Gauge wiring questions Options

[lmcchesney]

I'm trying to add a temp gauge and ammeter meter to the 914. The gauges are from the FLAPS and not VDO's.
The temp gauge and sender instructions says to insert the sender into head source with wire connected to back of gauge. I wanted to test the gauge and be sure the readings are correct. My understanding of these gauges are that they produce a mV current which is read by the gauge. Thus, I'm thinking the sender creates the current but you need a ground (casing of the sender and case of the gauge) to complete the circuit. Thus for a test I placed a jumper cable from the case of the sender to the case of the gauge. Placed the sender in boiling water and waited. The gauge does not move after an extended period of time. Thoughts?
Also, the schematic for the ammeter describes connection of one of the poles of the gauge to the large amp wire from battery to the starter solenoid and the other pole to the "Hot Wire". My understanding is that the gauge is placed in line with alternator to the battery. What am I missing?
Thanks,
L. McC

[smrz914]

Temp:
I'm not sure how the guages work but there should be a connector for ground, +12v, and signal.

The sender can have up to 2 wires, ground and signal

Amps:
Current is measured in series like a short string of X-mass lights all the current needs to go through the guage (or at least the current that you want to measure).

If you connect the amp meter between the alternator and the battery you will be measuring the current that goes between those two points, which is not necessarily the amount of current the car is using. for example the guage would show nothing if you are siting in your car with the radio on but the engine not running because there is no current going between the battery and alternator.

Hope this helps.

[lapuwali]

On the temp gauge, are you talking about a head temp gauge, or an oil temp gauge? Most head temp gauges use a thermocouple, most oil temp gauges use a thermistor. Two different operating principles involved.

Thermocouples generate a current on their own. Two wires from the sender to the gauge, and most such gauges don't require a +12 or a ground connection, except to power a light. Thermistors are just resistors that vary their resistance with temperature. Typically, the place where the sender screws into is the ground, which is also the car's ground (the engine, usually). +12 applied to the post on the sender means there will be a voltage between the sender and ground that varies with temperature, which is what these gauges measure. So, there has to be +12 and a ground connection to the gauge, and one wire between the gauge and the sender (the ground is common).

Take the ammeter back if you can and buy a voltmeter instead. Ammeters inline between the battery and the alternator require the same (heavy) gauge wire to run all the way from the alternator, up to the gauge, and back to the battery, and it must be in SERIES, not parallel (thus, ALL of the current from the alternator to the battery has to run through the gauge). This means you've got 50-90 AMPS of current running through that gauge, which is very bad. Ammeters are useful for relatively low current setups (older cars with generators or low-amp alternators), and running the cable isn't so bad when the alternator, battery, and the dash are all relatively close together, unlike the 914 (unless you've moved the battery to the front trunk).

A voltmeter will measure battery voltage, with almost no current running through it, so you can use plain 18 or 22 gauge wire. The voltmeter should read about 12v with the key on, engine not running, about 13-14v with the engine running, and about 9v while cranking. All sufficient information to keep an eye on the charging system. If the volts are below 13v while the engine is running, either the battery is hosed or your alternator isn't doing the job. If it's too high, your regulator has failed. If it's below 12v before starting, your battery is dying.

[machina]

i noticed that the westach amp meters use a "shunt", does that take the place of running the heavy gauge wire?

dr

[lapuwali]

Partly. The meter actually uses only milliamp currents, and most of the current goes through the shunt resistor. One could design a system where big shunt resistor is near the alternator or battery and the meter connected only by small wires to carry the mA current to the dash. However, the shunt has to be matched to the gauge, or it will misread. Most off-the-shelf ammeters simply mount the shunt in the gauge housing, so you have to run huge wires to and from the gauge, carrying fire-starting levels of current.

If you really want to know how all of this works, look here:

http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

If Westach sells a separate shunt/gauge system, good on them. However, voltmeters are cheap, easy to get, easy to hook up, safe, and tell you everything you need to know about the charging circuit, and tell you something about your battery the ammeter doesn't.