Dodgeb4ya Posted December 5, 2014 Report Share Posted December 5, 2014 (edited) Why would the fuel gauge on a 1947 chrysler go higher as the car goes faster, actually all the way to full and past at 40MPH? Also just revving the engine-takes a minute or so for gauge to rise to full or above. The car was professionally restored. The tank is a 1/4 full. Sender is wired correctly-blue wire to#1.... yellow/black to #2. Gauge is accurate at slower RPMs. A separate ground wire was used to ground the sender to the body and frame. Generator output voltage 7.85 volts. Has anyone ever seen this happen? Thanks, Bob Edited December 5, 2014 by Dodgeb4ya Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted December 5, 2014 Report Share Posted December 5, 2014 Bob. This is a rare occasion for you to ask and not answer a question and I thank you for the many, many questions you have answered. So I spent some time attempting to find an answer for you. I came up with the following. This may or may not be your problem but you might have a look at the sending unit as well as the wiring to the sending unit. In Chrysler applications, the sending unit increases resistance as the fuel level drops and decreases resistance as the fuel level goes up. When the fuel tank is empty, for example, resistance is high (around 73 ohms). High resistance reduces the current that flows through the fuel gauge, producing little or no movement in the needle. When the fuel tank is full, the sending unit has low resistance (around 8 ohms) so more current flows through the fuel gauge. This heats up the bimetal strip causing maximum needle deflection. Now the needle moves all the way to the full mark. A shorted sending unit or a short in the wiring between the sending unit and gauge would reduce circuit resistance causing the fuel gauge to read full. 1 Quote Link to comment Share on other sites More sharing options...
TodFitch Posted December 5, 2014 Report Share Posted December 5, 2014 Is the ammeter showing anything odd? I am wondering if the ground connection between the body, the frame and the engine is bad which could lead to ground current from the generator (attached to motor) going through whatever path it can find (fuel gauge system). 1 Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted December 5, 2014 Report Share Posted December 5, 2014 (edited) as the car is under acceleration and there is or could be a slight flex of the body, it could be easily grounding out the sender wire some where along the path from the gauge to the sender itself...once at speed or not moving at all, the strain/flex is gone and all returns to normal...I would check this by simply running a set of temporary wires to the sender and repeat said acceleration of the car and needle movement... Edited December 5, 2014 by Plymouthy Adams Quote Link to comment Share on other sites More sharing options...
Merle Coggins Posted December 5, 2014 Report Share Posted December 5, 2014 Does the generator output voltage remain steady through the RPM range? Could an extreme voltage change cause this phenomenon? Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted December 5, 2014 Report Share Posted December 5, 2014 Merle...the gauge itself has a built in 5 volt regulator...it seems to me that by pegging the meter...you have a wire grounding out somewhere..still going to stick to torque relational pinch Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted December 6, 2014 Author Report Share Posted December 6, 2014 I have not taken the sending unit out or gauge out. The fuel gauge will move from 1/4 tank which the customer says should be about right to above full by just sitting in the car and revving the engine to about 2000 RPM. Let off on the gas and it will slowly drop back to 1/4 tank. The amp meter works normally and smoothly ...start the car and it charges to about 20 amps for maybe 2-4 minutes and drops back to maybe 5-10 amps max while driving. Stays at zero shut off. I did run a separate body ground to see if a change occured-non did. I did find out the whole instrument cluster - gauges and speedo was supposedly refurbised. Yet the speedo does now not go above 40MPH!! Quote Link to comment Share on other sites More sharing options...
Frank Elder Posted December 6, 2014 Report Share Posted December 6, 2014 So if you have short, couldn't you disconnect the ground wire/wires?....and see if you are still getting a reading, if so you would have a grounded hot wire/component wouldn't you? Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted December 6, 2014 Author Report Share Posted December 6, 2014 I will pull one wire at a time to watch and see for gauge reading changes-both at the tank unit and at the gauge. The car also has had a new Rhode Island complete wire harniss put in it. The shop that did the restoration on the wood and body I guess after 3 years was ready to be done with it! No energy left to fix the fuel making gas gauge. Quote Link to comment Share on other sites More sharing options...
TodFitch Posted December 6, 2014 Report Share Posted December 6, 2014 The slowly part is probably due to the natural dampening built into the dash unit, takes a little time for the thermostatic elements to heat up and cool down. So at a constant 2000 RPM with the transmission in neutral will cause the issue? If true then at least you have a test condition to verify the fix once the problem is found. If the instrument panel has been refurbished there is a chance that not all the wires go replaced properly. Though I am having a problem coming up with a wire swap that would cause that issue. As Tim pointed out, the symptom is like one of the wires either being open or grounded. He suggested a slight shift in the body but if it happens when still that seems less likely. Also, if it has a brand new harness it is unlikely that a wire's insulation would be worn through already. Certainly a puzzler. Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted December 6, 2014 Author Report Share Posted December 6, 2014 The car is parked and in nuetral and the fuel gauge will rise with RPM's and also do it while driving faster. Driving under 20 MPH gauge stays at about a 1/4 tank. Quote Link to comment Share on other sites More sharing options...
