Keeping needles and numbers on the vehicle's dash true

July 29, 2014
While there are still late model vehicles out there sporting the basic gauge package, sender-driven magnetic gauges are as archaic now as bimetal units were 20 years ago. Just about every instrument cluster is computerized.

In the same year the Titanic broke in half and sank to the bottom of the North Atlantic, George H. Townsend, president of Moto Meter Inc., obtained exclusive Boyce patent rights to manufacture radiator cap-mounted and dashboard-mounted engine temperature indicators.  The year was 1912.  Boyce Moto Meter instruments dominated the American automobile industry for about 20 years.           

Just about any old car buff has seen one of these hood-ornament type temperature gauges on vintage vehicles.

The most rudimentary clusters of old had nothing to show the driver except a speedometer, a fuel gauge and those simple warning lights that alert the driver when oil pressure is low, brake pressure is unbalanced or temperature is high. In the 1960s, some domestic platforms even had a blue or green warning light letting the customer know the engine was still cold. Some instrument clusters included a strategically marked vacuum gauge that was intended to help the driver save fuel. The first vacuum gauge I ever saw was in the middle of the dash on a yellow 1965 Impala SS my friend Alan was driving back in the late 1960s. Some supercharged vehicles have a gauge that displays both vacuum and pressure.

Early speedometers had alarms that could be set to warn a driver if he or she was exceeding their desired top speed, and one 1963 Oldsmobile Super 88 I rode in had an indicator bar that would change colors from green to yellow to red, depending on the speed. The problem with that one is that some yo-yos (like the guy I was riding with that day) just loved watching that bar turn red, which happened at 80 mph.

This is the vacuum gauge with which some 1965 Impala SS models were equipped. Because gas was just 20 cents a gallon back then, this gauge had more “cool” appeal than anything else.

 “Full Instrumentation” packages included gauges for temperature, fuel and oil pressure along with an ammeter for charging system status (replaced in the late ’70s by a voltmeter, which confused older ammeter-accustomed drivers). Some of today’s heavier duty light trucks boast a transmission temperature gauge along with a built in hour meter function embedded with the odometer software to measure the amount of time the engine has been running instead of recording only miles and oil life, because so many work trucks spend hours idling on the job and that time needs to be tracked for maintenance.

Computerized instrument panels are the order of things now, with processing power that would have been considered science fiction when I was in high school. Anybody who drives a Ford Fusion is right at home with watching the instrument cluster boot up and fan out on each side of the speedometer with so many display options in those wings it’s hard to keep track of all of them.

This 1999 Ford Lightning cluster came in dark, and when I got it apart, I was kind of stunned that it had no bulbs and the cluster had to be replaced to get the illumination back.

While there are still late model vehicles out there sporting the basic gauge package, sender-driven magnetic gauges are as archaic now as bimetal units were 20 years ago. Just about every instrument cluster is computerized, even on low-cost vehicles. And where there are instruments, needed repairs are inevitable, but knowing how to make the repair isn’t sufficient without accurate troubleshooting procedures – how many times have we replaced a gauge or a sender and discovered that we had misfired on diagnosis?  In today’s world, you need a good scan tool and a pass through programmer, too.

Illumination and Warning Lights
Cluster illumination is probably the least common symptom we have to deal with, but it does happen, and on at least two vehicles I’ve serviced with illumination concerns, the cluster itself needed replacing when it went dark, those two being a late 1990s Ford Lightning (which has no bulbs that can be replaced) and a 2003 Jeep Wrangler I wrote about a few years back, where the instrument illumination circuitry inside the cluster is weaker than the 10 amp fuse the engineers placed in series to protect it. A short to ground anywhere on the orange instrument panel feed wire on one of those Jeeps can darken the cluster permanently without blowing the fuse.

This 2006 Explorer cluster has a bevy of warning indicators in addition to the message center. When working on one of these that has malfunctioned, you have to find out where the signal is coming from that triggers the lamp – whether from the network or a hard-wired input.  Keep your schematics handy.

