SL-C Manual: Electrical

The SL-C has two distinct electrical systems: one to operate the engine and related controls, and the other to operate everything else. We’ll call them the engine and chassis wiring systems, respectively.

Some SL-Cs were shipped with a custom version of the popular Infinitybox multiplex electrical system, including a 3-cell system with integrated security, wireless remotes, and an SL-C-specific wiring harness that makes installation of a powerful electrical system as simple as routing and fastening the cells and the wiring harness, and plugging the connectors into the loads.

The Infinitybox system comes with a comprehensive installation manual that goes through a step-by-step installation process, including where to mount key parts like the battery, the cells and the harness. For builders who want to do advance planning before the delivery of their kit, the Infinitybox manual may be available separately from Infinitybox. Because the manual with the Infinitybox system is so specific, the details aren't repeated here.

You can read more about the Infinitybox system, including available options at the Infinitybox site.

Older cars that were not shipped with an Infinitybox system can be retrofitted with the kit from Superlite Cars at extra cost. 

All cars before late 2011 or early 2012 shipped with a traditional wiring harness from American Autowire. This system is viable, but requires more understanding of wiring, electricity basics, and is not a plug-and-play solution like the Infinitybox system, nor does it have the reliability and sophistication of the Infinitybox solution.

This section documents wiring a car this way, but a builder considering using a traditional wiring system should be familiar with wiring in general, and automotive wiring in particular. If you are not comfortable with a traditional wiring harness, Superlite or Infinitybox can provide the new Infinitybox system with integrated harnesses specific to the SL-C.

If you are not confident in your ability to correctly wire a safe in a safe manner, do not attempt this task without professional assistance. Wiring errors can cause fires which can result in substantial property damage and personal injury and death. You are responsible for safely wiring your car.

The harness has oil and fuel-resistant wires, and they are marked about every 6 inches or so as to their function. This makes it easier to understand where every wire should terminate, and also makes debugging easier.

Every wire on the harness is unterminated. This is the norm for universal harnesses. It will make your task easier if you examine every load (e.g., lights, horn, fuel pumps, etc.) for the correct connector and source these first. That way you can make one trip to the store for connectors.

Before you begin routing the harness, make up a worksheet of every load you intend to power. When you have identified every load, start by removing excess wires from the harness, one at a time. For example, since you probably won’t have a power window system in the car, you could remove that wire from the harness (unless you planned another use for that circuit). To do that, carefully pull the affected wires, one at a time, through the cable ties, and remove them from the fuse panel by gently tugging on the wire as it enters the fuse box. You can save the wires as spares if desired.

This is also the time to move the wires that power the engine computer and related harness to the rear of the car from the front part of the harness.

The next step is to locate the fuse panel in a place that is easy to service, and centrally located in terms of wire runs. For most cars, that will be under the dash, on the left side. The fuse box has legs that are designed to be bolted through to a mounting panel. You can mount the panel upside-down so it is attached to the upper dash panel, or make a small bracket to allow it to face the driver’s legs, mounted on the side kick panel.

Mount the horn relay near the horn.

The large harness branch is to be routed to the front of the car. Uncoil it carefully and route each wire to the intended load. You will need to make a hole in the front of the chassis to run the wire coil through. Be sure to use the proper size grommet, and seal the opening with silicone or other appropriate sealant.

Terminate each wire to the appropriate load only after you have finished the routing completely. It’s best to wait until you have run all the wires, protected the cables with sheathing, and secured the cables to the chassis in a way that will prevent chafing and damage from excessive heat.

You can route the rear branch through a hole drilled in the chassis near the fuse box. Passing the wire bundle through the kick panel and running it down the driver side chassis rails above the coolant tube to the rear of the car is the most common solution. Use grommets and seal all openings made in the chassis or body panels.

You will need to plan for relays if you use the old, traditional harness. Most cars will use a dozen or more relays to control everything (lights, horn, starter, etc.) and so you should have a plan to locate all of them. Relays need to be near the switch load, so they will be located all over the car. 

For those who need help with traditional harnesses, several web sites have useful information, including Painless Wiring, the Ron Francis wiring site and others.

While the chassis wiring system controls everything else in the car, the engine wiring system controls the engine operation, including the fuel delivery, and spark control.

The only connections needed between the chassis wiring system and the engine controller are a constant 12V and a switched 12V that follows ignition. The chassis harness provides a switched wire for the engine controller's fusebox; you need to wire a constant 12V wire to it as well. This easy to do if you have mounted the battery in the rear as suggested. The constant 12V can be a small wire (16-18 gauge) run directly from the battery, and should be fused with an inline fuse like this one.

Because builders can select any engine from a wide array of sources, and may elect to use injection or carburation, the kit does not include a specific engine wiring system. If you are using a GM LS-series engine an obvious choice is the GMPP engine controller kit, as seen on the GMPP web site. Select the controller that is appropriate for your engine.

With the popular LS-series engines, in almost all cases, it is easier to remove the intake manifold and turn it around 180 degrees in order for the intake opening to point to the rear of the engine, and that is what these instructions assume. If you do that, you will need to take the harness apart, and re-route parts of it as shown below:

• MAP Sensor from front of engine to the rear.

• MAF sensor from the front of engine to the rear.

• TPS sensor from the front of engine to the rear.

• Oil Pressure sensor from the rear of intake to where you have mounted your oil pressure sender (see Senders section below).

For more information on rotating the intake manifold, see Engine – Intake Manifold section above.

In the 2nd photo below you will see that the engine wiring harness fuse box and ECU have been mounted to the right side of the engine on the front wall of the engine bay. For LHD builders, this should be mounted on the other side, as the rear Infinitybox Powercell is mounted there. As is common with his manual, some pictures, like the ones below, show parts on the car that are not part of the standard kit.

