrongly connected.
If it still reads 'hot' then the new sender is faulty or the wrong type.

Alec
 
The gauge only climbs with the engine running -- at least initially. If I go out, turn the key and not start the engine, the gauge will read fully cold.

If I run the engine, the gauge will be fully hot within about 30 seconds. If I then disconnect the wire, it will go cold.

If I reconnect the wire, turn the engine off, then turn the key (but leave the engine off,) the gauge will immediately come hot.

I have the same results with both the old sending unit and the new one I just put in.
 
Hello Sammy,
well it is definitely the sender that is at fault, the rest of the system is behaving as it should. Unless, as a long shot, the terminal on the wire is touching earth when you connect it?

regards,
Alec
 
how hot is the air blowing out of the heater?
 
Hello Sammy,
Philman may have a good point and we've missed the obvious. You say the hoses are warm and cold, but what about the engine?. You could have a stuck thermostat or, less likely, a sheared water pump drive.

Alec
 
I've already replaced the thermostat and the temp sending unit.
 
You guys have used up all my good guesses, sure sounds like a short but only doing it when the engine is running kind of shots that in the head. Wish I lived closer, sounds like a interesting problem. Keep us informed. Wayne
 
SammyB, Wayne here again, thinking about your problem, I have one thing you might want to try. First off does you fuel gauge read high, if so you could have a bad voltage stablizer, it gives a control voltage of about 10 volts to these two gauges and when it goes amuck these gauges read either high or not at all. Just unplug the wire to the temp sending unit and check out the voltage, if more than 10 volts you have liftoff on the problem. Wayne
 
My fuel gauge has never worked /ubbthreads/images/graemlins/wink.gif ... Wayne, I think you just gave me my next "to-do" in troubleshooting this.

Thanks...will let you know what I find.
 
If you do that test you will probably want to use an analog voltmeter. On a TR4 at least that isn't a real 10V but a fluctuating voltage (0 to 12) that sort of nets out to 10V. A digital multimeter will produce some strange results.
 
Hello Sammy,
Ok, are you then happy that the engine is not hot and the gauge is wrong?, I go back to my earlier post, the fault is the sender.

regards,
Alec
P.S., the easiest way to check the gauge supply voltage is to disconnect the wire at the sender and measure it there. That saves getting to the rear of the gauge.
The voltage stabiliser is a very crude device, basically a bi-metal strip that makes and breaks rapidly giving a portion of the real voltage.
 
Okay,
I checked voltage on the sending unit wire, and it read 9.8 ish. I also took the suggestion and checked for heat in the engine bay. While it took the needle about 30 seconds to sweep to hot, it took about another minute for the heat to come to the defroster. It was a warmer day today, and I had run the car earlier. The interesting thing was that the radiator hoses were still cold.

Since the sending unit is brand new, I'm guessing the gauge is just fried.
 
[ QUOTE ]
Since the sending unit is brand new, I'm guessing the gauge is just fried.

[/ QUOTE ]

Or you got a bad (brand new) sender.

Okay -- someone check me on this as I do not have TR7 to try this on...

Can he connect an analog ohmeter to the sending unit and a ground to test?

If sender is okay -- ohmeter would read about zero ohms at start-up and then slowly rise as engine warms up.

If sender bad -- the ohmeter would act like his gauge and the needle swing up almost immediately.
 
Hello Sammy,
that voltage is about right so it is unlikely that the gauge is not serviceable.
The first thing to determine is the engine hot at the sender location?. The fact that both the heater and the radiator hoses are cold does not mean that the engine is not hot, it could be due to restricted or very poor circulation, as I earler suggested, a sheared water pump drive.
If the engine is not hot and disconnecting the sender wire makes the gauge read cold, then it is a faulty sender (new or not) or the wire terminal is touching earth when you re-attach it to the sender. It could be a section of the insulation has chafed through and touches the engine or a bracket when you re-attach it.
Fault finding is a process of elimination, and as I say, first determine if the engine is hot at the sender unit area, and take it from there.

Alec
 
[ QUOTE ]
Hello Sammy,
The first thing to determine is the engine hot at the sender location?.

[/ QUOTE ]

A laser gauge confirmed that at the sending unit it was still cold.


[ QUOTE ]
Alec

[/ QUOTE ]
 
I don't know if the TR7 is like a TR4, but I had the same problem and after changing everything you changed along with a new guage, I changed the voltage stabilizer and that cured the problem. The guage now reads the same as the infra-red temp gauge I borrowed.
 
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