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1985 720 testing cluster volt regulator


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what would I do without ratsun?


 my 1985 720 DX 4x4 5 speed and I have been in Hawaii now approaching one year and surprise now that my old once beat to shit desert truck has seen more rain in a year than in two decades before it has developed an electric issue. I watch my gauges well and noticed the temp gauge creeping higher than its normal under half way mark then settle down lower at red lights then later back on the road get to high for comfort not trusting the gauge but not trusting myself either so pull over let it cool go straight home as it does similar rising falling then rising further freaking me out. once there I saw no signs of over heating coolant level, overflow reservoir, oil all seemed and smelled normal so I went to testing the sender in the intake and it reads with the ohm meter when I tested the temp sender wire for voltage I got random levels flashing on the meter from under 1 volt to 9-10 volts but never steady. shortly after the fuel gauge just shot to past full and stayed when the engine was running or with it off and key turned over. I did start the engine with jumpers ran to the intake and temp sender itself wire unplugged and read the resistance as the engine got to temp it fell linearly and leveled off slightly higher as the thermostat opened. 


 I read some posts about the regulator behind the fuel gauge that regulates the water temp gauge as well and got to them cleaning the entire cluster assembly along the way and I found continuity between all the wires in and out of the regulator and gauges though it does look a little cooked at some of the solder points and a few cracks in a component I'm unfamiliar with. I am not all that competent when it comes to wiring so I may be missing something obvious. but once I had it reassembled there was no improvement with the temp sender wire still sending erratic voltages with high spikes the fuel gauge seemed to work but the volt meter bounces just slightly as well and the turn signal indicators in the cluster don't illuminate or cut in and out. the alternator puts out 13.9v battery sits at 12v engine off it is believed to be original but the gauge behavior exists even with the engine off so wouldn't that rule out the alternator? I have photos of the Water temp and Fuel gauge regulator. they are on Flickr I will have to add them another way

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I am with Charlie on this since the temp comes back down when you slow down. I would suspect the radiator is restricted on the inside first, fins second. Look into the radiator and look at the tube openings. Any crust around the edges of the tube is just the tip of the iceberg. The radiator can be cleaned with muriatic acid or CLR. Out of the truck of course. The fins rarely clog but in Hawaai they corrode. The fins may look ok but there is not enough metal left to transfer any heat. Poke at them with your finger and see if they bend or crumble.

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With engine off, try to spin the fan blades with your fingers. They should turn smoothly but with firm resistance and should stop as soon as you finger stops pushing them. You should not be able spin them freely.


The clutch fan is fluid coupled and when moving air at about 1,500 fan RPMs the effort is more than the coupling can sustain and though the water pump can spin faster the fan cannot. At these speeds the vehicle movement is also enough to push air through the rad for cooling. There is also a thermostat in the clutch fan that senses an over-heat and releases more clutch fluid internally and the fan will increase speed to about 2,300 RPMs max. If the fluid coupling is slipping more than designed to do, not enough air will be pulled through the rad. It will over heat driving around town but can idle perfectly.


If the fan is firm to turn, it's probably fine.




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Thank you guys for the input. Checked through the rad closely and its in great shape its about 3 years old and not a cheap platic tank one but copper and brass fan clutch feels good too rad has shroud and i can feel plenty of flow at idle thermostats opening as it should. Now the gas gauge is going to past full and im getting intermittent issues with the turn signal indicators also the voltmeter needle bounces just slightly even with engine off. Is there a chassis ground inside the cab anywhere? Ive been getting water leaks through the seam in the cowl into the drivers footwell worried i have a bad ground somewhere causing electrical gremlins

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Get a volt reading across the battery cables while running. Is it jumping around???


Ground as far as the gauges are concerned shouldn't matter.  Ground for the gas and temp are the senders in the intake and gas tank.

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Check body to ground wire it is located at the back of the valve cover and connects to the body. Often gets disconnected and never hooked back up. Corroded body mounts make for poor grounding. You are now in a salt air environment cleaning grounds can sometimes really clear up mystery electrical issues.

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good call bottomwatcher I went through all the body to engine grounds and cleaned them up and their contracting surfaces seems to have done the trick gauges are accurate and stable only testing was running in the driveway after cleaning but feeling good about it. thanks again for everyones input 

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