"Buick Trademark(s) used with the written permission of General Motors"
Home Forums Garage Classifieds Reference

Tech Reference Page #20

Aftermarket Gauges

Authored by Wes Vann, updated March 5, 1999

"data logging" information to be added latter

Opening comments;

When in doubt, disconnect the battery! Use your head! Don't call me if you short out something and damage it!

This is going to be general data and rather basic due to the fact that there are several manufacturers of quality gauges.

This is a long "text" page and there are "jumps" to the desired sections at the "directory".

What you need to know and recommendations;

1) If you don't already have, and know how to use, an ohm meter, buy one! Have somebody show you how to use it, even if it's some silly computer nerd that doesn't know anything about cars. You really need to know how to work one of these things!

2) Buy the factory wiring diagram for your car! If you are "on the cheap", you can get just the wiring diagram and not the full factory manual. If you are really cheap, you may be able to find it at the local library (you may have to ask if there is a certain branch that carries a large automotive section). Please don't ask on the web for somebody to just send you one, people make a living selling them and they don't cost that much.

3) I personally don't want to get into the discussion of whether mechanical or electrical gauges are best. It's more a matter of ease of installation (and some options) than anything else.

4) You have to use the correct sending unit for the gauge! You have to have solid (common) grounds and a good 12 volt source! You have to have the instructions that came with the gauge and have to read them!

5) I recommend that you always retain the "idiot" lights. It could save you an engine! It's more notable when a light comes on, then when a gauge pointer moves a couple degrees. In certain wiring conditions, the alternator light is needed in order for the alternator to work correct (it's all in the resistance of the bulb).

6) Although it may look "cool" to have a ton of gauges, the fact is that the more that you have, the less you will really monitor them! Don't believe me? Just look at the gauge panel in a dragster or formula one car. Do you really think the driver has time to look at any gauges other than maybe a tach? The usage of "shiftlights" proves that they don't even have time to look at a tach!

Directory; (and jump point)

  • 1.Oil Pressure Gauge (mechanical and electrical)
  • 2. Mechanical Water Temperature Gauge (mechanical and electrical)
  • 3. Electrical Transmission Temperature Gauge
  • 4. Ammeters and Voltmeters
  • 5. Fuel Level Gauges
  • 6. Fuel Pressure Gauges (mechanical and electrical)
  • 7. Tach
  • 8. Shiftlights (independent of tach)
  • 9. Vacuum / Boost Gauge
  • 10. O2 Sensor Gauge
  • 11. EGT (exhaust gas temperature) gauge
  • 12. Data Logging (it's going to take some time for me to get to this)

  • 1. Oil Pressure Gauges (mechanical and electrical);

    With a mechanical gauge, you will have to run a tube line to the back of the gauge. What normally comes with the gauge is a nylon tube and fittings. I'd recommend that you trash the nylon tube and replace it with a stainless steel braded line. This may seem like overkill, but you have a pressurized line going into the cabin of the car with hot oil in it!! If the tubing breaks, you will not only have a car that you can't drive home, but could end up with hot oil all over the carpet and your pants.

    On a small block Chevy, the hook up for the tube for a mechanical gauge or mounting of the sending unit for an electrical gauge is at the back of the block, on the drivers side of the distributor. This is the location of the idiot light sending unit. I'd recommend that you retain the idiot light and use a "T" fitting. You will need a short straight section of tubing and the "T" fitting. The reason for the straight section is so that the "T" will clear the intake manifold. The straight section shouldn't be too long, or you will have clearance problems with the distributor. I mount the idiot light sending unit pointing up and the gauge sending unit pointing towards the drivers door (so that it clears the firewall).

    On a big block chevy, the hook up for the tube or sending unit is on the drivers side of the engine. Look for the original idiot light sending unit. (I have never owned a big block, so the recommendations may change after it get reviewed by others) Just like the small block recommendations, I think you should keep the idiot light sending unit and use a "T" fitting.

    return to directory

    2. Water Temperature Gauges (mechanical and electrical);

    If you get a mechanical gauge, it will come with a long tube attached to it. It's called a capillary tube. This tube can't be cut or removed, or the gauge is trash!

    When hooking up a temp gauge, I'd recommend that you find a location on the intake manifold where you can mount the sending unit or end of the tube. There are normally several areas where this can be done and the fittings that are required come with the gauge.

    The temperature idiot light is normally mounted on the side of one of the heads and I'd recommend that you leave it alone.

    The temperature in an engine isn't the same throughout the engine. In other words, a temperature reading at the intake manifold isn't the same as at the head. I believe that the head is a few degrees hotter. It's nothing really important, just something to keep in mind.

    One of the reasons that I like electrical temperature gauges is that it's a cleaner installation under the hood (without the long capillary tube) and if you have to pull out the engine, you don't have to remove the sending unit (possible water leak latter).

    Another reason that I like using electrical temperature gauges is that I can wire in an additional sending unit (with selector switch) to look at the transmission temperature.

    One word of concern on both types of gauges, they rely on the fluid flowing past the sensor. If the water level is low, or you forgot to put the water back in after pulling the engine, the gauge will read cold!!

    return to directory

    3. Electrical Transmission Temperature Gauges;

    This is the same gauge as what is used for water temperature! It just has a different face on it. The heat range may be different.

