1989 Nissan Sentra Temp gauge wont work

Which one are you checking? the coolant temperature sensor or the coolant temperature sender/gauge

the coolant temperture sender/gauge. The gauge on the dash that tells you how hot your engine is running.

The sender/gauge unit is located above the oil filter, is this the one that you're bridging? You might wanna change it out/check the wiring and connections behind the gauge

I have already changed the sending unit. WHat is making that clicking sound when i bridge the wires?

You could be back feeding these components thru the computer .

Thanks for your quick response. Guess what! i just cant seem to read the schematic diagram u sent. I know the gauge works, for when i pull it out of the instrument panel, run a current through it, it moves. just dont know what to do next. Could it be a bad computer?
thanks

Ifit has 2 wires you are shorting out he coolant sensor and can do damage to teh cars computer. The one Raz spoke of above the oil filter (1 wire) is the temp gauge sender.ground the single wire to the engine block, the gauge should rise to H if not there is either a problem in the wiring between the sender and dash or the gauge is bad, if it goes to H, check your coolant level if you have already replaced the1 wire temp gauge sender, they cant read air, only fluid

There is a one wire sensor above the oilfilter and that is the oil sensor. How do i know? well when i unplug it and ground it, the oil light comes on. The temp sending unit has two wires. Please find attached a pic of it. img]https://www.2carpros.com/forum/automotive_pictures/231480_CIMG0002_1.jpg[/img]

going off what the locator tells me for the sender. Temp senders DO NOT have 2 wires. coolant sensors do.If there isnt a temp sender only a coolant sensor, then the ecm tells the gauge what to read, the problem may be there

OK. cool. thank you for the info. So now i know. it is the coolant sensor. ok so i will have to change the ECM. i want to go to the junk yard to get one. HOwever, what do you look for when choosing an ECM? Mine has a big number 56 written on it. do i go by this number or by the serial number? do all the numbers on the old one match the replacement?
Some additional info. There is also a sensor right above the thermostat housing which turns on the radiator fan and this one works. Those are only the two sensors that deal with temperature i have seen.
thanks again

All of the numbers should match

I read that on older cars like mine there are three temperature sensors: one for the fan, one for the coolant and one for the ECM. Is this true and can you tell me where, (if it exists) the ECM gauge is?
thanks

i meant the ECM sensor

Ok you were right! problem solved. the sending uint, the unit responsible for sending the signal to the temperature is different from the coolant temp which was what i kept changing. You were so right the sending unit is above the oil filter. All i had to do was clean it and the thing is now working. you guys are the best. thank you so much. i was about to purchase an ECM unit. Thank you again guys