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1967-1971 Ford Thunderbird and
1969-1971 Continental Mark III AMP Gauge Repair

Yes, they are actually supposed to do something!

Ford AMP Gauge History on the Thunderbird

Image: 1969 Thunderbird ammeter gaugeThe first use of an ammeter (AMP) gauge in the Ford Thunderbird came with the newly-designed 1964 models. Perched in the last pod on the right of the T-bird's new "jet age" instrument panel, the gauge advised the driver if the battery was being charged or discharged at any given time, and indicated the degree to which that activity was taking place. In order to accomplish this, Ford ran heavy duty (smaller wire gauge number) wires to and from the gauge, so that all of the car's electricity moved through the gauge, making it register. A flow of power into the battery moved the needle on the gauge to show a charging condition, and a flow of power from the battery caused the needle to move to the left side of the band, indicating a discharge.

When new, this design wasn't a problem, and effectively allowed drivers to monitor the charging system. As the cars aged, connections began to corrode and get dirty, and resistance built up. Resistance causes heat, which isn't a good thing for electrical wiring. The main area of concern on the 1964 Thunderbirds was the connection at the cowl, where the wiring entered the passenger compartment from the engine bay. Having all of the power flow through the AMP gauge meant the gauge itself had to be heavy duty, which was more costly to produce. Since it was normal on these cars for the power to fluctuate somewhat depending on engine speed and accessory use, it was not unusual to have the gauge indicate a discharge at idle, then move over to indicate a charging condition once underway. When the battery was fully charged with minimal accessory use, the needle would sit near the center of the gauge, indicating a fully charged battery and an alternator putting out minimal current for electrical accessories. Customers didn't care for all the movement, and became concerned when the gauge indicated discharge for long periods of time, such as when idling in heavy traffic.

To address customer concerns, and to cut costs, Ford made a change around 1967 that eliminated the need for heavy duty wiring inside the car and gauges that were capable of handling large amounts of power. These changes also minimized movement of the needle, eliminating customer concerns. Ford achieved this by running smaller diameter (higher numerical gauge) wiring to the AMP gauge. This kept the high voltage, high amperage wiring in the engine compartment, and allowed the AMP gauge to "sample" the power flow.

Current will follow the path of least resistance, which meant only a small amount would travel through the smaller wires to the AMP gauge. When new, all of the connections were clean and tight and resistance was very low, so some needle movement at the AMP gauge could be seen during periods of heavy charging or discharging, yet minor changes were barely perceptible. Most of the time, the needle sat near the center of the gauge, an indication that all was well, and customers seemed to like that.

Until, that is, time and use caused resistance to build up to the point where so little power flow was going through the AMP gauge that it virtually stopped working. At this point, the cars were a few years old and often in the hands of second owners, who likely didn't realize anything was amiss, or were reluctant to take the car in for repairs. Needle movement was so small that a disconnected alternator would not cause the gauge to indicate anything was in need of attention. Owners often didn't realize the problem was as severe as it was until the engine stopped running due to low voltage! (Don't ask how we know this.)

Ford AMP Gauge History on the Continental Mark III

Lincolns used AMP gauges in 1964 and 1965, but discontinued them during the 1966 redesign. Since the new Continental Mark III was built using the 4-door Thunderbird platform, it had a similar instrument panel arrangement and included an AMP gauge. While the gauges themselves are internally identical between the two car lines, they differ externally and therefore cannot be interchanged. The Mark III suffered with the same issue of needing more sensitive gauges, and while there are some detail differences between the two, the diagnosis and repair instructions are the same for both.

The Thunderbird and Continental Mark IV for 1972 eliminated all of the gauges except for a fuel gauge. Ford product planners determined that customers buying these cars didn't want to be bothered with watching gauges to monitor critical systems, they would prefer to only have a red warning light come on when there was actually a problem. In 1976, separate lights for engine temperature and oil pressure were eliminated, and the two different systems were combined using one "ENGINE" warning light. This meant that if the light came on, one didn't know if the problem was overheating or low oil pressure. The owner's manual stated that it was normal for the light to flicker at idle, too. Fortunately, this didn't last long. The Thunderbird began offering an optional gauge cluster with the new 1977 models.

AMP Gauge Diagnosis and Repair—Thunderbird and Mark III

Ford's service procedures to check for a non-operational gauge were quite simple: 1. Close all doors and turn off all electrical accessories; 2. Turn ignition key to accessory position; 3. While observing AMP gauge needle, turn on headlights; 4. Any movement of the needle toward discharge indicates a satisfactory gauge reading. Of course, with a little resistance in the circuit, everything could be OK and the needle still wouldn't move enough for the human eye to detect anything.

To repair the AMP gauge circuit, start by checking the fuse panel on 1967, 1968, and some very early 1969 models. There's a SFE 14 amp. fuse that protects the charging circuit. It's marked as space #12 in the fuse panel, although the diagram for the fuse panel on these cars is very confusing. (A TSB, number 109 dated February 14, 1969 advised of a change in the information on the fuse panel cover to correct the cornering lamp circuit information, and to update the decal to make it easier to read.) You have to check the sizes of the fuses to determine which is the correct position. If that fuse is bad, replace it and check operation of the AMP gauge. (Note: Disconnect the battery negative cable before doing any electrical work, and leave it disconnected until you're ready to test your repair. Be prepared to remove the cable quickly if you see a lot of sparking when you connect it.)

