Low voltage at gauges

[Gretz]

(I posted this in an older thread and got no response so here goes)

I recently added a voltmeter to the oil temp/oil pressure gauge and it's only displaying ~12.6V, which is nowhere close to the true battery voltage of 13.8V. A loss of 1.2V on the rest of the circuit seems excessive, no?

With the car running, I used a multimeter to check battery voltage at the battery, which was 13.8V. I am only measuring ~12.6V from power to ground on my oil pressure and oil temp gauges, so I am dropping ~1.2V somewhere else in the circuit(s) and I'm wondering if that is normal.

How much voltage drop should I expect across wiring connected to gauges? e.g. Switches and related wiring from battery to gauge and then subsequently from the gauge to ground?

Over the last 5 years I have cleaned up all of the grounds, most recently (less than a year ago) I cleaned up the one behind the master cylinder, which is believe is the location all the gauges use. Also, I previously installed a Classic Retrofit fuse panel, and all of those connections seem good.

[John W.]

I wouldn’t expect that much of a voltage drop. Try measuring the voltage at different places along the circuit from your gauge back to the battery.

[7783911]

is you voltmeter reading from a switched source?

[MBAtarga]

What is the measurement with the ignition switch turned to accessories only?

[Gretz]

I need to measure that, along with a number of other things. On my car the fuel pump runs with the key turned only to RUN. So i assume there will be a slight drain from that as well as standard accessories. So far I have only checked the voltage difference between the power into the gauge and the shared ground on the back of the gauge, and found it to be consistent with the voltage displayed by the voltmeter gauge.

Another interesting observation: when a turn signal is on/flashing, the voltage on the voltmeter dips about 0.5 volt each time the signal illuminates. Is this expected behavior?

Also, my stereo has a voltmeter, which usually displays about 0.5V higher than displayed on the voltmeter gauge.

I have been poring over the current flow diagram to try to understand what else is on the circuit and what may be causing the problem. So far (as I understand the diagram), it looks like positive goes from the battery to the ignition switch, through a connector, to the tachometer, then on to multiple gauges. It looks like the gauges are all on parallel legs and should drop whatever voltage is left over after the ignition switch and tach. Please correct me if I misunderstand any of this.

I will be measuring: voltage drop upstream (battery + to + on gauge), voltage drop downstream (gauge - to battery -), voltage with ignition switch in RUN position. I am also going to check all the connections at the ignition switch, tachometer and the connector. Please let me know if you guys can think of other things to check, or other devices in the circuit.

[wazzz]

You said your battery voltage was 13.8 when measured at battery terminals. That is the only voltage of interest and the one you want to monitor.

Voltage drops happen in low voltage / high current electric systems like cars. The only way to get a true and steady measurement that is not dropping .5V when signals are on is by connecting the voltmeter with its own dedicated power line to a switched power that is as close as possible to battery +. That's the ignition switch downstream terminal.

As for the ground line, double check the ground point you are using. Turn on accessories and engine, and check voltage drop between the ground point you are currently using for your voltmeter and the battery negative terminal. If you find anything more than .5V, pull a dedicated brown wire from voltmeter negative terminal to battery negative terminal. Then you should get a 13,8V reading on your dash voltmeter when your DMM reads the same at the battery terminals.
No need for heavy gauge wires for these lines, as there will be near zero current in them.

[Gretz]

Just to confirm, all the instruments ground to the point behind the master cylinder, correct?

[PeteKz]

Gretz, see the response I posted on the other thread. From this thread it really sounds like you didn't find a good power source or ground point. .5V swings when the signals are running points strongly in that direction (pun intended).

[jwakil]

For the voltmeter I tapped off the cigarette lighter if I recall, and I would see around 14 when driving with nothing on. If I turned my lights on it would drop to 13.5 and with lights and AC it would go down to 13. Of course at idle I would get much lower. Recently I upgraded to the classic retrofit fuse panel with the modern flat fuses. Suddenly I’m seeing about 0.5V higher in all cases, and the drop of lights and AC on vs off is much less, and much less drop at idle vs high RPM. I’m quite happy with it.

