Temperature sending unit

other_shoe · Nov 22, 2010

I finally got my cooling system all back together and started it up. Things seem to be working well, but after about 10 minutes the temperature gauge is now pegged on the cold side as if it is not getting a signal.

Since I overheated the car and the current unit is at least ten years old, I'm thinking I should just replace the sending unit before going on a hunt for suddenly broken wires or a failed gauge. Is this reasonable or is there something obvious that I should be looking for first?

Thanks in advance.

TuneUpsNSuch · Nov 22, 2010

Hnmmmmmnm

First, make sure that you have proper electrical grounds between the battery and body as well as the engine and battery. Next, with the key on and the engine off, take loose the temp. sender wire and with it held away see that the gauge is at one end of it's sweep, then ground the sender wire and make sure the gauge goes to it's other travel limit. Only short the wire long enough to check the sweep of the gauge. That is the quick test for the gauge and circuit. If the gauge performs normally then your problem is either a bad sending unit, poor connection at the sending unit or the sending unit is not making a proper ground. Sometimes people will put teflon tape on sending unit threads, but for one wire senders this can interfere with the ground circuit of the sender. If you have an ohm-meter, you can look up the temp. resistance specs for the sending unit on your car. With that spec in hand you can place one meter probe on the sender terminal and the other at the battery ground and see whether or not you are close to spec. If not, move the probe from battery ground to the engine and read the meter again. If still no good, move the ground probe to the sending unit base. If the reading becomes good when you touch the engine, then it's a bad ground to engine, if the reading becomes good when you touch the sender base, then the threads of the sender are not making good contact with the engine. Also, check the voltage coming down the sending unit wire, some systems (older analog type) use a voltage regulator for the gauges and should read about 8 volts.

quiz on monday . . . . LOL

other_shoe · Nov 22, 2010

Wow. Thanks for the quick, detailed, and very helpful advice.

tx65coupe · Nov 23, 2010

It looks to me like he pretty much summarized that. I don't really have anything to add other than this. You can take the sending unit and put it in a pot of hot water and using the multimeter see if the reading changes.

2+2GT · Nov 23, 2010

I have done some experimentation along these lines, and have yet to find a replacement temp sender that's even close. If yours is dead, find a nice used or NOS Autolite or Motorcraft.

other_shoe · Nov 23, 2010

I used all of this advice and a couple of long wires to run part of the test. After I found a good ground while sitting in the driver's seat (mounting screw for the courtesy light, which burns plenty bright), I found the gauge works.

That said, the voltage coming down the wire to the sender fluctuates wildly. It peaks in the 6 V range but drops into millivolts every second or two. I hadn't found the specs on resistance before I had to run some errands (daily driver is getting badly needed new tires and alignment), so in order to not have things dangling reattached everything. Now, I'm getting a reading on the temperature gauge again. It pulled from pegged cold into the low range. This is plausible since it's in the high 30s here, and I was doing highway driving with fan that always rotates with the crank. The radiator and hoses did not feel hot.

2+2, I have no reason to believe that the current sender is of reliable stock. Indeed, when the car overheated this summer, I would have liked it to have gotten much higher on the meter before puking fluid, but it was the hottest day of the year and the thermostat seized. Still, it was at 3/4 when I turned the wipers on and took evasive action to get to the roadside.

At this point I think I have some old wires that are not reliable, a sender of dubious quality, and likely some problems with the voltage from the instrument cluster. I'll post on the last in a separate thread since the problems extend beyond the gauges, but the short version is that the cluster lighting intensity and frequency of the turn signal varies with engine speed, while headlight intensity does not.

Sorry for the long post and thanks for all of the responses.

tx65coupe · Nov 24, 2010

You may have a problem with the instrument cluster voltage regulator. If so, I suggest getting or making your own electronic one.

