Installing Ammeter in 911

[MyOld911]

Has anyone put a VDO ammeter in their 911. I have a 1988 Carrera 3.2 and would like to hear some advice or thoughts. I know the basics but am looking for a "clean" install.

[walt]

ammeter or voltmeter? I saw recent post of one mounted on a radio delete plate. Have also seen a few mounted on the center console and on the knee pad.

Post 8 here:
https://forums.pelicanparts.com/porsche-911-technical-forum/1073108-car-radios-obsolete.html

[Trackrash]

Not sure why you would need an ammeter. A volt meter makes sense, and I am guessing that is what you are talking about.

[john walker's workshop]

Ammeters are a potential fire hazard.

[Jack Stands]

A voltmeter will give you all the info you really need and me much easier to install.

[911obgyn]

Ammeter has to have the entire load of the fuse panel run through it to be accurate. John walker is always right. So far as I can tell.

[MyOld911]

I have a volt meter installed, but I would like to have a better picture of the electric loads put on the system. Why is it a fire hazard if its properly installed?

[RDM]

I'm interested in this too. My 240Z waaay back in the day had an ammeter that I found very helpful.

Is it posible to measure the current inductively, perhaps, rather than directly?

[930cabman]

EZ fix: pickup a voltmeter that plugs into your cigar lighter. No wiring and works great

[Jonny H]

Originally Posted by john walker's workshop

Ammeters are a potential fire hazard.

Old school ones where the current goes through the gauge, potentially yes.

More modern ones use a shunt resistor inline and measure the voltage drop across it.

[MyOld911]

Thanks for the advice. I never considered the fire potential and that is something I am not willing to risk just for info on my electrical system. My car is 32 years old with 256,000 miles, runs like a champ and has zero rust. Now is not the time to be stupid. Thanks again for all your thoughts.

[engineerdave]

I mounted a voltmeter and ouside air temp gauge in the space previously occupied by the ash tray. I strongly recommend against using the VDO 2" Vision voltmeter; i tested several examples and concluded that they are worthless: inaccurate and with hysteresis. The VDO OAT meter works well.



I found this meter on Amazon and for half the price of the VDO unit it is accurate with repeatable readings. My only objection to it is that it is heavilly damped, but so was the VDO meter. This is installed in a '73.5 911T that I use as a daily driver.

I disagree that the voltmeter will tell you more than the ammeter. They tell you different things. The voltmeter gives an indication of the charging history of the battery but tells little about what is happening at that moment. For knowing what the charging system is doing in real time, there's no substitute for an ammeter.

In the air-cooled 911s (unless the harness changed in late air-cooled 911s) there's no good single place to pick off an ammeter measurement. You will need one sensor on the cable from the starter to the battery and another around the leads from the battery to the rest of the electrical system. These two points will measure everything except the load on the circuit for the rear window defroster, which comes directly off the alternator output terminal.

I built some circuitry to compute the value of the battery charging current from measurements at these two sites. I display these on the voltmeter, with 12 v reading meaning zero current, 9 v reading meaning either 15 or 60 amps net discharge rate and 15 v reading meaning either 15 or 60 amps net charge rate. A switch underneath the meter panel (not shown in the picture) chooses between volts, amps high sensitivity or amps low sensitivity. I used Hall-effect devices to measure the currents. I have found this very useful. This was part of a much larger project involving a rebuild of the air conditioner. In the picture the meter is showing about a 2 amp drain on the battery from the car radio (engine is not running).

I've had this system in the car for only a relatively short period of time but so far it appears to be performing well. I am willing to supply information to anyone who is interested once I have enough experience with it to be sure that it doesn't do unexpected things (by the end of summer 2021).

addendum 3 October 2021

I have 6 months of experience with this system now. It's been very stable over that period of time and over the range of temperatures that it's been exposed to (mid 50s to about 105 degrees fahrenheit ambient, range of temperature of the electronics, which are located in the front storage compartment, about 59 degrees to about 90 or 95 degrees) and is doing exactly what i wanted it to do.

I was reminded of one benefit of having an ammeter in the car recently, when the Bosch voltage regulator failed. I was able to recognize and correctly diagnose the problem almost immediately, before it would even have shown up on a voltmeter and long before any damage was done. If I had depended on the original instrumentation in the car (idiot light) I probably wouldn't have known about it until the batteries failed.

[targa80]

I did a post right after installing a VDO voltage and amp meter where the ash tray was located.

https://forums.pelicanparts.com/porsche-911-technical-forum/956542-vdo-volt-amp-gauges.html?highlight=Vdo

[911obgyn]

I would not want that much amperage going through the dash of an old 911 unless it was done with a high degree of engeneering.

[911pcars]

Originally Posted by 911obgyn

Ammeter has to have the entire load of the fuse panel run through it to be accurate. John walker is always right. So far as I can tell.

