Auto and Truck Repair and Advice
Serving the Gainesville and Hawthorne Florida Area
Auto and Truck Gages and Warning lights
IS THAT LIGHT IMPORTANT?
Ideally you should never have any abnormal warning lights on your dash. The "fasten seat belt" light should be off, because everyone has their seatbelts on. Turn signal lights should blink normally and cancel when the turn is done.
What about a CHECK ENGINE LIGHT? How important is that?
A general rule of thumb: RED LIGHT: STOP!.... YELLOW LIGHT: CAUTION!
A red light means something bad is wrong. Stop as soon as possible. If you ignore a red light and keep driving you may cost yourself a lot of money, or even kill yourself!
A yellow light is less urgent. The better your car runs, the less urgent the light is. For example, if your check engine light goes on when you are on vacation, and the vehicle still runs well and gets acceptable fuel mileage then you can probably finish your vacation and take it in to your regular mechanic.when you get back in town. A red oil pressure light on the other hand is cause to stop as soon as safely possible! A red "brake" light can be from 3 things: parking brake left on, low brake fluid, or off center proportioning valve caused by loss of pressure in the brake system. Once again if your brake pedal is firm you might be OK, but check the fluid level and operate your parking brake a couple of times to see if the light goes off. An amber "ABS" (Antilock Brake System) light warns of a problem that won't affect you under normal conditions, but if you start skidding on a slick road your antilock brake system may not work properly. Ditto with an amber air bag light: it's no problem as long as you're not involved in an accident!
NORMAL GAGE READINGS AND WARNING LIGHT ON CONDITIONS
The two vital factors monitored on a vehicle are engine temperature and oil pressure. Normal temperature range is between 165 degrees F and 220 degrees F. Oil pressure should be between 20 and 80 pounds. At highway speed the engine should have 40 or more pounds oil pressure. Generally the more oil pressure the better, although too much (over 80 PSI) can cause some problems. Temperature warning lights usually come on at 220 to 250 degrees F. Oil pressure warning lights come on if pressure goes under 6 to 10 pounds.
The alternator light (or ammeter or voltmeter if a gage) isn't as crucial as the oil pressure and temperature light, but a charging system problem can leave you on the side of the road just like an empty gas tank can. Thus nearly every vehicle ever made has had an oil pressure, temperature, and charging system light or gage, and a fuel gage. Modern cars have lights and gages for so many functions that they look like a pinball machine when you first start them up! Even so, the "vital" ones are the temperature and oil pressure.
"IDIOT LIGHTS"
There are a couple of different attitudes toward gages versus lights on a vehicle. Gage lovers like to know EXACTLY what their oil pressure, engine temperature, etc. are. Gage lovers know what readings are normal for their cars and check things out if the readings get too low or high. For example if you have a temperature gage you can see your car "getting warm", possibly from a coolant leak, and check into things before the car overheats severely. By the time a temperature warning light comes on a car is severely overheated. I even knew one guy with a "oil user" car who knew when to add a quart of oil by watching his oil pressure gage go down!
The advantage of the "Idiot light" is obvious: it is hard to keep track of a lot of little white needles on the dash while driving. People who don't know anything about cars can be confused by gages or will ignore them.
But ANY IDIOT will notice a bright red light on the dashboard!
Personally I like systems that have gages combined with a single "CHECK GAGES" red warning light. I have also seen gages that have a red warning light on the face of the gage. This gives you the best of both worlds.
BASIC GAGE AND LIGHT TYPES
DIRECT
A direct gage connects directly to whatever it measures. Direct temperature gages have a metal tube with a liquid or gas in a bulb at one end and the gage at the other. The bulb is inserted into the water jacket of the motor and the metal line is routed through the firewall and under the dash. A direct oil pressure gage has a line of either metal or sometimes plastic running engine oil pressure from the engine oil passages through firewall to the gage. A few direct gages are original equipment on vehicles, especially oil gages on trucks. Most direct gages are aftermarket custom installations.
ADVANTAGES AND DISADVANTAGES OF DIRECT GAGES
A direct gage gives an instant and reliable reading. This is important especially in an oil gage. Many electric gages have a delay between a change and the gage registering the change. This was especially a problem with the older thermo-electric gages. An electric gage can give a bad reading if you have a bad battery or a charging system problem. An electric gage only works when the vehicle is running.
