My temperature gauge on my 1987 F-150, 4.9 doesn't work. I thought it was the temp sending unit. I replaced it but it did not fix the problem. I ran my code scanner on it and I found 51 "signal voltage too high" for the sending unit that I just replaced. Do you think the gauge in my dash decided to break? I have 172,000 miles. Any ideas? Thanks

 

Take the lead off the sending unit and ground it, the temp gauge needle should peg out. If it doesn't, it's bad.

What is actually wrong? Does it show anything?

 

There are two sending units. One for the computer, and one for the dash gauge. My guess is that you replaced the wrong one. The dash sending unit is on the passenger side of the block just under the exhaust manifold.

 

Well I drove the truck the last few days and watched the temp. gauge on my dash. It does move. It is very slow and with 85 degree temps and the a/c on all the time the motor should be warming up fast. The gauge is reading low (on the cold side of the gauge). I was not aware of a sending unit for the dash gauge. Does it screw into the block under the exhaust manifolds? I do have the 4.9 six. Thanks for your help guys.

 

Then you have this issue...

https://www.ford-trucks.com/forums/s...d.php?t=375870

 

Mustanggt221 after reading the four pages of your link I came to the conclusion that the factory gauge blows. Thanks.

 

I got a 51 once also, that will happen if you pull the wire off the sender(the sender near your t-stat housing) while the igniton is on. Clear the code, and I hope you don't see it again. The real gauge sender is under cylinder #6 intake manifold, in the side of the block. You can barely reach it while on your back under the truck, and you sure can't reach it while sitting on top the engine. If you can reach the one wire sender,, try pulling the plug off the electrode a few times to scratch up the plug and electrode. You may just have some dirt/corrosion in the connector. If you have any luck reaching up there try cleaning the electrode and connector with some q-tips and isoprpyl alcohol or electrical contact cleaner.

PS my gauge works fine, it is stock, original, and seems to be steady, I have to occasionally clean the sending unit plug off, but it works OK. Lets assume that your gauge is OK, and patiently try the easiest and cheapest repair methods first.

 

Cool. I will try to clean it up. Thanks.

 

You didn't get the point of me giving you that link? It's not the sender or gauge, it's air pockets stuck in the cooling system. The stock gauge is ok...it's not as good as an autometer but good enough...

 

Air pockets were one of the many ideas thrown into that thread. Thanks for your help.

 

I didn't read that good link well enuf either, It is a great theory, and basically is implying that the cooling systems of many trucks develop air bubbles.

Has anyone actually looked at the sender postion and the cavity that the sender screws into to?

Of all the nooks and crannies an air bubble could develop, why would it form there, and stay there, when there are more likely places for air to get trapped such as the top of the engine, or upper radiator hose? The sender seems to be strategically placed in the middle of the engine, is it possible that the engine designers selected that God-forsaken location for a sender because there was no chance of air getting trapped there??

My gauge flucuates from day to day, but larger meter errors that persist for days and weeks may be sender wire related.

 

I still say its a grounding/corrosion issue. Recall that the sender is 1 wire which means its passing a voltage thru to the block (ground). And where is the block getting ground? That depends on whats running the engine. If the ALt is charging the battery then the engine is running off the alt which means ground path is thru the block to the alt brackets to the alt. If the battery is running the engine (idle?) ground path is thru the block, to the battery Neg cable on the block. There are several connections to check and clean.
When's the last time you pulled your alt and cleaned the mounts?
Just my 2 cent rant...
Popa Tim

 

I love this site when people are putting out good ideas and trying to solve problems!

 

Thanks 924. This has become one of my favorite sites too!
Popa Tim
ps - i think the Schwinn 12 speed is hilarious

 

Popa Tim, I hate to say it but I know it's not the grounds...

I took my cap off and topped the system off a little more, I got some airbubbles to come out and the temp is reading better. More than 1/2 the time it's reading correctly now.

Pretty much everything in the engine bay is brand new because I just re-did it all, no ground problems, and my autometer doesn't even ground that way.

It's a perfectly reasonable thought as to what is wrong, I just happened to be in a different position to show otherwise...

Also, if it were the ground...it would flicker and vary randomly on my needle. I know it's not a ground because it never hangs in the middle, just on the low and high side...(think voltage and temp at the same time).

It never reads say 150...it'll read 200ish, then 120ish. It's consistant, it makes me believe that the gauge is reading perfectly correct, it's just that what it's measuring is not constant. The air bubble is probably 120ish and the water is probably 190-200ish.

You were probably thinking....bad ground = low voltage to gauge = low reading...makes sense...

but with the stock gauge you can't tell that the temp is varying, but it is. It's not just "reading low". A real gauge shows the temp variations. Since my gauge fluctuates, if it were a bad ground...that would have to mean that the voltage is spiking...and if it were spiking, it wouldn't just read 120 and 200, it would read all over the place.

Sounds logical, right?

The air will not get trapped anywhere other then being in a cavity, where the water flow doesn't propel it further down the system. The sending unit is in a little cavity, perfect place.

I'm still trying to work this out myself, but at this point I feel if I can get more air bubbles to come out of there like I did last time...then it may get better like it did the last time...til it gets fixed....I feel like I'm on the right track.