This is the OEM manual printed by Chrysler, not a Chilton's or Haynes. As such, it is the source for information reprinted in third party manuals, with the information in those often being an abridged version of what's in the OEM manuals.
This manual covers the early B100/200/300 vans. Because many of our older Class C motorhome are build on a version of the B300 van, I now have an additional source of information for helping people on this thread.
Manuals like this one are hard to get your hands on. So, I want to extend a very heartfelt, public thank you to ronfisherman.
1970 Explorer Class A on a 1969 Dodge M300 chassis with 318 cu. in. (split year)
1972 Executive Class A on a Dodge M375 chassis with 413 cu. in.
1973 Explorer Class A on a Dodge RM350 (R4) chassis with 318 engine & tranny from 1970 Explorer Class A
I'm with you ... this is baffling.
This is starting to look very much like an electrical problem. So, you may need to have or borrow a multimeter.
One possibility you can check fairly quickly ... look at the back of the gauge (yes, easier said than done) and see how the gauge is connected. If there is one or two wires connected directly to the back of the gauge, I may know the problem.
Most people don't know that older OEM gauges operate on 5VDC. On the instrument cluster printed circuit board is a voltage regulator that reduces the current to the gauges from 12VDC to 5VDC. I've seen cases where people have wired a 12VDC supply directly to the gauge, bypassing the instrument voltage regulator.
Because you've not indicated any erroneous reading on your oil pressure and fuel gauges, I do not suspect the instrument voltage regulator.
Think I'll drive it home so I can work on it during the week. Just have to ignore that gauge for now and try to trust its ok for 20 mins.
Wow, I didn't know there were 5vdc gauges. If someone hooked up the positive wire directly, then it would explain the gauge maxing out quickly as the voltage exceeds 5vdc.
Eyeteeth,
So, you just need to measure the voltage at the gauge to see if it exceeds 5vdc when the engine has warmed up. Actually, it should be reading about 2.5vdc when the engine is warmed up (assuming it is a 5vdc gauge).
Anxious to hear what you find.
TreeSeeker wrote:
Griff,
Wow, I didn't know there were 5vdc gauges. If someone hooked up the positive wire directly, then it would explain the gauge maxing out quickly as the voltage exceeds 5vdc.
Eyeteeth,
So, you just need to measure the voltage at the gauge to see if it exceeds 5vdc when the engine has warmed up. Actually, it should be reading about 2.5vdc when the engine is warmed up (assuming it is a 5vdc gauge).
Anxious to hear what you find.
Griff,
Wow, I didn't know there were 5vdc gauges. If someone hooked up the positive wire directly, then it would explain the gauge maxing out quickly as the voltage exceeds 5vdc.
Eyeteeth,
So, you just need to measure the voltage at the gauge to see if it exceeds 5vdc when the engine has warmed up. Actually, it should be reading about 2.5vdc when the engine is warmed up (assuming it is a 5vdc gauge).
Anxious to hear what you find.
I suspect the 5VDC gauges are a throwback to the days of 6 volt starters and batteries. It'd be easier for manufacturers to add a instrument voltage regulator rather than replacing two or three gauges. (Or, it was easier than getting the gauge manufacturers to change their production lines.)
Even the aftermarket gauges are still 5VDC ... each one has a resistor or voltage regulator inside the case to allow them to be connected directly to a 12VDC supply.
I recall the days when people were converting 6 volt cars to 12 volts. A lot of people had "issues" with their dash after the conversion. (I used to jump a 12 volt battery to my '47 Willys Overland wagon to start it ... and occasionally had to replace the starter and other parts because of it.)
Griff wrote:
You can also read the voltage at the sensor, which may be easier to get to than the back of the gauge.
You can also read the voltage at the sensor, which may be easier to get to than the back of the gauge.
Won't the sensor read 0-12v? Only after the voltage goes through the regulator would it read 0-5v, right? If so, then you have to read it at the gauge.
If it reads more than 5v at the gauge, then the regulator has been bypassed. If this is the case, then the 0-5v gauge would be maxed out when the temp sensor was sensing the middle temperature (normal running temperature). Normal running temperature should be sending about half of 5v to the gauge, or about 2.5v. At least all that makes sense to me.
The potential is 12v on the ground side of the sensor and the supply side of the regulator. Between the regulator and the gauge, it should be around +5v. Between the gauge and the sensor, it should be at or somewhere below +5v.
The relationship between engine temp, sensor resistance, voltage, and needle position is not linear. So, the voltage at operating temperature is not necessarily +2.5v. As long as it's somewhere between zero and +5v, the circuit is working correctly. (The gauge is designed to compensate for the non-linear voltage.)
Hmm, I just now realized what you were saying before.
If you connect a meter to the screw terminal of the sensor and the positive side of the battery, it will read 0-12v. That is what I originally thought you were suggesting. But now I think you were saying to connect the meter between the sensor and the sensor wire to the gauge (disconnected from the sensor).
If this connection reads more than 5v it is bypassing the voltage regulator which explains why the meter is maxing out. Actually, if it is reading more than 3 volts something is probably wrong.
Yes, I agree that is much easier than removing the instrument panel.