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Everything from Hood Ornaments to Tail Lamps 1967-1971 Ford Thunderbird Alternator gaugeAT LEFT: It's likely you'll never see this much of a charge reading on a 1967-1971 Ford Thunderbird Alternator Gauge. By design, if you ever do, chances are there's something wrong!*
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Vol. 2, No. 7
January 5, 2004

Ford Ammeter Gauges: Do They Work?
by the Automotive Mileposts Staff

Image: MILEPOSTS Garage

Image: Troubleshooting/Tech Tips Series

You may call them Alternator Gauges, Amp Gauges, Amp Meters, Charge Indicators, ALT Gauges, or simply Ammeters, but they all basically tell you the same thing: whether or not your electrical system is doing what it's supposed to do. Generally speaking, when there's enough power going into the battery to keep it fully charged without overcharging it, everything is OK. When more power is going out of the battery for long periods of time, it's bad. Some cars have lights that come on to tell you there's a problem. We've always thought that was strange: place an additional burden on the battery by lighting a light to tell you there's a problem. Seems a "ALT OK" light would be better. When it goes out, the rest of the car is soon to follow.

When Ford started installing Alternator Gauges in its cars around 1964, most were designed so that all of the power in the electrical system passed through the gauge. This allowed the gauge needle to move around quite a bit, according to which accessories were being used. When the cars were new, this design worked fine and was relatively trouble free. As the cars aged, connections within this circuit began to corrode and build up resistance, which caused heat. Under certain circumstances, and over a period of time, this heat could damage the gauge to the point it would fail. This could mean there was no electrical flow through the circuit, which effectively shut down the car. Sometimes, the back of the gauge itself would melt and warp, not a repair anyone would want to deal with, but minor compared to other - much more serious - potential problems.

In a worst case scenario, the potential for an electrical fire was very much a possibility. Around 1966 or 1967, Ford redesigned this circuit and installed an Ammeter Gauge in many of its cars that used gauges instead of warning lights to notify the driver about charging system health. A shunt wire in the circuit allowed only a small amount of electrical power to flow through the gauge itself, which meant the needle didn't move very much. Only very strong charging or discharging would cause the needle to move, and even then the movement was so slight most people didn't notice it.

This resulted in customers and service technicians questioning how to tell if the gauge was operating correctly. Ford released a Technical Service Bulletin (TSB) to advise service personnel on the proper procedure for testing the gauge.

Here is the abbreviated official procedure outlined in the TSB:

  • Close all doors and turn off all electrical accessories
  • Turn ignition switch to "Accessory" position
  • While closely observing Ammeter Gauge, turn on headlamps
  • Any deflection of the needle towards "D" (discharge) means the gauge is fine

Normally, pressing on the brake pedal to activate the brake lights or opening a door to turn on the interior courtesy lights should also cause the needle to deflect, but it's rare to find one today that actually does anything.

If you want to troubleshoot your gauge in the hope of making it work well enough to actually see some needle movement, here are a few things to check. First, make sure all of the electrical connections in the charging circuit are clean and tight, start with the battery cables and include all of the plugs in the wiring harness going to the gauge. Also make sure the connection at the printed circuit on the gauge cluster itself is clean and tight.

Some cars have a fuse to protect the gauge, so be sure to check the fuse to make sure it's good. Normally, the Ammeter Gauge is all that particular fuse protects, so it's pretty easy to not realize it's blown, since the needle doesn't normally do much anyway.

That's about all you can do at this point to attempt a fix. Even when the cars were brand new, the Ammeter didn't register much movement under normal operating conditions. Why Ford continued to use this design in many of its cars for several years after this problem was reported is a mystery. Models affected, among others, include the Ford Mustang and Thunderbird, Mercury Cougar, Continental Mark III, and Lincoln Continental.

We've seen several suggestions for fixes, but so far we haven't verified that any of them actually work in the long term, without potentially causing a problem. If we do find something that works, we'll be sure to let you know. In the meantime, keep your battery cables clean and tight, and make sure your alternator belt is in good shape and properly torqued. It probably wouldn't hurt to check battery voltage from time to time under different load conditions with a volt meter to make sure the voltage readings are where they should be. Normally, a reading somewhere between 12.3 and 14.6 volts is considered good, with readings on the lower side under heavy electrical loads. If you get a 12.3 reading with the engine idling and no accessories on, your charging system isn't keeping up with demand, and will need attention quickly. And you don't want to see a reading higher than 14.6-14.7 for long periods of time, as that likely means your battery is getting cooked by your alternator or regulator. If you check your battery voltage at regular intervals, at least you know that everything is OK - or not OK - with your charging system at that particular point in time. Trust us, it's not fun to find out the hard way that there's a problem in rush the a major intersection. Don't ask us, we'll still be too traumatized to talk about it!

*Update: See 1967-1971 Ford Thunderbird and 1969-1971 Continental Mark III AMP Gauge Repair for steps to restore ammeter operation.

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