74 F100, 360 2wd, The temp gage wasn't working when I got the truck or so I thought, I put a new sending unit in it and a gage from another f100. I grounded out the lead wire and it pegs to hot. So I'm told that means the gage is good? Next I checked the thermostat, it's a 195. So next I did this test on the sending unit. I removed the sending unit, grounded the body and attached the lead wire. Put the sending unit in a thermos cup of water that started at 210 and was down to 194 by the time I took the pic. This is as close to normal operating temp as the gage got. I would like to keep the stock gages but know at 195 it should be in the mid normal range atleast. Any ideas guys? It's either a bad sending unit (it's new but even new ones can be bad) or the gage is bad? I"m open to any ideas. One thing i just thought of, would having pipe tape on the threads of the sending unit prevent it from getting a good ground and cause a low reading because it changes the resistance? Heres the pics of what I did. and where the gage sets at 194.

 

Sounds like your troubleshooting it on track, I would replace the sending unit again and go from there. Here are some words of wisdom from a fellow FTE member that I saved a whole ago.

"Temp sensor trouble shoot:
Assuming it's an electrical sensor and not a Bourdan tube (long skinny tube from the sensor site all the way to the gauge) probably one wire to the sensor? If so, check resistance through the sensor. Connect the + wire of the multimeter to a 12+ line, then the ground wire of the multimeter to the wire post on the sensor, engine cold.

Turn the engine on, and as it heats up you should see the resistance changing. If it doesn't, your sensor is probably shot. On a Ford factory gauge I think I read elsewhere that they go from 20-70 ohm resistance, aftermarket I have no idea.

Also pull the wire off the temp sender and turn the ignition to "run" position. What does the gauge do? If the needle doesn't move then the sender is bad. If it still swings all the way over to "H" then check for a short in the wiring between the gauge and the sender."

 

Try your test with the other gauge out of the truck. If there is resistance between the sender and the gauge, it will read lower. Just a thought.

 

Thanks, I guess I'll try another sending unit in the morning. The current sending unit moves the gage up about 1/4" but not into the normal range. So I know the resistance is changing. What I"m not sure of is.. the resistance changing as much as it should or is the gage not responding the amount of resistance beign sent?

 

I doubt that the thread tape is causing problems..just use a thin strip on the stud end/elec connection end to make sure.

 

I'm going pick up a new sensor this morning and run the same test on it, if it gives the same result without the tape on the threads, I'll know that part is working as well as it can and what temp does to the gage needle. After that, I have a spare dash cluster, I might plug it up, to see it works and if the gage reads any different. I've got to take it out eventually anyway to work on the dash lights for the turn signals working intermediately.

 

Sounds like you could have some ground gremlins in your dash too - the 20-70 range sounds right, so when you dead-grounded the wire, you could still have 20 ohms resistance through the ground and have it peg the gauge - and that much resistance is a little less than half the gauge sweep, putting your gauge about where it is in relation to where it should be (assuming halfway is about 180? not sure on that). Maybe try a jumper to the cluster ground, see if that changes it?

 

Thanks for all the help guys, not sure what did it but the old thermostat said 195 and I put a new one in at 195, tried a new temp sensor and got the same result out of the water test I did to start. After I put it all back together and was making sure the stat didn't leak, I put the temp probe in the radiator and the with the 195 the stat opened up and maintained a consistent 199 temp. Next surprise was the temp gage is not readying just under middle of normal. Seems to be working just as it should now, comes up to temp, doesn't boil over and the gage works. Thanks again everyone.

 

I'd be looking at a ground issue. It's very easy to get the plug not seated quite-right when plugging the cluster in. You also need a good ground from cab to block. The gauges run on ground (or resistance to ground)

I had problems with my gas gauge that went away when I ran a new ground from cab to block

 

I'll look into that, I've got a new sending unit in my gas tank and the only goes up to about 1/2 full when it's topped off, then it goes to E after what should be about 1/2 of a tank left. very frustrating, I grounded the gage and I know it works like it should, I really hate to drop the tank again, but i think I'm going to have to to rule our a bad new sending unit or bad float that is sunk.

 

Couple of ideas after the fact:
1) If you think the thread tape is affecting the ground, just jumper the outside of the sender to ground. No need to pull it, if you see no difference it is not a ground issue at the sender.
2) Check resistance to ground at the dash end of the wire and and at the sender end. Should be very close values, if not you have a problem in between.
3) Gas sender - you can adjust the float rod to raise or lower the float - right now it sounds like its total travel bottoms out at mid-tank. If you take the filler hose off you can reach in with a dowel and raise/lower the float. Adjust the float level so that it goes more or less top to bottom. If you want to get sophisticated, you can make a couple of jumpers from the sending wire underneath the truck and a ground, and actually observe what the gauge says after each bend of the float rod. Be sure to disconnect the jumpers before you pull the sender!!! Sparks around gas fumes are very bad things and you could wind up charred.