Frequently Asked Electrical Questions

Got a short? Need a fuse. Chanting “ohm” got you down? Click on an item from the list of frequently asked electrical questions below. You might find the answer to amp you up.

Truck electrics can be tricky but they don't have to be.

These Answers May Help You The Next Time You Wire Your Truck

  1. Is soldering better than crimping?
  2. How important is a good ground?
  3. How do I figure the DC rating of a switch if I know the AC amperage rating?
  4. What is a fusible link?
  5. What is a maxi fuse?
  6. Is a fuse better than a circuit breaker?
  7. Can I use 6-volt gauges with a 12-volt system?
  8. How can I test my fuel gauge?
  9. What is an easy way to check for a draw that’ running down my battery?
  10. Should I run relays with my halogen headlights/ taillights?
  11. What’s the difference between an SPST and an SPDT relay?
  12. Should I use an ammeter or a voltmeter?
  13. Do I need to use a relay with my electric water pump, electric fuel pump, and cooling fan?
  14. I want to install Chevy Fuel Injection on my truck. Should I use a mass airflow sensor (MAF) or a speed density (MAP) TPI system?
  15. When do I need a ballast resistor?
  16. How do I connect an SPDT relay?
  17. What’s the toughest GM fuel injection swap you’ve seen?
  18. Will disconnecting my GM fuel injection EGR valve make more horsepower?
  19. What does a neutral safety indicator do?
  20. Do I have to run a speed sensor with GM fuel injection?
  21. What is a vehicle speed sensor?
  22. What is VATS?
  23. Can I use any PROM in my fuel injection computer?
  24. What is a cold-start injector?
  25. Is GM Tuned-Port Injection better than throttle-body injection?

1. Is soldering better than crimping?

Soldering requires experience to get it right. Too much heat can make the connection brittle, and can reduce the strength and durability of the joint. A good crimp is just as good as a solder joint, and the average person stands a better chance of doing a crimp properly than doing a solder joint properly.

2. How important is a good ground?

You can never ground something too well. We recommend grounding the battery to the engine, the engine to the frame, and the frame to the body. A common problem is with taillights. If the ground is bad on a three-wire taillight socket, the bulb will seek a ground through the other power circuit, causing backfeeds and lights being on that shouldn’t be. An engine not grounded well causes trouble when the starter is engaged. This makes things like the shifter cable and throttle linkage carry several hundred amps of power, and that can actually weld things together from the heat generated.

3. How do I figure the DC rating of a switch if I know the AC amperage rating?

Multiply the AC amp rating by 1.8. A switch that has a 10-amp rating at 125 volts AC would be rated at 18 amps at 12 volts DC. Use a 20-amp fuse or circuit breaker to allow for voltage surge. A good rule to remember is when voltage goes down, amperage goes up.

4. What is a fusible link?

A fusible link is made of material that will melt when overheated, and this occurs with a short or with an electrical load too great for the fuse link to handle. This keeps the rest of the vehicle wiring from melting. The fusible link wire is usually 6 inches long and two wire gauges smaller than the wire it’s protecting.

5. What is a maxi fuse?

This is the modern replacement for the fusible link: a large blade-style fuse in a fuse holder that will blow like a regular fuse when the circuit load gets over the rated amperage. A maxi fuse is much easier to replace than a fusible link. All new cars use them, so replacements are available in parts stores everywhere. The photo shows a maxi fuse holder with the cover removed, a standard fuse in the foreground, the maxi fuse, and a complete Maxi Fuse assembly with cover installed.

6. Is a fuse better than a circuit breaker?

When a circuit has a short in it, heat is produced. A circuit breaker is designed to open and then reset. They have been known to overheat and stick closed, which is like having no fuse at all. A circuit breaker can also cycle rapidly, allowing heat to build up in the circuit and do damage. When a fuse blows, the circuit opens and stays open until the fuse is replaced.

7. Can I use 6-volt gauges with a 12-volt system?

Yes. A voltage reducer can be used to power the gauges, or they can be converted to 12 volts. Be sure to use an electronic voltage reducer on the gauges and not one of the ceramic-type reducers used on heaters.

8. How can I test my fuel gauge?

A fuel gauge uses battery power on one terminal, and the fuel tank sender provides a ground on the other. The gauge can be tested by observing if the gauge goes full travel when grounding and ungrounding the sender wire. If the gauge doesn’t go to “empty” during testing, the gauge is bad. If the gauge test is OK, the sender is probably bad. This same test works for an electrical temperature and oil pressure gauge.

9. What is an easy way to check for a draw that’ running down my battery?

A slight current draw in a car with a computer or memory radio is normal. Something is wrong if the battery goes down overnight. To find the problem circuit, first disconnect the positive cable on the battery. Hook a test light between the cable end and the battery terminal. If there’s a draw, the light will illuminate. Next, start pulling fuses one by one until the light goes out. When the test light goes out, that’s the fuse with the current draw. Take into account that if you have the door open with a door-actuated dome light, the light won’t go out until the dome light fuse is pulled. Try shutting the door first and see if the test light goes out or goes dim. It should, or that may be your problem.

10. Should I run relays with my halogen headlights / taillights?

Modern halogen headlight and taillight bulbs can draw a lot of amperage, and can overheat the circuit breaker in some headlight switches. That can leave you in the dark. You can take the load off the headlight switch by installing a relay for the lights. The switch will only be turning the relay on and off, and the relay will be taking the load.

