Voltage gauge reading low

[Michael]

Problem that has annoyed me for a decade. My voltage gauge always reads low. My multimeter shows my battery at 13.4-.6v at idle and around 13.2-3 with AC engaged.

My gauge however will show about 12.7 at idle and about 11.5v with AC on. Even less with lights on etc. It's always been this way but I know my charging system is fine and I know the gauge is known to be quirky. Is there anyway to recalibrate it or even pull the needle and reposition it to a more accurate location? At this point I just may buy a new gauge unit as my temp gauge sometimes will peg and get stuck at 12 o'clock when I turn the ignition off. I know these 2 share a ground. Is it possible a corroded ground could be the culprit?

[jackb]

The fuel pump shares a ground w/ the instrument cluster. You can reground it at the inertia switch to one of the bolts that secures the body to the frame by the drivers right leg. Probably won’t make a huge difference at the gauge though. Neither would a new gauge- the ground wiring across the whole car is so shitty that what the gauge gets at that location is accurate. As long as you know you are charging correctly at the battery/alternator, and you know your gauges baseline, so be it.

[Ron]

Corrosion or any bad connection along the volt gauge's circuit, ground or feed, could cause the gauge to read low. (Wiggle the IP plugs, check the fuse/socket.)
I would guess it has as much to do with our quirky gauges as anything else, as you say. So common that it wouldn't be worth trying a new unit, imho. There are ways to easily trick the needle to move lower (but not higher) so that it reads correctly at a given voltage, but that would throw readings at other values way off. So I'd agree with just live with it.

Now if you knew a talented painter who could change the "13" to a "14"....

[dmcnc]

Dedicated ground wire spliced on to the binnacle connector for the gauge did the trick for me in terms of gauge accuracy (for an analogue gauge). Separately, Dave M's Idle ECU has the AC bump, which keeps the idle 100 RPMs above the normal setting while the compressor is engaged. That gives my car enough kick to stay above the 12.8 waterline with everything turned on at idle.

[Ron]

That's a good idea...bypasses all of the old connections etc. You might do the same thing for the positive side (but taken off of the (W)hite wire feeding Aux Relay 2 instead of the battery).

[Bitsyncmaster]

Yes moving the ground on the inertia switch to frame ground will make your gauge read about 0.3 volts higher.

[Michael]

Thanks for all the suggestions. I'm going to tack this onto my fall tune up list.

[Jonathan]

The fuel pump shares a ground w/ the instrument cluster. You can reground it at the inertia switch to one of the bolts that secures the body to the frame by the drivers right leg. Probably won’t make a huge difference at the gauge though. Neither would a new gauge- the ground wiring across the whole car is so shitty that what the gauge gets at that location is accurate. As long as you know you are charging correctly at the battery/alternator, and you know your gauges baseline, so be it.


Yes moving the ground on the inertia switch to frame ground will make your gauge read about 0.3 volts higher.

Not sure that any of us are planning to ever need this safety feature in the event of a crash, but would changing the grounding on the inertia switch change the way it is supposed to kill the fuel pump if the inertia switch goes off?

Just wondering how that might work. Safety, or security too, if you happen to use the inertia switch plunger as an extra "don't steal my car" measure.

My tach gauge does all kinds of wonky things bouncing around so getting in there to improve the grounding around the instrument cluster is on my winter to-do list as well.

[Bitsyncmaster]

Cutting the ground wire and moving it to the frame ground does not affect the operation of the inertia switch. It just moves the fuel pump ground current off the wire going to the gauges. The inertia switch still switches that ground open in a crash.

You cut the black wire not the black/purple wire.

It will also up the voltage your fuel pump gets by the same 0.3 volts. The problem is the OEM harness used a small gauge wire on the that ground connection to the gauges and when the 7 amps of fuel pump current passes throught that small gauge wire the voltage drop makes that ground point at 0.3 volts when it should be 0.0 volts.

[jackb]

Exactly what Dave said.
Here’s a link to a guy discussing him doing it, for the same reasons as you:
https://www.riumplus.com/delorean-fuel-pump-re-grounding/

[David T]

As long as we are talking about the inertia switch, it is a primary safety item. In addition to shutting off the fuel pump in the event of a major deceleration crash, it also unlocks the doors if your door lock circuit is hooked up and working. Make sure you have complied with SC-06-7/82 dealing with relocating the switch and installing the updated switch with a white splotch of paint. On a lot of Deloreans there were problems with the fuel pump electrical circuit. Some of the most "creative" wiring solutions seem to get employed by PO's to get the fuel pump to run. Clean the mess up and make sure it works by actually testing both the fuel pump circuit AND the door lock circuit. As for the ground, it is one of the many deficiencies in the electrical system, not having a good grounding system and undersizing the wiring. When the fuel pump starts to go bad it overloads the circuit on both sides, the feed AND the ground.