Direct gages have the edge on accuracy and reaction speed, but they are a pain to install. the oil gages they sell today come with a tiny plastic tube which you connect to your engine oil pressure journals somewhere. Trouble is, this little tube has to thread its way past hot exhaust parts and a lot of sharp sheet metal on its way under the dash. one little slip and it can pour all your oil out in seconds! The temperature gages have a somewhat fragile copper line usually wrapped with a steel coil with a thermal bulb on one end. once again routing can be difficult. if you break the copper ine you have to replace everything: the whole gage assembly.
SPEEDOMETERS
Most newer vehicles have electroinic speedometers, but until recently most cars had mechanical speedometers. With a direct speedometer a cable runs from the transmission or front wheel (VW Beetle) to the speedometer unit mounted in the dash, often called the "speedometer head " . A shaft inside the speedometer unit attaches to the cable on one end and has a magnet attacheds to the other. As the cable spins, the magnet spins iinside a cup attached to the speedometer needle. A spring pulls the needle towards zero. As the cable spins the magnet pulls the cup around with it and moves the needle over to indicate speed. A gear on the shaft in the speedometer unit turns the odometer, measuring miles traveled.
WHY ELECTRIC?
Electric gages are by far the most common original equipment on vehicles today. An electric gage just requires a wire or two going to a sensor and connected to the dashboard. Although sensors can leak, you aren't piping oil pressure all over your car like with a direct oil pressure gage. Modern electric gages have a pretty good respose time , so it isn't as big of an issue as with older thermo-electric gages.
TYPES OF ELECTRIC GAGES AND SENSORS
SENSOR TYPES, OR SENDING UNITS
The sensors which operate electronic gages or warning lights are sometimes called "sending units". The sending units for gages and lights are different, even though either sensor will fit on your vehicle. A sending unit is usually either a varible resistor or a switch to ground. It gets its ground from the engine block where it is screwed in.
A light sending unit provides its wire with either an open circuit or a closed circuit to ground: it's just a switch. The gage sending unit feeds a varying resistace to ground back to its gage.
in any case, most all sensors operate by providing either a variable or an "on/off" circuit to ground.
One exception is an electronic speedometer sensor, usually called the vehicle speed sensor.. It has a magnet inside a wire coil and is usually linked by a gear to the output shaft of the transmission or transaxle. As the magnet spins faster, more current is produced in the wire coil. This current goes to the speedometer, where it moves the needle. A digital speedometer uses the same sensor, but a computer takes the analog electric signal from the speed sensor, converts it to digital, and displays it on the dash. ELECTRIC GAGE TYPES
Most all electric gages get a varying ground provided from a sensor or sending unit. A low constant voltage is delivered to the gages via a VOLTAGE LIMITER, which is normally part of the dashboard assembly, or incorporated into the fuel gage.
THERMO-ELECTRIC GAGES
The earlier electric gages were thermo-electric. These gages had a bi-metalic strip connected to the needle. A bi-metallic strip bends back and forth when heated or cooled. A thermo-electric gage has an electric coil which heats this bi-metallic strip, thus moving the gage needle. The problem with thermo-electric gages is that it takes a bit of time for the bi-metallic strip to heat up and move, so there is a slight time delay in the readings from these gages.
BALANCED COIL GAGES
More modern gages are balanced coil type, much like an analog voltmeter dial. A magnet is deflected by a coil, moving the needle according to the current. These gages react almost instantly so the only delay in reading is the time taken for the sensor or sending unit to react.
TESTING GAGES AND LIGHTS
It helps to have 2 people to do this:
Since most gages and operate by grounding the gage or light they can be easily tested with no special tools. Disconnect the wire from the sensor or sending unit. Have an assistant BRIEFLY touch the disconnected wire to a good ground. A bent paper clip works well for "probing" an insulated terminal.
If it is a gage, it should "peg", or go all the way to one side or the other. If it is a light, it should turn on when grounded.
If the gage or light responds in this way, then your gage or light problem is probably from a bad sensor or sending unit.
EXCEPTIONS TO THIS TEST
This test DOES NOT APPLY to the charging system light or ammeter or voltmeter. It also is not meant for any computer sensor or for any light powered by a computer, such as the "CHECK ENGINE" light, Air Bag Warning light, or ABS (Anti-lock brake) warning light.
The Economechanix Cyber Mall
We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry!
Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.