temp and oil pressure gauge problems

[kohlbeezy]

so i swapped in a 4.0 out of a 91 expo in my 85 ranger. ever since the temp and oil pressure gauge do not work properly. they get pegged hot and high as soon as its started. i replaced the both oil pressure and coolant temp senders. i thought maybe because the block wasnt grounded and i just fixed that issue. it only had the head ground. both gauges still do not work. when i take off the coolant temp wire, the gauge goes to cold. when i plug it in it goes to above hot. the wires are still in the factory loom so i dont think it could be grounding out on anything. is there something i am missing?

 

[SenorNoob]

Couple questions.
Do they go to max when the key is turned over to run or after the truck starts?
85 dash or from the donor?

That should help anybody else who looks figure out what might have happened.

 

[RonD]

Assuming 1985 dash is still in place.

The '85 oil pressure gauge is probably a Real Oil pressure gauge, which is good
You will need a real oil pressure sender for that, google: oil pressure PS60

That is a universal sender, it is larger so you may need an extension or adapter.
Or look on your old engine and use that sender.
In the mid-80's Ford switch over to an oil pressure switch instead of a "variable resistor" sender(PS60), what this means is that when oil pressure is above 6psi the switch closes and Grounds the gauge, on a real gauge this would send it to max/high.
Ford added a 20ohm resistor to the back of the gauge, so when switch is closed it would show about 1/3-1/2 on the gauge, if needle varied it would be because of voltage not oil pressure.
You could add a 20ohm resistor to your current sender and it would do the same thing, but most people prefer a real gauge.

Coolant temp gauge is a puzzle but could be reverse sender, not sure on that year, does needle move down as engine warms up?
If so then it is a reverse sender, get your old sender, or get a "new" old sender

Fuel gauges on the '85 would be reversed, they changed in '86, and since you didn't mention fuel gauge issue I "assumed" you still had the '85 dash/gauges

 

[kohlbeezy]

No sorry. It's the dash from the 91 and complete wiring harness. Not spliced at all. All other gages work.

 

[kohlbeezy]

Couple questions.
Do they go to max when the key is turned over to run or after the truck starts?
85 dash or from the donor?

That should help anybody else who looks figure out what might have happened.

They go max when key is on or engine is running. 91 dash no splicing of harness. Gauges don't move while engine is running.

 

[RonD]

Well that is odd, I thought it was '86 but it was '89 when Ford switched to (empty)16-158(full)ohm sender/gauges
Pre-'89 Fords were 73-10 Ohms so reversed, unless you changed the fuel pump assembly the gauge would work in reverse.


When you remove the temp sender wire the gauge goes to cold, it should go below cold, try grounding that wire, gauge should go to max, above hot.

Same with oil sender, but it is an on/off switch, and has a resistor in the circuit, so if wire is disconnected it will still show 0 oil pressure not below like temp gauge, and if grounded gauge should show 1/3-1/2 not full.

Only thing that comes to mind is that another circuit is using those gauges as a ground, electricity always finds the easiest path to ground, that's why a tail light might glow weakly when there is a missing ground at another light, the tail light is being used as the ground so it glows as power passes thru it, anyone who has wired a trailer knows this sometimes frustrating phenomenon, lol.
Temp and oil gauges might be "glowing" because of a loose or missing ground on the dashboard wiring, just a guess.

 

[kohlbeezy]

the fuel pump assy is from the 91 expo. i just installed everything from the expo into the ranger.