Inoperative indicator lights are another concern that isn’t too common, but some of those lights are pretty important. For example, on some Fords the voltage passing through the battery light turns on the alternator voltage regulator, and if the wire leading from that bulb to the alternator gets shorted to ground or cut, the alternator won’t work. If the wire is cut, the charge indicator light won’t work, and neither will the charging system. If it’s shorted, the light will be on all the time, but replacing the alternator won’t fix the problem. Most newer cars have the Powertrain Control Module (PCM) between the battery light and the alternator turn-on terminal, but a wiring problem on that circuit can still drive an unwary tech to needlessly replace the alternator.

These hour meters are a relatively new feature and obviously get their information from the network and the PCM about engine run time.

On mid-90s Ford four wheel drive vehicles, the 4x4 low light can blow or lose its power feed, and that will confuse the PCM into remaining in 4x4 Low shift schedule, meaning the truck will shift all the way to its top gear at about 30 mph. I’ve seen at least one transmission rebuilt because of confusion related to a blown fuse feeding this indicator lamp.

I’ve also seen two inoperative Malfunction Indicator Lamp (MIL) lights over the past couple of months because of bad PCMs. Once I saw a high beam indicator blinking like a turn signal flasher on a brand new Aerostar, but placing my finger on that tiny bulb socket stopped the flashing and so I removed the bulb holder and tweaked the little contacts. On a Jeep Grand Cherokee, I saw a high beam indicator flashing a strange binary code and after the hotline had me replace the $550 instrument cluster under warranty, I discovered that the $125 Body Control Module (BCM) was really causing that concern.

Speedo Stuff
Back in the 1980s and ’90s when cables drove speedometers and we had to replace an instrument cluster, at my shop we’d take the odometer drum out of the original speedometer and drop it into the new unit so those red tenth-mile numbers wouldn’t lower the trade in value of the vehicle. Granted, there was a sticker you could put in the door, but there’s nothing to keep a driver from peeling the sticker off, and nobody wants to lose 25 percent of what their car is worth because of red tenth digits.

This is another recent feature, and to repair this, you need to know what the system is using to determine the driver’s alertness. Some of these watch the driver’s face and some detect weaving.

Cable driven speedometers were a pain sometimes, because they’d make cable-related noises while the cable was spinning and the needle would quiver, necessitating the removal of the speedometer, extraction of the cable from its tube and lubrication and reinstallation of the cable.

The speedo needle can also be indexed wrong. Once I drew a ticket on a new Tempo that was reading 20 miles lower than accurate and the woman’s complaint was that the car seemed noisy at road speed. It was no wonder, since she was driving between 80 and 90 mph everywhere she went, and who knows how she escaped getting a speeding ticket. 

Speedometer needles on those older, cable driven units are mounted on a shaft that rides in a sintered bronze bushing in the center of the part of the speedo that is spun by the cable. Also a part of the needle/shaft assembly is a copper coated drum and a tiny spiral spring. The needle fits on the shaft as an interference fit and can be popped off.  The spinning part with the bronze bushing where the needle shaft rests has on it a bar-shaped magnet that spins inside the drum without touching it. As the magnet spins, the ferrous metal drum tries to follow the magnet, working against the tiny calibrated spring and the attached needle indicates vehicle speed.  The bronze bushing sometimes gets dry and tries to grab the needle shaft, bouncing the needle around all over the place.  Whenever I ran into that concern on a Taurus or Windstar, I simply disassembled the speedo and lubed the bushing with a drop of motor oil, and it was a permanent repair. Replacing the $350 speedometer was only a temporary fix, because the new unit would fail the same way within a few thousand miles. 

Electronic speedometers replaced the needle driven ones in the late 1990s, and speed sensors die, so watch for that, but make sure you replace the right sensor. That plastic output shaft speed sensor on the side of a Dodge RAM transmission doesn’t feed the speedo. That signal comes from the differential speed sensor, and on some Ford platforms that signal passes through the four wheel Antilock Brake System (ABS) unit on the way to the cluster, and sometimes the ABS unit has to be replaced for an inoperative speedo.

These old Chrysler printed circuit clusters can develop loose pins and cause unwary technicians to replace all manner of parts. We soldered this one to take care of a crazy gauge that would deflect when the turn signals were engaged.

