Temp Gauge swings toward high-side and back to norm with 3 sec

-
T

TRWRacing

FABO Gold Member
FABO Gold Member
Joined
Nov 3, 2015
Messages
412
Reaction score
105
Location
Chicago
500 miles on a mild 360 9:1 compression with stock cooling system and 180 t-stat 50/50 conventional coolant: stock radiator and fan clutch. Only seems to happen when moving low speeds 10-15 mph swings rapidly towards high scale and than just as quick back to normal. It happened the same other my other stock 340 as well. Ammeter and fuel gauge stay normal

It will idle in the driveway all day long at the low end of the scale.

Here is the gauge after a 12 mile 40 min. drive:

IMG_1921.JPG
 
Once? Or keeps on cycling. Two types of thermostats, reverse poppet and graduated. Some graduated thermostats do just as you describe then settle in to normal cycling.
Poppet type open and close completely so you would see cycling.
 
Mine does the same thing. Both the temp and fuel gauge swing up and then back down within a few seconds. Seems like it only occurs when slowing down coming to a stop. I changed the instrument cluster voltage regulator and it didn't fix it. Interested in this one.
 
It happens once in a blue moon, but not every drive. It might even happen when I am not even looking. I had a 1971 Dodge Demon 318 back in 1989-1994 that would the same thing in the summer, but not doing winter months. I have seen it happen twice this summer. I am running a Mildon 180 high flow thermostat, but last year it happened with a stock style 195 in the 340 as well. It happens after driving for 20+ min.

My fuel gauge does not change position. I recently changed to the thermostat to the Mildon 180 high-flow. The 360 had a previously had the same 195* degree stock style as the old 340.

1973 Dodge Dart Sport H code
 
Last edited:
Wild needle swings are a symptom of failing instrument voltage regulator. I've seen this quite a few times. In most cases this temp gauge swing doesn't happen at cold start and warm up. It happens only after water is hot, and ignition switch is cycled off then on again. Anyway...
Eventually the limiter contact points will stick permanently, and something will die. Maybe the one gauge?, or all of the gauges?, or hopefully only the limiter itself.
 
I would think if the IVR was failing the fuel gauge would act up as well.

I would try another sender, and most you guys know I don't like "throwing parts" at a problem. If that doesn't fix it, wiring problem, bad gauge unit itself

See if you can see the fuel gauge act up.........at all......it's the IVR
 
67Dart273 said:
I would think if the IVR was failing the fuel gauge would act up as well.

I would try another sender, and most you guys know I don't like "throwing parts" at a problem. If that doesn't fix it, wiring problem, bad gauge unit itself

See if you can see the fuel gauge act up.........at all......it's the IVR
Click to expand...

And the fact that it does it quickly "3 seconds" and goes back to normal means it's not the motor temp varying, but the gauge for whatever reason.
Could be the IVR or sender or it could even be an intermittent grounding of the gauge sender wiring.

Not speaking to you Del, as you know all this already. :D
Just adding to the info and eliminating thoughts of a thermostat or motor heating problem.
 
You would think so. The key is in the way a OEM limiter works... At switch on the limiter throws a lot more current to warm up the gauges,,, and when the engine is cold that's 1 gauge. The fuel gauge supplies the only "path of least resistance", so the current goes that way. Fuel gauge response is about the same but not considered "acting up". Once the temp sender is hot the limiter finds 2 paths of little resistance and puts out even more current through 2 gauges. Consider which is the shortest path to ground.
I wont argue. Diagnose first, then replace the limiter... soon. Good luck with it.
 
I'll add this although not exactly the same. Our first 67 fish... this example has 3 thermal instruments on the mechanical limiter which is inside the fuel gauge. Once in a blue moon all 3 of those instruments would climb up just a little bit and very briefly, then drop right back down to where they were. I decided the charging system voltage regulator was the culprit only by noticing a very slight difference in the high beam indicator brightness. It wasn't enough difference to be noticed in the ALT' gauge or anywhere else really. Replacing that regulator didn't cure it. Cleaning and tightening everything about the inst' panel did cure it for a year or so ( That limiter did eventually die ).
I can only suspect the limiter was suffering weak ground ( where that fuel gauge contacts the inst' housing ). I aint seen it all. Just sharing what I have seen.
I'll stick with my diagnosis, Your mechanical limiter is the fault whether internally or in its connections. Maybe the spade terminal of noise suppression cap' was omitted which can cause a weak connection. Seen that before also. let us know what you find.
 
Thank you for all the help. Electrical stuff is not my expertise. I am assuming that I have to drop the the steeeing column and remove the instrument cluster to get to the voltage limiter on a 1973 model.
 
Some have or will go at it from under the dash. Not me. Ensuring that extra spade terminal from the noise cap' stays in place adds to their challenge. And yes you would need to lower the steering column to pull the panel. Entire procedure is detailed in service manuals. Many of those are free downloads at Mymopar.com
 
If the "points" stick closed in the factory voltage limiter. The gauges that are powered by the limiter will swing to right and peg. Your gauge supply voltage has now gone to a full 12 volts. This can quickly damage the gauges if it remains there.
I have played with these dash voltage limiters on the bench before using a DC power supply and a Fluke digital VOM with a peak/ averaging feature . When they are powered up they put out 12 volts. Then the points open and the voltage to the gauges drops to zero. Then the points start to close and open very quickly over and over. This produces a 0 volt to 12 volt square wave. I have found that you end up with about a 4 volt average signal powering the gauges.
 
My temp gauge does not peg, it will swing rapidly to just about an 1/8" short of the high line of the normal scale. It then returns to where it was originally all within 3-5 seconds when moving 10-15 mph. I have noticed this 3x within 500 miles over the course of 2 years.

I have not noticed the fuel gauge changing during this time, but the the temp gauge has never pegged.
 
I caught my fuel and temp gauges swing up for a few seconds and then return to normal. My assumption at the time was that the limiter points hung up monetarily. In you case with the temp gauge swinging up and not the others then the voltage limiter could be ruled out as the cause.
 
You must log in or register to reply here.
-

Back
Top