89GT Temp Gauge

Fixin1up · 06-06-2001, 04:48 PM

The temp gauge wasn't working when I bought the car so I replaced the sending unit next to the distributor. After that, the temp guage worked but my voltage went down. The next day I went to start the car and the battery was dead. The battery is brand new. I unhooked the wire from the sending unit and the voltage goes right back to normal. Any thoughts on this would be greatly appreciated.

MDM · 06-06-2001, 10:58 PM

The factory gauge sends a positive electrical charge down the single wire to the sender, which is grounded to the motor through the passage it is threaded into. The sender acts as a resistor, giving different readings based on how hot it is. Since your voltage gauge shows a drop when your water temp sender is connected, the problem could be rather significant electrically, especially when you factor in the dead battery. Rather than spending time tracing the wires and pinning it down, you could remedy the situation and greatly improve reading accuracy by axeing the factory setup and installing a nice 2 5/8 autometer electrical gauge.

Fixin1up · 06-07-2001, 12:12 AM

Thanks for the info. Ordering a gauge tommorrow. Also the fuel gauge reading is off. When the tank's full the needle is way above the F and at empty it's a little above the 1/4 tank mark. Is this an easy fix?

QuantumMotorsports · 06-07-2001, 12:29 AM

You can fix that prob pretty easy. Do you have a digital multi meter or an ohm meter? You need to find out the resistance of the sending unit when it is at full and empty. Mine is approximately 100 ohms at empty and 10 at full. To change the range, just find either a resistor or potentiometer (variable resistor) and put it in series with the sending unit (splice it into the wire coming from the sending unit to the gauge) This will raise the resistance of the total circuit and bring the gauge down. If your range is similar to mine (mine is from an 88 turbo coupe thunderbird) you should probably either get a 4.7 ohm resistor (it's a common size at radio shack) or a potentiometer around that range would be better. You can usually find a 10 ohm "pot" at radio shack. If you need help on exactly how to hook it up, just post again and I'll draw you a diagram. Good luck. You might be able to fix the water temp sensor with an ohm meter too. But if you don't mind having an aftermarket gauge, go for that.

------------------
Michael Black
Quantum Motorsports
Norman, Oklahoma

1988 Merc Cougar 5.0 HO, Ported and Polished E7TEs, 2.25" offroad H pipe w/ 2 chamber flowmasters, T5 tranny
New E/T!!!
14.626 @ 94.94mph, 2.194 60' time, a little better than last time. Keep in mind this thang is heavy (3590lbs) Finally 14s!!!!

06-06-2001, 04:48 PM	#1
Fixin1up Registered Member Join Date: May 2001 Posts: 7	89GT Temp Gauge The temp gauge wasn't working when I bought the car so I replaced the sending unit next to the distributor. After that, the temp guage worked but my voltage went down. The next day I went to start the car and the battery was dead. The battery is brand new. I unhooked the wire from the sending unit and the voltage goes right back to normal. Any thoughts on this would be greatly appreciated.

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06-06-2001, 10:58 PM	#2
MDM Registered Member Join Date: Feb 2000 Location: Flemington, NJ USA Posts: 166	The factory gauge sends a positive electrical charge down the single wire to the sender, which is grounded to the motor through the passage it is threaded into. The sender acts as a resistor, giving different readings based on how hot it is. Since your voltage gauge shows a drop when your water temp sender is connected, the problem could be rather significant electrically, especially when you factor in the dead battery. Rather than spending time tracing the wires and pinning it down, you could remedy the situation and greatly improve reading accuracy by axeing the factory setup and installing a nice 2 5/8 autometer electrical gauge.

06-07-2001, 12:12 AM	#3
Fixin1up Registered Member Join Date: May 2001 Posts: 7	Thanks for the info. Ordering a gauge tommorrow. Also the fuel gauge reading is off. When the tank's full the needle is way above the F and at empty it's a little above the 1/4 tank mark. Is this an easy fix?

06-07-2001, 12:29 AM	#4
QuantumMotorsports Registered Member Join Date: Jul 2000 Location: Norman, Oklahoma, USA Posts: 597	You can fix that prob pretty easy. Do you have a digital multi meter or an ohm meter? You need to find out the resistance of the sending unit when it is at full and empty. Mine is approximately 100 ohms at empty and 10 at full. To change the range, just find either a resistor or potentiometer (variable resistor) and put it in series with the sending unit (splice it into the wire coming from the sending unit to the gauge) This will raise the resistance of the total circuit and bring the gauge down. If your range is similar to mine (mine is from an 88 turbo coupe thunderbird) you should probably either get a 4.7 ohm resistor (it's a common size at radio shack) or a potentiometer around that range would be better. You can usually find a 10 ohm "pot" at radio shack. If you need help on exactly how to hook it up, just post again and I'll draw you a diagram. Good luck. You might be able to fix the water temp sensor with an ohm meter too. But if you don't mind having an aftermarket gauge, go for that. ------------------ Michael Black Quantum Motorsports Norman, Oklahoma 1988 Merc Cougar 5.0 HO, Ported and Polished E7TEs, 2.25" offroad H pipe w/ 2 chamber flowmasters, T5 tranny New E/T!!! 14.626 @ 94.94mph, 2.194 60' time, a little better than last time. Keep in mind this thang is heavy (3590lbs) Finally 14s!!!!