If something goes awry with the seat belt alarm module on vehicles equipped with one (and I’ve seen this happen), the speedo can begin to read speed and then drop to zero when it reaches about 30 mph. I’ve also seen a bad junction box cause that problem. Sometimes when the speedo goes dead on a computerized cluster, a simple reboot (battery disconnect) will bring it back online.  Here’s a tip: If the odometer works but the speedometer doesn’t – or vice versa – the speedo has to be the problem.  It’s as simple as that.

With the shaft at rest, point the needle at the index mark, tap it back on, and then lift the needle above the peg to put it back in place.

Other Needles
Amongst the other needles, fuel gauges are the most likely candidates for failure. Because the fuel tank sending unit is in the most hostile environment and has mechanical moving parts, it’s the hardest working component. On some vehicles, the fuel gauge sender feeds its signal to an electronic module (like the Rear Electronics Module on some Ford minivans), which then forwards that information to the cluster, Personally, I don’t like having a middleman between the sender and the cluster. When the sender fails, the gauge will continue to display whatever reading was there when the failure occurred.

On older magnetic gauge units, you can disconnect the tank sending unit and the gauge will go to full, but slosh dampening electronics typically keep it from happening quickly unless you switch the key off and back on between test steps. If you short the sender wire to ground, the gauge should to empty. If you can find the ohm range of the sender, you can find ohm reading that corresponds with a half tank and use a potentiometer or special tool to send the half-tank signal and see what that does to the needle.

When a gauge drifts into the danger zone, it’s wise to check the datastream to see if the engine is truly running hot. If the same sender feeds the gauge and the PCM, it might be time to break out the Infrared Temp Gun for some strategic spot checks.

GM sending units from around the turn of the century wear out frequently and you can buy just the sending unit from the dealer, but the parts store only sells the whole fuel pump module.

Oil pressure gauges are either transducer or contact switch driven, depending on the platform, and you need to find out what kind of sender you have and how the signal makes it to the cluster in order to troubleshoot those effectively. Using a scan tool can test the gauge whenever you can, but know how to test the sender too.  Starting in 1987, Ford put magnetic gauges on most of their vehicles, but the transducer didn’t work well with those gauges, so a 22 ohm resistor was placed in series with the gauge and the transducer was replaced by a switch that closed its contacts with oil pressure.  These gauges read in the center and don’t reflect true oil pressure, and Ford has continued to use that system to this day.

Important note: When you’re troubleshooting an oil pressure or temp gauge, it’s wise to use your tools and know how to see what is actually going on in the engine’s innards before assuming the problem is with the gauge – if the gauge is telling the truth, oil and metal could be at the root of that reading.

Temp gauges nowadays frequently get their info from the Instrument Cluster electronics by with of a signal from the PCM, which is monitoring the ECT or Cylinder Head Temperature, and those sensors work very hard and are swimming in hot coolant, so they can fail and misreport the temperature. GM units can fail so as to make the temp needle swing all over the place while you watch.

This schematic illustrates the way so many new vehicle clusters are wired. Note that there are four fuses feeding the cluster, and one of them is a keep-alive.  Sometimes F150s like mine have clusters that begin to kill the battery through that keep alive wire, so watch for that

Voltmeters, when present, are pretty solid and almost never give trouble.  Some platforms have a “Check Gauges” light to get the driver’s attention when he or she isn’t watching the panel and something drifts into the danger zone. Chrysler has had that for decades, and now Nissans have a warning triangle that illuminates for the same reason.

With today’s clusters, even the mildest oxidation on terminals can cause gauge anomalies. On some late-90s/early-2000s Jeeps we had to remove the instrument cluster and clean those tiny pins even though they didn’t look dirty in order to get the cluster to stay alive.

There was a time when gauges could be replaced separately within the cluster, but those days are just about gone. Once again, you’d better have a pass through programmer if you plan to replace the cluster on most late model high end vehicle GM instrument clusters are some of the most notable that are prone to fail and they, like so many others nowadays, have to be flashed when installed.

About the Author

Richard McCuistian

Richard McCuistian is an ASE certified Master Auto Technician and was a professional mechanic for more than 25 years, followed by 18 years as an automotive instructor at LBW Community College in Opp, AL. Richard is now retired from teaching and still works as a freelance writer for Motor Age and various Automotive Training groups.

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