[Gretz]

I performed all the basic measurements, including connecting power directly to positive at the battery and negative directly to negative at the battery. Connecting directly to battery ground only improved the voltage at the gauge by 0.1V/0.2V. Connecting the power directly to the battery positive essentially fixed the problem. Voltage at battery was 13.85V and voltage at gauge was 13.75V. So I knew it was a power problem. All of the connections at the other instruments looked fine.

Next, I checked the power going to the gauges from the ignition switch. I found this sketchy connector installed by a previous owner:



It looks like someone must have cut that wire and spliced in another wire at some point and then later removed that additional wire. So I went ahead and soldered in a new connection. Now when battery voltage is 13.8.V, I am seeing about 13.29V at the gauge. Also, the gauge does fluctuate with the blinkers on, but only by about 0.2v.

So the current status is about 0.5V lower than battery voltage is displayed at the voltmeter gauge. I couldn’t see a good way to check the output voltage (or anything else) coming from the ignition switch due to the connectors being integrated into the back of the ignition switch.

What do you guys think? Where else might I find an issue with the power that would drop the additional voltage? Since I know that I’m dropping 0.1/0.2V going to ground, that means I’m dropping 0.3/0.4 from the battery to the gauges. Maybe that’s just what I should expect based on the power passing through the ignition switch and other gauges?

[wazzz]

If everything else is working fine I would live with it and keep in mind a 0.5V drop when monitoring voltage on dash instrument.

[PeteKz]

Since someone else mentioned fuses, let me put in a regular plug on this thread too. The older open "bullet" fuses get corroded and cracked over time. If you can't remember the last time you replaced the fuses, or they are starting to distort the metal strip, replace them with new ones. A gentleman on the Mercedes list I participate in recommended replacing them at least every 10 years. We called it "the Marshall plan" after him. Just doing that solved at least 50% of other people's electrical gremlins.

[Gretz]

Thanks, that’s always good advice. Fortunately I have already installed Classic Retrofit mini-blade fuse panels front and rear.

I also noticed that my positive battery terminal clamp needs a refresh so I ordered the parts from our host and they should be here this week. Not terrible but not great. Here’s the current status:

[PeteKz]

Thumbs up on the plastic blade fuses. Way better than the OE ones. That battery clamp wasn't stock. Someone replaced the original terminal with that aftermarket cheapo clamp. I'll bet that replacing with original clamp and wires will solve other electrical problems you don't even know you have.

[7783911]

Do same for - ve, even consider a 0 or 2 gauge, use ring eyelets to attach - ve to post, I run my +ve from a switched source as a direct wire versus using one in the dash and have no accuracy issues

[Gretz]

Thanks for the great input, folks.

Ordered this negative cable from our host:
https://www.pelicanparts.com/More_Info/91161189900.htm

And this positive terminal clamp:
https://www.pelicanparts.com/More_Info/90161131100.htm

As well as some Rennline copper terminal ends:
https://www.pelicanparts.com/More_Info/RENEL276.htm

Hopefully all of this will bring back some of that ~0.5V that I am still missing.

[Gretz]

Yesterday I reterminated all the positive connections. I started by removing the old terminals and stripping away some insulation. I found that essentially all four cables had some level of corrosion. I used a vinegar/salt mixture and then baking soda to clean up the cable ends a bit. Then I tinned the new ends and installed new ring terminals.

After everything was connected I started the car up and checked voltage. I’m now getting 14.3V at the battery vs 13.8V previously. The analog voltmeter displays that same voltage now, as does the stereo head unit. So it looks like I have solved the electrical issues.

For good measure, I ordered a new negative cable from our host. The cable is great quality but is about an inch short due to my Optima battery. So I’ll need to find/make a slightly longer one. Porsche part is about 8” and I need about 9-10” (so many jokes). That shouldn’t be difficult, but I doubt the replacement will make much difference since the current one is in decent shape with no corrosion.

Thanks to everyone for the advice.

Here’s the new positive terminal:

[PeteKz]

Good to hear the resolution. Thanks for the follow up.