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

[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.