Jeff Balazs Posted December 6, 2014 Report Share Posted December 6, 2014 Have you tried swapping in a different voltage regulator? Also I would look for loose, dirty or corroded connections. I have also found that old ignition switches can sometimes create a path for odd faults like this. Just some ideas. Jeff Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted December 6, 2014 Report Share Posted December 6, 2014 I'm thinking also along the lines that Jeff is. Generators have more output as they turn faster and a weak regulator, either the main reg or the one on the gauge, may not be able to fully control the additional output. At your 20 mph and below, the gen output may be low enough for them to work and give an accurate reading. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted December 6, 2014 Report Share Posted December 6, 2014 Would you not see any additional generator output on the amp gauge? Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted December 6, 2014 Report Share Posted December 6, 2014 man stated voltage tested to be 7.85 volts....the gauge meter changes in regard to ground more so than to voltage....and it is a known fact that these voltage regulators in the gauge can and will handle 12 volts...the visible side effects of 12 volts is a faster rise time due to faster rise/fall due to the increase in switch rate..this is often viewed as a slight flicker of the gauge needle.. Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted December 7, 2014 Report Share Posted December 7, 2014 If the fuel gauges rises as the engine rpm increases while parked, you should be able to take body flex out of the equation. What is the voltage reading at the higher rpm compared to the rpm needed to run at 20 mph? What happens if the reg on the gauge is no longer functioning and getting the full line voltage? Just a theory on my part, asking questions that may lead to the answer Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted December 7, 2014 Report Share Posted December 7, 2014 I believe if there was higher voltage over and above the 7.85...Bob would have made such observation known in post #1..he is very thorough in his testing.. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted December 7, 2014 Report Share Posted December 7, 2014 Not sure what transmission is in this car but Is there a change in state of the transmission solenoids at high engine RPM's and or at 40 + MPH? Something must be changing state and going to ground at high engine speeds and is there possible back feed to the fuel gauge when this occurs? Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted December 7, 2014 Author Report Share Posted December 7, 2014 Thanks guys for all the info...I will be looking at the car in a couple weeks as per the owners call. The car is a 47 Chrysler woodie 4 door and does have the M5 transmission with the governer and solenoid which works flawlessly. I have also considered that maybe the IGN stud on the back of the fuel gauge could have been rotated ever so slightly by the wiring harniss installer changing the gauge calibration according to this National repair manual information. Not positive on this issue but something also to consider. To check for this issue I'll substitute a new dash fuel gauge . All the wiring on the back of the dash is clean and new and I could not see any corrosion at the terminals with out removing wires. The car has 50 miles since the frame off resto.. Voltage running at idle is about 6.5 idling.. off idle to say 1200 is about 7.20... and revved up at 7.85V. I could not determine the 6 volt supply (feed) wire to the guage or IGN terminal. Couple pics of the back of the cluster and a chrysler fuel gauge Fuel gauge... Thank you for all the input which I will need to get this taken care of! Bob Quote Link to comment Share on other sites More sharing options...
TodFitch Posted December 7, 2014 Report Share Posted December 7, 2014 I don't see star washers on the studs in your last image. . . Probably is not the cause of your problem but could cause issues down the road as the star washers do a much better job at keeping good electrical connectivity than standard lock washers or no washers at all. 3 Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted December 7, 2014 Author Report Share Posted December 7, 2014 The guage on the far right is an original 1946 chrysler as from the factory I took out of a parts car and just cut the wires. It shows #1 and #2 wire locations. It was a working gauge. I will check washers under the owners wiring. Quote Link to comment Share on other sites More sharing options...
Jeff Balazs Posted December 7, 2014 Report Share Posted December 7, 2014 (edited) The bulk of my DC troubleshooting experience comes from operating fishing boats in a marine environment.....so you should take it with a grain of salt. In that environment I learned early on that you won't always be able to see evidence of corrosion or dirt. Items like sealed ignition switch's were very prone to this and caused all sorts of problems. They typically lasted about 2 years and were trash after that......even when they appeared to be working. And I always carried a brand new spare. I broke a few of these sealed switches apart just to see what was going on and it was often a real mess inside. Any switch or solenoid can pick up increased resistance over time due to the effects of corrosion, dirt and wear. The same is true of wiring connections. They can look OK but the actual connection or mating surfaces should still be taken apart and cleaned. You would be surprised at how often this gets overlooked. I know we are not talking about boats here but there are some similarities when you factor in the age of the components in these electrical systems. I don't know how much work it is to swap the regulator and an ignition switch on this car but that is where I would start. The coil in the gauge or the gauge itself might be the problem but I would rule out these others first. Jeff Edited December 7, 2014 by Jeff Balazs Quote Link to comment Share on other sites More sharing options...
Frank Elder Posted December 13, 2014 Report Share Posted December 13, 2014 Progress......? Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted December 13, 2014 Author Report Share Posted December 13, 2014 (edited) I will not see the car till next month when It goes in for a fluid coupling diagnosis that seems to be out of balance. It's a car that had two different resto shops work on it and they get 1st chance to fix their possible remaining problems... Including the fuel gauge. I will let ya all know what the trouble issue was once it's fixed. I might end up actually fixing it. I got a call to look the car over and note out things that might not be as from the factory. He wants to show it at this years Pebble Beach! Bob Edited December 14, 2014 by Dodgeb4ya Quote Link to comment Share on other sites More sharing options...
Frank Elder Posted December 14, 2014 Report Share Posted December 14, 2014 Pebble beach...that's quite an honor in the wine and cheese circles...I found a pebble in my tire once, lol..that is about as close as I will ever get. Thank's for the update. Quote Link to comment Share on other sites More sharing options...
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