11. What’s the difference between an SPST and an SPDT relay?

A single-pole single-throw relay completes a circuit when power is applied to the relay. A single-pole double-throw relay can switch a connection from one circuit to another depending on whether the relay has power to it or not. An SPDT relay can disconnect a circuit when power is applied to the relay. An SPDT relay can also work like an SPST relay and complete a circuit when power is applied to the relay.

12. Should I use an ammeter or a voltmeter?

Voltmeters are much easier to install. Ammeters require large-gauge wire, and older ammeters don’t have enough amperage range to operate safely with modern high-output alternators. A voltmeter is hooked up as easily as a light bulb and gives a very accurate indication of your charging system condition.

13. Do I need to use a relay with my electric water pump, electric fuel pump, and cooling fan?

These components draw a lot of amperage. Using a relay puts the high amperage through the relay instead of the switch. All the dash switch does now is turn on the relay, and switch life is increased. We suggest running only one component from each relay.

14. I want to install Chevy Fuel Injection on my truck. Should I use a mass airflow sensor (MAF) or a speed density (MAP) TPI system?

Chevrolet had three basic TPI systems, other than the LT1. These were the 1985 TPI, which used the same intake manifold system as the 1986-1989 but had its own ECM and other electrical parts that were only used that year. There is also the 1986-1989 mass airflow (MAF) and 1990-1992 speed density (MAP). The 1990-1992 MAP system is a little easier to install and has a cleaner appearance because the MAF sensor doesn’t have to be mounted in front of the intake like on a 1986-1989 system. Driveability is the same for both systems when installed correctly. As for which year to use, the best advice is to go with the system you have the parts for.

15. When do I need a ballast resistor?

These were used with 12-volt points ignition systems to reduce voltage so the points would last longer. If you have a points ignition system, you’ll need a ballast resistor. If you have an electronic ignition such as a GM HEI, you won’t.

16. How do I connect an SPDT relay?

This diagram should explain things.

SPDT relay

17. What’s the toughest GM fuel injection swap you’ve seen?

Replacing a 1990s Chevy pickup V-8 TBI with either MAF or MAP TPI. The truck instrument cluster requires a certain input signal, the TPI requires a certain input signal, and the transmission requires a certain input signal. “Input signal” refers to the signal from a speed sensor, and GM changed the type of speed sensor signal several times during the years these vehicles were produced. This combination here produces the most headaches. In most cases, the TBI system being removed probably ran as good as the TPI system being installed.

18. Will disconnecting my GM fuel injection EGR valve make more horsepower?

Aside from getting you in trouble with state emissions laws, in most cases, disconnecting your EGR valve won’t make more horsepower on a stock GM engine, and we don’t recommend it. The computer may think something else is going on and do everything from retarding the timing to setting a Check Engine code.

19. What does a neutral safety indicator do?

This is an input that tells the computer whether the vehicle is in Park or Drive. It controls engine idle speed and EGR function. This computer input is looking for a ground in Park and no ground in Drive. You can use either the original neutral safety switch for this or a toggle switch or you can hook up to the parking brake switch so the input has a ground when the brake is on and ground when the brake is off.

20. Do I have to run a speed sensor with GM fuel injection?

A speed sensor helps set the engine’s idle speed, like increasing idle coming up to a stop sign. A speed sensor is needed with a 700-R4 transmission to help the computer control converter lockup. It affects driveability and emissions regardless of the transmission used.

21. What is a vehicle speed sensor?

A speed sensor tells the computer if the vehicle is moving and how fast it’s moving. A speed sensor can be attached at the speedometer cable connection on the transmission. Sometimes the existing speed sensor in the transmission or speedometer can be reused with a new fuel injection system.

22. What is VATS?

VATS is the abbreviation for “vehicle antitheft system.” This was a system used on some GM cars as early as 1988, which used a resistor in the ignition key. When the key was put in the ignition, the system would look for a specific resistance value from the key. If the system saw the wrong resistance value, it would not let the engine start. Some 1990-up TBI, all 1990-1992 MAP TPI systems, all LT1 systems, and some MAF TPI systems had VATS. When an engine that had VATS is transplanted into another vehicle, the VATS still operates if using the stock PROM chip. To get rid of the VATS, a PROM chip that has had the VATS portion of it removed has to be substituted for the stock PROM. If you have a no-start problem with your new fuel injection system, and you have spark but no fuel, the VATS could be your problem.

23. Can I use any PROM in my fuel injection computer?

For unknown reasons, all GM computer proms from TBI to TPI use the same-style socket. While the PROM you have may plug right in, it might not work correctly. For proper operation, be sure you have the correct PROM in your computer. Aftermarket PROM manufacturers can custom-build a chip for your engine if you tell them what kind of non-stock parts you have in the engine.

24. What is a cold-start injector?

This is the “choke” for TPI systems. The 1985-1989 GM TPI had a ninth injector controlled by a thermo time switch that added extra fuel during cold start. In 1989, GM did away with the ninth injector and just added extra fuel to the existing eight injectors for cold start.

25. Is GM Tuned-Port Injection better than throttle-body injection?

Both of these are big improvements over carburetion. TBI is great for driveability and gas mileage. Power is very good with a stock TBI system. TBI is much more sensitive to engine modifications than TPI. We recommend TBI for a daily driver and TPI if you want to modify the engine and make lots of power.

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