An ammeter measures the current through the circuit at the point of the instrument's insertion into the circuit. Do you want to measure total current flow from the battery at all times? If so, you need to install the gauge in series with a main source cable using equivalent size wire (it become part of the circuit). That's kind of unwieldy unless using a shunt. A shunt allows an ammeter to read total current without using large battery cables as it's circuit.

http://www.measurementest.com/2010/08/how-to-measure-current-using-shunt.html

Originally Posted by 911obgyn

I would not want that much amperage going through the dash of an old 911 unless it was done with a high degree of engeneering.

The current that normally runs through the dash or any part of the car is whatever it is, with or without an ammeter installed. An ammeter just measures existing current flow through the circuit it's connected to. It doesn't demand current like an electrical load.

Basic question again: Why do you want an ammeter?

Sherwood

[dtxscott]

I can see the usefulness of an ammeter. It can give early warning to systems that are drawing higher than normal current like failing batteries.
Passing full system current through the meter or dashboard is very dangerous and will lead to undesirable affects in the electrical system.

The proper way to do an ammeter is to install a very low ohmic shunt in series with the battery.

Something on the order of 0.1 Ohms. The current through the shut creates a voltage across it. A meter that reads the voltage and displays it in amps is the best, safest way to do this.

This is the same method used in the brake light circuits for 928s and in industrial applications like this one.

https://www.amazon.com/100A-Analog-Current-Needle-Ammeter/dp/B07H3P8R7B/ref=sr_1_22?dchild=1&keywords=amp+meter+with+shunt&qid=1621950416&sr=8-22

[targa80]

Originally Posted by 911pcars

……………………

Basic question again: Why do you want an ammeter?

Sherwood

I will try to answer your question with a long winded response since I have installed a voltage and amp meter into my car even though many on this web site with knowledge say don’t do it.

Reasons:

1. I had issues with my charging system that slowly drained my battery during driving without the low glowing indication from the idiot light. This left me stranded twice, fortunately I was close to home and only needed a jump. The system was putting out a charge but not enough to supplement the demand from the motor running. It should be noted that as long as I can remember while driving at night during idling the lights would dim. Sometimes I would see a low glow on the idiot light. The idiot light would go out and the headlights/dash lights would brighten with increased rpm’s. This was a normal operating condition to me for the system with the old original Marchal alternator installed in the SC’s.

2. Installing just a voltmeter can provide an indication of the health of the charging system based on the voltage reading. Anything over 13 volts should be good and anything below it might not be so good.

3.Installing an amp meter in series with the battery and the rest of the electrical system would include the starting current (+60 amps) and is not necessary. The amp meter only needs to monitor the current draw while the engine is running. I installed the amp meter between the alternator output at the starter and load at the battery using 6 AWG stranded cable with a 30 amp in line fuse this bypasses the high current demand of the starter by eliminating the battery starter cable from the amp meter circuit. I routed the cable on the same path as the battery starter cable with a detour into the cabin for the connections to the amp meter. The cable is spot tied in place and additional wire covering is used near any bends or entry points into the cabin.

4. I now have the capability of monitoring the charging system without relying on the idiot light and provides me one more indicator of the health of the car just like the other gauges do.

5. The main reason I did this mod was because of personal preference. I grew up in the days when an amp meter was common prior to the introduction of the idiot light and I just wanted my car to remind me of the days when life was just a little simpler.


I have heard all of reasons why you shouldn’t and if you do not have the technical capability then you should not do this because you could burn up your car. In my own case I am confident in my DIY capabilities and I am willing to assume the risk based on those capabilities.

[GH85Carrera]

Since this is my car, I think it is the perfect instrumentation for my needs.

Calibrated temp gauge, and the volt meter built right into the tach. That gets rid of the silly shift light, and I KNOW the alternator is doing what it needs to do. Ant is acts as a backup to a failed fan belt. If the volt meter drops to not charging, I would pull over as soon as possible, and check the fanbelt is in place. The fan is way more important than the voltage.

Just my opinion, and it is my car set up the way I want it.

[911pcars]

Other than the starting system, there are no factory current specs for the many circuits being used, especially when each is used only as needed (fan motors, vehicle lighting, accessories, etc.). System current is thus - variable.

Since this is your car, you can record baseline reference current use during normal operating conditions, then use that to monitor the overall system. However, what would you do if you noticed a slight drop or increase? Isolate and test each component in a circuit? If you isolated and measured nominal current previously, you could compare. Otherwise, all you have are your personal reference points as to fan speeds, illumination levels, motor speed, turn signal rate, cabin/seat temperature, injector/relay/solenoid response, horn tone, etc. That’s really getting into the weeds, but YMMV.

Monitoring available voltage is a more broad indicator of circuit efficiency. If 12 volt loads are getting 12 volts, electrical loads will either work, sort of work or not. Current levels are also affected, but the only constant in a 12 volt circuit is 12 volts. Slightly more, ok. Slightly less, NG.

S

[7783911]

I replaced my clock with a VDO 2.25 voltmeter fit into an old clock housing so it looks original. Previously I had a small voltmeter attached beneath the ashtray and before that in my center console (removed now)