Dave,
Did you ever resolve the issues with the gauges going down as you drive. I seem to be having this exact issue and could use some guidance as to the remedy. Thanks so much

Hey Russ

The gauges have metal cases and I have removed cleaned and reinstalled the ground.  The grounds all go to a central ground stud, with the voltage regulator.  I have a good regulator case ground but also ran a wire from the case to the ground stud, and then tried 2 other ground locations to ensure it was not the ground I was using.  

I have not actually checked the sensor resistances but I'm sure they are fine as they read and function perfect until I move.  The tank reads perfect and moves properly as I fill up with gas.   Last year I swapped in a spare temp sensor I had laying around and this did nothing.  I'm thinking its the voltage regulator or after as it's a problem affecting both sensors, so it's anything that they share. 

 

Kevin,

Very good to know, my next move will be to track down the electronic version !

 

Thanks for all the input

Dave

My car (a real 1970) had this problem years ago. The faster you went the lower the fuel gauge would read, then you stopped at the lights and watched it come back up!
Long story short- the voltage stabilizer in these cars is mechanical, (a bimetal strip) and sensitive to external vibration. I binned mine in favour of a 10V electronic one, and it's been fine ever since.

Worry about rising temps and falling oil pressure...check the cooling system and add an auxilliary rad and oil cooler if necessary.  Both can be easily blocked off in "normal" weather/temps.

Dave:

Do your gauges have metal cases or plastic? If they are metal check if they are grounded to the same ground as the voltage regulator and that the voltage regulator case has a good ground and not intermittent.

Both gauges that are having bad readings work with resistive sensors. There is a resistor in the fuel tank and a resistor in the temp sensor screwed into your engine. Have you put an ohm meter on the sensors to see if their resistance changes?

Russ

Ben: My car does the same thing. The fuel gauge drops, the faster I go. As if it is road speed related.  I also did the same tests as you did, with the same results.  I have a mechanical water temp gauge.  and  my odometer is broken, if that means anything.  Thanks for any help fixing this problem

OK so a little more background.  I bought this mini (my first 1976 1275) 4 years ago and this has been going on ever since (well almost since the day I bought it because someone had taken out the voltage reg and had both gauges being fed 12v when I first bought it so i guess its been going on for 3 1/2 years.  Everything has gone wrong with this mini ("new" engine rebuild) failed 3 months after I bought the car, rad failed, fuel lines rusted through and leaking, heater core leaking, etc, etc, etc so this gauge issue has been the least of my concerns but now knock on wood it runs great and I'm not scared its going to break down every 5 min so I wanted to solve this last little mystery after 4 years of drive for 1 hr and repair for 2 hr.

Yes the fan belt is tight.  I ran 2 wires from the voltage reg to test as I drove just now and here is the latest results.  Battery with car off 12.75v.  Start the car and get 13.65v at alt.  Car idling and have 13.65v into reg. and 9.90v coming out.  Start to drive and voltage into reg. goes to 13.75v and out stays exact same at 9.90v.  Still gauges drop and at 70 kph still have 13.75v in to reg. and 9.90v out.  Run another ground to frame from reg. and no difference but this is the interesting part.  I disconnected the speedo and had the same results as before.  But reving the eng to the cruising rpm and not moving did not change the gauges, they still were accurate.  So then I drove up to 120 kph and kicked it out of gear and coasted with rpm at an idle and gauges remained down until I slowed below 40 kph and then started back up to the proper spots as I continued to slow.  

soooooo....it's like its wheels turning not engine turning that is doing this but how ?????   I'm sure it has to be a ground not going were it should but talk about strange!  

thanks for all suggestions as at this point nothing is to crazy !

Dave 

Tighten your fan belt first.

Dave,

I think you should be seeing a solid +10VDC out on your voltage regulator, REGARDLESS of input voltage (ok, within some limits, but basically 11v-15v in = 10v out)

Is there any chance it's hooked up incorrectly? (and thus, your replacement is too?)

Was the replacement a KNOWN good regulator?

Is your alternator putting anything out?

I am just thinking waay outside the box, that if your battery starts to get lower, and if your alternator has no output, the Vin to the regulator would be so low that the Vout cannot meet the expected 10v regulated voltage...but I am getting ahead of myself.

Take your VOM and ground the negative, black lead and attach the other (red) lead to Vout on the 10V regulator.  You should always see 10v out.

At least, that's the theory.

Hi I was wondering if anyone else has had the same problem I have and that is the temp and fuel gauges drop back when I drive.  I can run the car at an idle and everything reads correctly but as soon as I start driving the fuel and temp drop with speed.  When I come to a stop sign they come back to proper readings and again drop as I drive away.  They both drop with speed up to about 40-50 kph and then stay and do not go any further down.  The temp moves from between N & H to between C & N and fuel goes down about 1/3 tank.  The slower I go (less than 30kph) the less they drop

So far..

I checked voltage reg., it was fine but replaced it with another and no change.  I cleaned all eng and dash grounds and even ran a new ground for the voltage reg. and no change.  My speedo bounces around so I disconnected it thinking maybe it was something strange like causing the bad ground.

Next I might have to tee in a wire to the voltage reg and have it hang out the dash and test voltage while driving to see if I'm losing 12v supply or 9v out.  Anyone else had this ?  Maybe a bad ground I'm not seeing or a charging system that can't keep up under load ? 

Thanks Dave