    My car has a TH350 transmission and when I installed the Autometer gauges, I got an extra sending unit to match the water temperature gauge. (it has to match!!) The sending unit came with the brass fittings and I shaped one so that I could solder it into the transmission pan. You have to make real sure that the sending unit will clear everything both inside and outside the transmission. I have a toggle switch that I can use to select which sending unit the gauge looks at. There is also an indicator light that lets me know when I am looking at the transmission, in case I forget.

    As I said about the water temperature in an engine, the transmission fluid temperature is different depending on where you take the reading. My engine temperature (as read at the intake manifold) normally is about 180 to 190. The transmission temperature (read at the pan) only gets close to the engine temperature when driving up a hill. I have no idea what the temperature of the fluid is in the line going to the cooler (mounted in the radiator).

    return to directory

    4. Ammeters and Voltmeters;

    There is a big controversy as to which is best. (Voltmeter is the answer)

    Ammeters work by reading the current in and out of the cars electrical system. Everything has to go through the meter! (there were meters on the market that used a "shunt" to get around this, but I haven't seen one for quite some time) It's like the water meter to your house, all the water that goes to your house has to go through the meter. If anything goes wrong with the meter or it's hook-ups, you have major problems!

    Ammeters require that you run heavy gauge wiring through the firewall and to the gauge. You have to modify the wiring loom in the engine compartment. If the wire shorts out at the firewall, you could get an electrical fire.

    It could be said that an ammeter will indicate how much current an alternator is putting out. It's true, however the output is also a function of the batteries charge.

    A voltmeter only requires a connection to a "12 volt" point in the loom. This can be simply done at the fuse block. On a car's electrical system that is working correct, you should see about 14 volts when the engine is running (and not at idle for older systems). With the engine turned off (or the alternator dies), the voltage put out by the battery would be less than 12 volts.

    So, both type gauges will give an indication of failure. However, the voltmeter is simpler to install and doesn't pose the potential "shorting out" problems.

    return to directory

    5. Fuel Level Gauge;

    It's best if you first go to the "techref" that covers factory gauges.

    Technical Reference #18

    Just keep in mind that just like all other electrical gauges, the gauge must match the sending unit!

    return to directory

    6. Fuel Pressure Gauge;

    Don't ever run a fuel line inside the passengers compartment! Not only is it against all sanctioning bodies (NHRA and such), it's stupid!!

    You can get what is called an "isolator" for mechanical fuel pressure gauges if you need to mount the gauge in the passengers compartment. It's nothing more than a rubber diaphragm that transfers the line "pressure" to another fluid type in the line that goes to the gauge. The isolator that Autometer sells already has the fluid in the line (it's capped at the factory) and they don't tell you what it is. (I'd bet it's just brake fluid, but was told by a friend that it's just a water and antifreeze mix) When you install the unit and the gauge, you have to be careful to not loose any of the fluid.

    It appears that isolators can only handle up to 15 psi. This rules out using one on a vehicle with fuel injection.

    You can get electrical fuel pressure gauges that go above the 15 psi limit of an isolator, if running fuel injection.

    return to directory

    7. Tach;

    Tachs are easy to hook up, but there are several different configurations of the cars wiring that have to be taken into account.

    You have to read the instructions!! I'm not going to tell you how to wire a tach due to not wanting to risk giving the wrong information.

    Autometer has a great page in their site that goes into several wiring configurations for their units. You can follow the following link and the go to their "tech tips" section.


    Some tachs come with a "shiftlight" attached. What is done here is that you can set an rpm where the light will come on, telling you when to shift. This helps with the consistency of your shifting while drag racing (and also validates the idea of keeping "idiot" lights). The lights are such that you don't have to be looking directly at them, they are pointed at you.

    return to directory

    8. Shiftlights (independent of tach)

    You can get shiftlights (see above) that are independent of the tach (or you don't have a tach??). In order to set the rpm setting, Autometers unit has little "plug-ins" for the desired engine speed.

    Wiring of a shiftlight would be the same as a tach.

    return to directory

    9. Vacuum / Boost Gauge;

    A simple pressure gauge that could be used for reading engine load (vacuum) or blower boost. I can't seem to find a listing for electrical gauges of this type.

    return to directory

    10. O2 Sensor Gauge;

    An oxygen sensor is mounted in one of the exhaust headers. It can tell you if your air/fuel ratio is correct. The sensors are the same as used in an EFI car. I contacted Autometer and if you already have the sensor for the EFI, you can just attach the gauge to it and you will not mess up the readings to the computer.

    Some units are a standard type "sweep" gauge and others are nothing more than a group of 5 LED lights.

    I'd really like to have one although I wouldn't mount the gauge permanently. I feel that it would be a great tuning aid, well beyond trying to read the spark plugs. Ever try reading the plugs while driving down the freeway at 65.

    return to directory

    11. EGT (exhaust gas temperature) Gauge;

    This is more of a "dyno room" type thing, although it would be of value in a race car.

    Where I've normally seen this sort of sensor is in monitoring the temperature of the exhaust right as it comes out of the head. Normally all eight exhausts are monitored and logged at the same time.

    This will give an indication of the "equality" of the cylinders. In other words, it's possible that due to a bad intake manifold design, one of the cylinders isn't getting as good an air/fuel mixture as the others.

    return to directory

    12. Data Logging;

    I've got to get back to you on this one due to not knowing exactly what is on the market.

    Basically, it's having a unit (computer) log all the data fed to it during a "run". The type of data is only limited by what sensors are mounted. The quantity of data is only limited by the units memory. The data is down-loaded for review latter.

    As an example, you know that the clutch was slipping if the engine rpm doesn't jive with the rear wheel rpm.

    return to directory