How do you know it is not accurate?

 

Who are you and what have you done with Dave? I don’t know who you are but you’re certainly not him! You’ve just implied the factory gauge could be accurate.

Are you holding Dave as a hostage? Will there be a ransom demand? Ever seen the movie “Ruthless People”?

 

Why would it show full scale all the to right of the gauge, and the truck is not over heating, I used my digital temp gun, and the all points I test, engine, hoses, raditor, water pump housing, water flow in the raditor nozzle, all reads less than 190 degs while idling but the dash gauge show hot (near full scale)

 

Do you have the heat/AC fan running? These trucks have an odd quirk with the coolant temperature indication. If the cab is not properly grounded, running the fan causes the gauge to indicate hot.

The cab sits on rubber bushings. It requires a dedicated ground cable. It runs between the firewall (near the wiper motor) and the engine block. It’s a bare braided wire.

 

For the possible symptoms when the cab is not properly grounded:

Pour yourself a strong coffee or other drink of choice and peruse this LarryThread™ for more details. The fix was eventually documented in post #59:


https://www.ford-trucks.com/forums/1...l#post21073752

 

All the dash guages for these years are 73 ohms to the left, 10 ohms to the right. For instance 73 ohms is empty, 10 ohms is full on the fuel gauge. All Ford gauges worked the same ever since they went to 12v electrical systems. They changed it in 1987 for these trucks to something different.

Here's a chart that I picked up for a old Mustang. It should be valid. Don't expect the values to be dead on from vehicle to vehicle, but this should be in the ballpark.

 

Im the 3rd owner of this truck, apparently the prev owner changed the temperature sending unit, and install the wrong unit. I found one locally, part 2-8331, the documentation claims to meet the oem oem ford specs, and it seems to track closer to my digital temp gun.I only did a idle test today, and idling, with no ac, no heat on, 90 degr, it was now showing about 75% of the dash gauge. I need to take to drive the truck to make sure it works on the road next.

BTW, my fluke measured 230 ohms, with ODA temp @ 90 degf prior to installing it, and the old sending measured 266 ohms @ 90 degf

 

I'm very confused by your resistance readings. Per the chart above, at 90F, you should see about 75 ohms. Where exactly are you measuring the resistance? Was this on the bare sensor on your workbench? Measured from the connector on the dash?

If reading somewhere between 230-266 ohms, the dash gauge should point far to the left, as if the key was switched off. Not sure how you got those very high resistance values, yet the gauge is pointing 75% to the right. That needle position would correspond to approximately 20 ohms, not the 230+ you've recorded.

Did you check for the cab ground as previously suggested? It's right out in the open and easy to spot.

If your new temp sensor is actually reading 230 ohms at 90F, it is defective. Do you have another meter for comparison? If you take it back, don't get a replacement from the same source. It could be from the same potentially detective batch. I recommend a genuine Motorcraft SW2328 instead. They are available from many online sources, such as RockAuto, Amazon, etc.

 

You can look at the chart and roughly see the temperature versus resistance is not linear.

130 to 160 is 30 degrees. That is 78 to 50 ohms which is 28 ohms. 30 degrees versus 28 ohms, pretty close ohm/degree.

160 to 200 is 40 degrees. That is 50 to 26 ohms which is 24 ohms. 40 degrees versus 24 ohms, we are getting off.

90 degrees that he was at (a hot room temperature) would have a larger resistance than 78 ohms. 90 to 130 is 40 degrees. He got 230 ohms at 90 and 78 ohms is 130. That is a difference of 152 ohms. You would think it would be closer than that. That is 40 degrees versus 152 ohms. But we now see why Ford put "NORMAL" on the gauge and not numbers.

 

I think you will find most sender are not linear.
I have a fuel sender on a 70 AMC and when I had custom gauges made for it they told me they just did a set for a AMC fellow and ran into that.
They had the full / half / empty readings and told them to set them to the same for my gauge.
Oil / temp / vacuum was not an issue as they supplied the senders for them.
Dave ----