[Ron]

FWIW-
I'm not thinking Michael would be satisfied with his gauge showing 11.5 + 0.3 = 11.8V with the AC on.
Seems using the two dedicated wires mentioned would have the highest reading available using existing switched circuits. Bitsy?

[Bitsyncmaster]

FWIW-
I'm not thinking Michael would be satisfied with his gauge showing 11.5 + 0.3 = 11.8V with the AC on.
Seems using the two dedicated wires mentioned would have the highest reading available using existing switched circuits. Bitsy?



The 12 volt side that feeds the volt gauge comes from fuse #5 so make sure that fuse is clean. It may be some added loads on that circuit also may cause a voltage drop on the plus side. My car the alternator puts out 14.6 volts and the volt gauge shows that same voltage.

[opethmike]

Problem that has annoyed me for a decade. My voltage gauge always reads low. My multimeter shows my battery at 13.4-.6v at idle and around 13.2-3 with AC engaged.

My gauge however will show about 12.7 at idle and about 11.5v with AC on. Even less with lights on etc. It's always been this way but I know my charging system is fine and I know the gauge is known to be quirky. Is there anyway to recalibrate it or even pull the needle and reposition it to a more accurate location? At this point I just may buy a new gauge unit as my temp gauge sometimes will peg and get stuck at 12 o'clock when I turn the ignition off. I know these 2 share a ground. Is it possible a corroded ground could be the culprit?

You can fix this by doing an LS swap.

[Michael]

You can fix this by doing an LS swap.

But I'm not gay

[opethmike]

But I'm not gay

Not yet...

On an unrelated note, does this smell like chloroform to you?

[Michael]

Bi curious is not gay!

[hippieman9]

Problem that has annoyed me for a decade. My voltage gauge always reads low. My multimeter shows my battery at 13.4-.6v at idle and around 13.2-3 with AC engaged.

My gauge however will show about 12.7 at idle and about 11.5v with AC on. Even less with lights on etc. It's always been this way but I know my charging system is fine and I know the gauge is known to be quirky. Is there anyway to recalibrate it or even pull the needle and reposition it to a more accurate location? At this point I just may buy a new gauge unit as my temp gauge sometimes will peg and get stuck at 12 o'clock when I turn the ignition off. I know these 2 share a ground. Is it possible a corroded ground could be the culprit?



I am actually working on the same issue. I will be moving the ground wire from the inertia switch ground to the frame, I also replaced all the stamped "nuts" on the back of the insturment cluster with brass nuts. Seems to help the situation too. 6-32 is the size.

[Michael]

As luck would have it my alternator took a puke last week. Instead of buying another one I took it to a local shop that has been doing nothing but alternators and starters for the last 30 years. For 90 bucks I got it rebuilt and cleaned (and bought a new pigtail)

I have had my car almost 11 years and I have never been satisfied with my volt gauge at idle until today. Over the years I have been through 3 alternators. I got a SpecialT 110 trying to solve it and it's still in my bench now as a emergency spare. The one I had on it (and had rebuilt) was a DMCH unit from about 5 or 6 years ago.

62105

[steve]

As luck would have it my alternator took a puke last week. Instead of buying another one I took it to a local shop that has been doing nothing but alternators and starters for the last 30 years. For 90 bucks I got it rebuilt and cleaned (and bought a new pigtail)

I have had my car almost 11 years and I have never been satisfied with my volt gauge at idle until today. Over the years I have been through 3 alternators. I got a SpecialT 110 trying to solve it and it's still in my bench now as a emergency spare. The one I had on it (and had rebuilt) was a DMCH unit from about 5 or 6 years ago.

62105

I may take a Pic. and tape it over my gauge. Never had over 13.

[Michael]

I may take a Pic. and tape it over my gauge. Never had over 13.

What is weird is I have had three brand new alternators over the years (DMCH, SpecialT and aftermarket and not one of them showed the voltage at idle this locally rebuilt DMCH unit has.

I would suspect the old pigtail was suspect but I had not yet replaced it when I took that pic.

I don't know what the odds are of me getting 3 crappy nators in a row are but I'm leaving well enough alone. I am going by the parts store today and get some asbestos wrap for the wiring near the header.

[Bitsyncmaster]

What is weird is I have had three brand new alternators over the years (DMCH, SpecialT and aftermarket and not one of them showed the voltage at idle this locally rebuilt DMCH unit has.