I wouldn't start a new thread. This one already has background on the problem.

http://chris66dad.tripod.com/id29.html

http://cgi.ebay.com/ebaymotors/Mode...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

2+2GT · Nov 24, 2010

other_shoe said:
I
That said, the voltage coming down the wire to the sender fluctuates wildly. It peaks in the 6 V range but drops into millivolts every second or two. I hadn't found the specs on resistance before I had to run some errands (daily driver is getting badly needed new tires and alignment), so in order to not have things dangling reattached everything. Now, I'm getting a reading on the temperature gauge again. It pulled from pegged cold into the low range. This is plausible since it's in the high 30s here, and I was doing highway driving with fan that always rotates with the crank. The radiator and hoses did not feel hot.

The CVR is a bimetallic relay, which actually clicks on and off to produce an average 6-7 VDC. The gauges can't react fast enough to see that, but your voltmeter will.

Pony66 · Nov 24, 2010

After putting in 2 brand new units that didnt work at all, I bought one from NAPA and it works perfect.

2+2GT · Nov 24, 2010

Yeah, the hard part is paying for them.

other_shoe · Nov 24, 2010

2+2GT, thanks for the explanation.

tx65coupe, thanks for the links. I'll probably go the ebay route.

Autozone has the senders for about 7 bucks, but I want to find the specs on the resistance before I start trying to find one that works.

2+2GT · Nov 24, 2010

other_shoe said:
2+2GT, thanks for the explanation.

tx65coupe, thanks for the links. I'll probably go the ebay route.

Autozone has the senders for about 7 bucks, but I want to find the specs on the resistance before I start trying to find one that works.

15-19Ω @ 100°C

TuneUpsNSuch · Nov 24, 2010

Having formerly owned a '69 GT and a '74 Ghia hatch', I have been down this road many times. The later 12 volt air core type gauges are a whole 'nother animal to troubleshoot . . . . keep it classic.

other_shoe · Nov 24, 2010

Thanks, 2+2GT. Luckily, my wife just got a new candy thermometer, so I get the old one with the dim LCD.

TuneUpsNSuch, I don't follow your point. Are you saying don't switch over to the electronic voltage regulator? Or just stick with the classic gauges? Or something else?

tx65coupe · Nov 25, 2010

I got my sending unit from Mustangs Unlimited I think. It might have been from CJ though. It works fine.

I got this thermometer and used some brass fittings and mounted it in the port on my thermostat housing.

http://cgi.ebay.com/ebaymotors/Smal...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

xj220 · Jul 27, 2011

Sorry to resurrect an old thread, but in terms of replacing the sending unit, how easy is it? Can I just unscrew the old one and put the new one in? Assuming it's cold and the thermostat is closed, I shouldn't have to worry about a huge amount of coolant leaking out, right? Pretty basic question, but I wanted to double check.

j69302 · Jul 30, 2011

You will have some coolant come out. The radiator is above the temp sensor, and can gravity feed through the lower hose which will by pass the thermostat being closed.

You can do it, but you will have a small mess. I decided to train he coolant level to the manifold level then just add a small amount to top it off once done.

65ShelbyClone · Jul 30, 2011

Like j69302 said, you'll end up with a coolant spill if you don't drain the radiator(at least partially) first. Just use a short wash tub or bucket so the car doesn't need to be raised. Then you can put the coolant back in the engine when you're done instead of it going into the environment and having to be replaced.

2+2GT · Jul 30, 2011

xj220 said:
Sorry to resurrect an old thread, but in terms of replacing the sending unit, how easy is it? Can I just unscrew the old one and put the new one in? Assuming it's cold and the thermostat is closed, I shouldn't have to worry about a huge amount of coolant leaking out, right? Pretty basic question, but I wanted to double check.

I have done this (on a stone-cold engine), and got away with it. Have a few paper towels handy.

Temperature sending unit

other_shoe

Member

TuneUpsNSuch

Member

other_shoe

Member

tx65coupe

Active Member

2+2GT

other_shoe

Member

tx65coupe

Active Member

2+2GT

Pony66

New Member

2+2GT

other_shoe

Member

2+2GT

TuneUpsNSuch

Member

other_shoe

Member

tx65coupe

Active Member

xj220

j69302

Active Member

65ShelbyClone

2+2GT

Similar threads

Temperature sending unit

Member

Member

Member

Active Member

Member

Active Member

New Member

Member

Member

Member

Active Member

Active Member

Similar threads

Privacy & Transparency

Privacy & Transparency