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Dr. Hamman
Dr. Hamman, Technician
Category: Classic Car
Satisfied Customers: 6286
Experience:  I have 30 years experience repairing, restoring, and customizing cars, and pickup trucks.
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Nissan/Datsun 1600: Just bought a 1967 Datsun 1600 and for

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Just bought a 1967 Datsun 1600 and for my first challenge I need to figure out how to get the fuel gauge reading correctly. I filled it up not 3 days ago and it's already at 1/2 a tank. I couldn't have driven more than 30 miles since I filled it up.

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.



It has been over 20 years since I have worked on one of these, but I do remember them. You need to determine if the problem is with the fuel gauge or the sending unit in the fuel tank. Basically the fuel gauge is an ohmmeter reading the resistance of the sending unit in the fuel tank. The sending unit is a variable resistor attached to a float that moves up and down with the fuel level. When this system acts up you can start by either checking the sending unit or by checking the fuel gauge. To get into this you need to understand the relationship between the two parts, that I just described, and know the operating resistance of the system. The sending unit should read 12 ohms full, and 88 ohms empty. So you can either get some resistors and attach them to the gauge wire going to the sending unit to see if the gauge responds correctly to 12 then 88 ohms of resistance, or you can measure the actual ohms of resistance of the sending unit with an ohmmeter. I like to check both looking for problems. The vast majority of the times there will only be one problem, like a bad sending unit, or a bad instrument. Occasionally there will be a bad electrical connection causing a problem, so keep an eye out for that. Because the sending unit is very dynamic, and exposed to gasoline it tends to wear, so the majority of the times when there is a problem, the sending unit will be what needs to be replaced. Still it is a very good idea to test the parts, to be sure. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.


I do not get paid for my work unless you rate at 3 to 5 Smiley Faces or Stars, a bonus is always greatly appreciated. If the answer is not clear, let me know what additional help you need, and I will be happy to assist you further.


Customer: replied 3 years ago.

Ok, well I used an ohmmeter and tested the leads directly on the sending unit. Regardless of the level indicated on the gauge, the resistance was around 66 ohm. When I got gas it took a little over 6 gallons to fill it and considering the fact that the tank holds around 10.4 gallons, the needle seemed to be at the right spot on the gauge. The car seems to be sucking gas at rate beyond what I think it should be. To go through 6 gallons of gas in 4 days and only driving it about 30 actual minutes each day seems excessive, no?

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.



Yes that sounds like super high rate of consumption, is it possible someone stole some gas? I would fill the tank, run it a few days, then refill it. By knowing the amount of gas it takes to refill it the second time you can calculate the mileage. If it is using a lot of gas, and it sounds like it is, I would check the carburetor to see if it is dumping excess fuel into the engine. Also has it been tuned up lately. If an engine is way out of tune it will start to burn way too much fuel. The last thing is look for leaks under the vehicle. Normally if you have a leak you can smell it, but remember gas evaporates real fast, so sometime you wont see a puddle. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.


I do not get paid for my work unless you rate at 3 to 5 Smiley Faces or Stars, a bonus is always greatly appreciated. If the answer is not clear, let me know what additional help you need, and I will be happy to assist you further.


Customer: replied 3 years ago.

And about the Ohms, shouldn't it be fluctuating? 66 no matter how full the tank is seems odd to me...

I didn’t understand that the resistance (ohms reading) was not fluctuating with the fuel level, my bad. The fuel level should fluctuate the ohms reading, that is what triggers the gauge to move. The sending unit should read 12 ohms full, about 38 ohms half full, and 88 ohms empty. The next step would be to remove the sending unit for closer inspection. If you need a replacement sending unit and cant find one there are places that rebuild most sending units. I would dig around on the internet if you need this service. One I have used is at the link HERE. They cant repair all types, but that is where I would start if I was looking.

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