If the AMP gauge doesn't work with a good fuse, you'll need to check the gauge itself. Pull it from the instrument cluster (consult Thunderbird Shop Manual for procedure) and check for continuity between the two contacts on the back of the gauge. If you have continuity, the gauge is good. If not, find a new gauge. Before installing the gauge, make sure the electrical connections are clean and tight. You may have to carefully bend the tabs with a small screw driver to make sure they have a tight contact. Also, gently move the needle back and forth manually to help clean the internal moving parts of the gauge. It's possible that dust and dirt over the years have contributed to the limited movement of the needle. Take the gauge inside and move the needle back and forth some evening while watching television.

While you have the instrument panel bezel off, make sure the large connector on the back of the printed circuit for the instrument cluster is tight. If you want to pull the cluster assembly out, you can clean the contacts in the connector as well as the printed circuit where the connection is made. Dust, dirt, and many years of use could make this connection a weak one, and that means resistance, which there wasn't much room for in this design. (See diagram below for cluster assembly connector.)

Image: Ford Thunderbird charging system schematicThe next areas to check are the connections in the engine compartment. Consult the diagram at left to locate the yellow highlighted circles. These are the connections that must be shiny clean and tight. Use dielectric grease on everything except the battery terminals and the instrument cluster connector when establishing the connections after cleaning them, to prevent future corrosion and to protect the connections from dirt. This is especially true of the connections at the junction block and starting motor relay, shown near the battery in the upper left corner of the diagram.

The next thing to check is the fuse link wire, identified in the schematic at left as an orange wire. The connections of this wire will be slightly different for 1967 and 1968 model Thunderbirds, as the schematic shown is for the 1969 model year. The difference is on the 1967-68 cars, the junction block has two studs on it instead of one, and the fuse link wire runs from one stud to the other, completing the circuit. In 1969, Ford changed this circuit to run all of the power through the fuse link, connecting the positive battery cable at the starter relay to the junction block, which provides power to the entire electrical system through a thick black wire with yellow stripe, identified as #37 in the schematic. Note that wire 37 passes through the connector identified as "E" on the schematic. This is located in the engine compartment, in the area around the passenger side hood hinge. It's very important that this connection is clean and tight.

The Role of the Fuse Link Wire

In the diagram above, the fuse link wire is colored orange to make it easy to see. The specifications of this wire are difficult to find in factory service literature. The purpose of the fuse link wire is to protect the electrical system. It is the weak spot, designed to sacrifice itself to save the rest of the wiring harness. Unlike a fuse that blows immediately in the event of a power surge or electrical short, a fuse link wire can withstand surges for brief periods of time without damage. This allows momentary spikes that don't harm anything to pass through the wire without causing the wire to burn up. Tube-type fuses are designed to blow immediately, circuit breakers are designed to open immediately then reset after a period of a few seconds, and fuse link wires heat up as a problem develops, eventually melting and opening the circuit. Fuses and fuse links must be replaced once they break the circuit.

Over the years, it's possible the original fuse link on your Thunderbird has been changed. It's also very possible that the replacement wire isn't of the same specifications as the original. This will add resistance to the circuit, and can also contribute to inoperable AMP gauges. The correct fuse link for 1967-1971 Thunderbirds and 1969-1971 Continental Mark IIIs is a No. 14 gauge wire that measures 7 inches. The replacements at auto parts stores usually measure 9 inches, and must be trimmed to the correct length before installation. Failure to do so increases the resistance, and can prevent the gauge from indicating charging system operation.

Production Change: Improved Indication Ammeter Gauge

As a result of continued customer complaints about the non-indicating AMP gauge, Ford made a running production change during 1970 Thunderbird production. This change occurred approximately mid-December 1969, and is covered under TSB 137, Article 2251, dated April 10, 1970. This TSB advised of the change in production to show improved gauge indication of charging system operation, and stated that the increased sensitivity gauges could cause the needle to swing to indicate a near maximum charge after the vehicle was started, or if the battery were in a low state of charge. So, ultimately, if a late 1970 Thunderbird or any 1971 model Thunderbird with a good AMP gauge can be located, these gauges will interchange in the 1968-1970 models built before the change in production, and may help to restore your AMP gauge to functionality. It's not clear if a similar production change was made for the Continental Mark III models.

The End Result

After going through your car, you may discover that your AMP gauge still doesn't appear to do anything. Chances are, there's still some resistance somewhere in the circuit that's causing a problem. However, we should advise you to not lose patience. Just relax for a few weeks and drive the car as you normally would, watching the gauge to see if the needle appears to move any at all. It's not unheard of for the gauges to just start working by themselves, after everything has been been cleaned up. Normally, you'll see very small movements at first, followed by increasingly larger sweeps later. Resist the temptation to install new wiring to the gauge, as some have suggested. This is not necessary and can cause problems if not done correctly. Clean, tight connections and the proper fuse link wire should go a long way toward restoring visual indication on your AMP gauge.

Note: We have observed AMP gauges that continue to appear dead, even though everything in the charging and ammeter circuit is working properly and is clean with good, tight connections. From time to time, these "dead" gauges will just start working again, sometimes months after the testing, repairs, and clean up tasks were performed. There is reason to believe that adding additional grounds may be helpful, although we can't verify that this is true at this time. Adding a ground wire from the battery negative cable terminal to the radiator support in front of the battery seems to help, especially since the headlights are grounded to the radiator support. It's important to remember that electricity not only has to be able to flow out of the battery through the positive cable, it must also be able to return to the battery through the vehicle grounds and battery negative cable. We feel this plays a role in the operation of the ammeter gauge in these cars.