I would suspect the old pigtail was suspect but I had not yet replaced it when I took that pic.

I don't know what the odds are of me getting 3 crappy nators in a row are but I'm leaving well enough alone. I am going by the parts store today and get some asbestos wrap for the wiring near the header.

Alternators should be putting out 14.5 to 14.7 volts. In cold temps the voltage should be higher until your engine warms it up. If you ground your fuel pump or inertia switch you should read about 0.30 volts higher on the gauge because it reduces the ground drop on the gauges. My DMCH alternator shows about 14.8 volts on the gauge.

[Michael]

Alternators should be putting out 14.5 to 14.7 volts. In cold temps the voltage should be higher until your engine warms it up. If you ground your fuel pump or inertia switch you should read about 0.30 volts higher on the gauge because it reduces the ground drop on the gauges. My DMCH alternator shows about 14.8 volts on the gauge.
Even ice cold and with a new batt/alternator, my gauge has never read as high as it does now. At best it would read 13 dead cold then gradually drop to around 12.5 to 12.8 and only hit 13 even when the revs are up. Whatever I have done, I like it. I have a theory that most people who sell alternators are either sourcing them cheap or having them built with cheap components.

If anyone is suffering similar problems you may want to try and have your present alternator rebuilt using quality higher end supplies. I told these guys to do whatever they needed to make it as good as they could and I was ok with any additional expense. I don't know if they actually had "a cheap parts pile and a expensive parts stash" but I am pretty happy with this thing.

[Michael]

Here is a screen grab from a cold start just a few hundred miles ago. Never got above 13v even ice cold and nothing on.(it only went lower when warm).

62116

And compared with yesterday's cold start and rebuilt alternator.

62117

[Bitsyncmaster]

Here is a screen grab from a cold start just a few hundred miles ago. Never got above 13v even ice cold and nothing on.(it only went lower when warm).

62116

And compared with yesterday's cold start and rebuilt alternator.

62117

I will run a test (I may have done this before) of what the linearity of the gauge is. I know I run at almost that 3/4 mark which I assumed was 15 volts.

[Michael]

I will run a test (I may have done this before) of what the linearity of the gauge is. I know I run at almost that 3/4 mark which I assumed was 15 volts.

If the markings are true, the upper mark should be 15.5v

[Bitsyncmaster]

If the markings are true, the upper mark should be 15.5v

This is an NOS gauge.

8 volt mark = 9.17 volts at 37ma.
1/4 mark = 11.15 volts at 47 ma.
13 volt mark = 13.30 volts at 55ma.
3/4 mark = 15.8 volts at 62 ma.
18 volt mark = 22.4 volts at 86 ma.

So the readings around the 13 volt mark are not to bad but elsewhere it's junk. I know the gauge in my car must have different calibration since I see it run pretty close to the 3/4 mark and my voltage is about 14.7 volts when running.

Note this gauge has a light blue calibration resistor on it.

edit:
I just thought of something. I was testing with the gauge laying flat on my table. Now testing with it vertical like in the car.

[Bitsyncmaster]

Test #2 with gauge vertical and light tapping on gauge after voltage change.

8 volt mark = 9.02 volts
1/4 mark = 10.82 volts
13 volt mark = 12.90 volts
3/4 mark = 15.62 volts
19 volt mark = 21.9 volts

So either the gauge is sensitive to being vertical or my test is not repeatable.

[Michael]

Wow, seems as long as you are within the acceptable range of a battery's voltage, the gauge is actually pretty accurate. (well yours is anyway)

[Michael]

Cleaned the main ground on the frame in the engine cradle. It didn't look bad and it barely gets wet but I decided to check it anyway. It had a thin dark gray layer of corrosion/sediment on the frame and fitting. I cleaned it with sandpaper and got another .5-.7v on my gauge.

Going to clean my fan grounds next time she goes in the air. It's been a minute since I have cleaned any of them.

[Jonathan]

Cleaned the main ground on the frame in the engine cradle. It didn't look bad and it barely gets wet but I decided to check it anyway. It had a thin dark gray layer of corrosion/sediment on the frame and fitting. I cleaned it with sandpaper and got another .5-.7v on my gauge.

Going to clean my fan grounds next time she goes in the air. It's been a minute since I have cleaned any of them.

Is that the one that’s together with the engine mount on the passenger side?

[Michael]

Is that the one that’s together with the engine mount on the passenger side?

It's on the passenger side but it's above the big engine ground. This one secures on the frame rail just below the expansion tank. I believe it runs either into the coil compartment or main harness(I didn't trace it). You will need a 13mm spanner for the nut on the bottom and a socket with a long extension for the top. Pretty easy to get to though.