Hello! I'm trying to verify the electrical plug "pin pattern" for a 1989 F-250. More specifically, I need to know if the plug-in for the fuel gauge sending unit is the same for gas and diesel??? The reason,,,,,,,,,,,I'm replacing both fuel tanks in my 1989 F-250 diesel and since neither tank had accurate fuel senders (the guage was always way off) I wanted to replace both senders during the tank repair. Problem is I'm having trouble finding new sending units for a diesel. The gas sending units are everywhere and since the tanks are the same, I know the sending unit will fit in the tank, but I don't know if my diesel wiring harness will "plug into" a gas sending unit????????????

 

The problem is the gas sending unit does not have a fuel return fitting. The diesel has a fuel return system, without a in tank pump. The fuel injected gas engines have a return type sending unit, but they also have a in tank pump mounted on the sending unit.

I believe LMC truck has the correct sending units for the diesels. But they are not cheap.

 

I'd still like to know if the electrical plug is the same??? The pictures of the gas units I looked at appear to have two hose fittings on the top???
Another angle to consider.........Since both of the sending units still work and they both provide inaccurate readings, is it possible the instrument cluster voltage regulator is the root porblem??? Another possibly related piece of information is my oil pressure gauge always reads low and I did replace that sending unit a couple years ago and it didn't effect the gauge reading. Plus my volt gauge reads really low when the heater fan is turned on?????

 

I don't know what you mean by inaccurate readings. Usually they fail completely. If you fill the tank, and the gauge stays pegged until you get to the last 5+ gallons and then falls rapidly, that's pretty normal for these gauges, and the new units can be worse.

If you are running out of fuel early, then most likely your pickup tube on the sending unit has broken off-another common problem on the diesels. Most people just install a fuel rated rubber hose with a v-cut on the bottom of the hose to extend it back down into the tank.

Your 89 does not have a gauge regulator like the older trucks did. Starting in 87, they did away with that and re-did the gauges so they didn't need it. They also changed the way the sending unit reads and it's value, so that's another thing to watch out for, the 86 and older fuel sending units will not work with your 89 gauge.

Some more info on your oil pressure gauge. Guess what? It's not a really gauge. Your sending unit is just a switch that makes contact when the oil pressure is over about 5-7psi. They put a resistor in the instrument cluster so the gauge will read half-way. So what does this gauge tell you? When you have over 7psi oil pressure, the gauge reads in the middle. If you have 30 psi oil pressure, the gauge reads in the middle. If you have 100 psi, the gauge reads in the middle. If you suddenly have 0 psi, the gauge reads 0. Great engineering isn't it.

I don't know if the sending unit plugs are the same, but I think that is the least of your worries. The only thing I have heard people are doing to try and get a cheap diesel sending unit together, is they are going to jcwhitney and getting a universal type unit, and retro fitting it to the original sending unit frame. A little bit of engineering is involved in that one though.

 

I posted a very similar issue in the IDI forum, as far as gauges not reading right. Mine seem to work as normal, but both read empty at half a tank. I thought it could be the units, but both go at the same rate. I already pulled the front unit and cleaned it up and did the v-notched fuel hose at the end. Some folks in the other forum have mentioned instrument cluster VR, and I've been wondering if there's a way to test it? (86 F350 6.9)

 

If the IVR is bad on your 86-down truck, all the gauges will vary up and down with rpm and electrical load. That includes the oil pressure and the temp gauges. So if your oil pressure and temp gauges read fine, I would rule out the IVR being bad.

 

The Volt & Oil gauges don't work great, but they do work. Wondering where to go from here? It doesn't make sense that both units would have bad readings at the exact same level (Putting about 9 gallons in every time they hit 'E'), and I'm not about to shell out $80 a piece, when the resistance readings on the gauge I pulled were within the specs I read here...

 

I forget, messing around with it you probably know more than I , but is the low ohms on your unit empty(I believe it's 0-90 ohms?)? If so, it relies on the ground bolted to the sending unit to create this 0 ohms. It then runs through the harness and is grounded somewhere. If this is true, and the ground was dirty, then you would never get a true "zero" or low reading, you would always have some resistance in the circuit, causing the gauge to read high. You could experiment by running a scrap piece of wire from the sending unit to a known good ground in the cab.

The more I think about it, I believe the 90 ohms may be empty. My theory still may be true though. Let's say 1/2 tank is 45 ohms. If you have 45 ohms resistance in the ground, the guage would read empty, but the tank is 1/2 full.