[Monkey Wrench]

Im kind of assuming but the temp sensor either changes resistance between its wires or maybe it changes resistance to ground. if its not too hard to remove you can put it in a pan of water, hook it to an ohm meter and watch the resistance change.. that way you should be able to see it changes as you heat a pan of water up.

if you know what the resistance range is and weather you have two wires with resistance between or one with resistance to ground , then you can always sub in some resistors and see what happens at the guage , maybe make sure the block is grounded, you can verify that with a jumper cable from the negative post to engine block. if the jumper cable changes anything with respect to the guage not working, you have a bad engine ground.

at home I have a little piece of kit called a resistance substitution box, you can make one up or there are commercially made ones, all it is, is a variety of resistors hooked to a multi position switch and all it does is makes switching between different resistors faster and easier.. you dont really need one but if you have a few resistors that are about within range you could sub the in to see if the problem is whith the gauge, if you do see the gauge react, then you might not need to go into the speedo unit's wiring.

if you had a spare sender in hand you can probably connect it to the wiring if you think you can access the wires and don' t want to remove it then just check it out of the cars engine block.. take a thermos of hot water to the car so you can immerse the thing in water of different temps while connected and not part of the engine.

it sounds like you found the wires as you are fiddling with the ribbon cable. you can check between there and the end of the wire for a break with both ends unhooked, measure resistance and check its not grounded by rubbing somewhere.

sometimes if I want to check continuity or check that I have the right wire in a bundle I just feed it 9V from a 9V battery at one end you could use a 1.5 volt too..

then go looking for the wire with 9V , no other wire will have 9V .. I just find it saves confusion.. then once you know you are on the right wire, you can check resistance and continuity and to see if it's either hot or grounded.. if both ends are disconnected then its not confused by some other current path.

sometimes I work on old antique electronics, modern resistors are pretty stable but old ones tend to drift up in value to higher resistance but they can go open too.. If they are all solderd to a circuit board then just placing a meter across it can be inaccurate because the board may provide some alternate current path, lowering resistance.. so its easy to know that is not the case if you un-solder one leg.. it is however impossible or some other alternate current path to RAISE the resistance..

If I put probes across a resistor that is in place and measure it and find more resistance than expected , in that case I know its a bad resistor.. because no alternate current path can raise the resistance.

If I measure the resistor and find it has less resistance than expected, that can be caused by an alternate path so unsoldering one end takes that ( alternate path ) scenario out of the equation.

most digital multimeters ( DMM"S ) do provide power via their internal 9V battery in order to read resistance in ohms. so if the circuit is sensitive electronics they may run on lower voltages so you can be a bit careful as to not provide ( 9V) power to any sensitive ( likely 5V) electronics inadvertently, when testing..

DMM's can load a circuit that is very sensitive to voltage, a scope or a vacuum tube voltmeter can usually test without loading the circuit.. most general car wiring is not very sensitive to things like that. I'd just be a bit careful if you go probing PCB boards, static shock can blow microchips too..

ribbon cables are notorious for having bad connections, they are generally ok if left alone but repeatedly plugging and unplugging may damage the microscopically thin copper traces on the mylar strip.. might not hurt to look with a magnifier or take a cell pic and expand it to see up close. a scratch across a trace can break continuity.
sometimes a DMM can come up with odd ( erroneous) readings, because it is so sensitive it can for example measure some odd voltage and display it, but that voltage may not have any appreciable current..

sometimes you can avoid that confusion by instead using an analog meter ( one with a needle) or even a battery or one of those cheapo car wiring probe testers that contains a battery. they are harder to be confused by.


Jfrahm is leading you on a good path, and he understands the particulars of 944 circuitry much better than I do. I hope my post didn't confuse things.