While not strictly a part of building the kit itself, many people use the popular GM LS-series engines in their SL-Cs and some have had problems making the tachometer work correctly. This section details what is needed to make either of the standard Koso gauges work correctly. It is also applicable to other instruments and tachs as well.

Failure to get any tach signal at all, or an inaccurate one is almost always due to one or both of two things:

1. The tach signal as delivered from the GM ECU harness is a low-voltage square wave, and typically not adequate for most tachs. The solution, as documented in the LS controller documentation, is to build a pull-up resistor circuit. This is easy to do, and, assuming the rest of the wiring is done correctly, will always make the tach operate. See the diagram below for details.

LS Series pull-up resistor network diagram

The bulkhead connector on the GM harness is used to source the tach output. Cavity “C” usually has a white wire, and is the Tach Out wire. The diagram above shows how you wire in a resistor to a source of switched ignition power to modify the normal output into a form that is compatible with the Koso and other tachometers. The wire marked “Pull High Tach Out” in the diagram goes to the Koso tach input.

You can get the appropriate resistor from your local electronics supply store. Known working resistor values include 4.7K ohms, and 5K ohms. 1/4 or 1/2 watt ratings are fine for this purpose. 

2. The second problem - usually manifested by a working but inaccurate tach - is that the GM ECU sends out a signal that is two pulses per rev, a signal that is often associated with 4-cylinder engines. The solution is to tell your tach that it is monitoring a 4-cylinder engine. Use the Koso documentation to set the gauge accordingly, or whatever tach you are using.

The harness is built assuming a battery mounted in the space behind the fuel tank, on the driver’s side of the car (for left-hand drive cars). Regular wiring systems can mount the battery anywhere, but close to the fuse box is desirable to reduce voltage drop.

The picture above shows an aluminum battery box bottom where the battery should be located when using the Infinitybox harness. This is also a good place for the battery if you are using a traditional harness.

If you plan to race, or run the car at track days, you may want to plan a battery cut-off switch. It should be wired to cut power from the battery and alternator, which should stop the engine from running. Most sanctioning bodies also require a battery to be mounted in a separate box to contain spillage in a crash.

Many racers use gel-filled batteries instead of the traditional lead-acid type to reduce the risk of spillage in normal use or in an accident. These are also great for the street.

Batteries are one place where significant weight savings can be accomplished, so select your battery with both performance and weight in mind. Most GM LS engines will need at least a 590 CCA battery, and higher compression engines will do better with more capacity in the charging system. For comparison, the standard Corvette battery in the C6 Z06 has 590 CCA, but the Optima 78-size battery (a more-or-less direct replacement) has 800 in the same size. In this case, more is better. Please note that a battery that started your engine in a Corvette or other car may not be adequate for use in an SLC, as the starter and flywheel will be different in an SLC, and that will affect cranking speed.

Experience has shown that high compression engines of large displacement when coupled with the Graziano transaxle and starter need lots of cold cranking amps. If you are having trouble starting the car and are sure you have proper wiring, including size, use a larger battery. One that has solved many starting problems is the Odyssey 079-2150 battery. This has 1150 CCA, and has started even the largest, high compression engines using the standard Bosch starter with the Graziano transaxle with no problems. Although it is heavy, it is the recommended battery for SLCs with slow cranking or hard starting.

Batteries are also available in reverse terminals, meaning that the position of the positive and negative terminals is reversed. There is no performance difference, but be sure you know where your positive and negative posts are before you buy a replacement battery, and confirm they are the same as the battery for which you originally wired the car.

Over the years, GM has manufactured two different physical designs for alternators, the original CS-144 case, which was used until the 80s, and the newer CS-130 design, which is mostly still used in current cars.
The CS-144 case is shown here:

The CS-130 case is smaller, and an example is shown here:

The pictures are not to the same scale.

Either of them can be sourced in a 1-wire or 3-wire model. 1-wire alternators are easier to install, but have traditionally suffered from low or no output until the engine sees a specific minimum RPM and thus are thought to be less suitable for street use. 3-wire alternators don’t have this issue, but the wiring is more complex.

If you are using a traditional style wiring harness (i.e., not using the GMPP engine wiring harness), the diagram below shows how to connect a 3-wire alternator:

If you are using a 3-wire alternator and the GMPP harness, the harness wire marked “alternator” should go to the “L” terminal on the alternator. That’s the only connection you need to make, aside from the battery post connector. You’ll probably have to acquire a “repair connector” (available at Alternator Parts  here or at local alternator repair shops) to get the correct connector to go to your alternator, and solder or crimp the alternator wire from the GMPP harness to the connector wire that goers to the “L” terminal. 

Don’t apply 12V to this connection at the alternator as it will cause the internal voltage regulator in the alternator to fail- the harness wire is already stepped down to the correct voltage, and does not need an additional resistor.

Of course, a simple one-wire alternator just has the one power wire, as the name suggests, running from the single alternator power stud to the battery. If you are using a one-wire, the "Alternator" wire in the GMPP harness can be taped off.

The Infinitybox manual, or the relevant Infinitybox tech sheet should be followed when using the Infinitybox system and harness.

If you make a custom cable from the alternator power stud to the battery, be sure to size it correctly. In general, a 4-AWG cable with good ends is safe for short runs. For longer runs, use a wire size calculator like this one to control voltage drop.

Mounting the alternators is important. Most of the front dress systems mount the alternator too high, where it fouls the chassis bars.

The optimal place for the alternator is down low, on the driver’s side (allowing the passenger side/low position for the AC compressor). Superlite has brackets available for most engines for this mounting option. Alternatively, some builders have had success with the front dress kit from GM that was originally designed for the Cadillac CTS-V as shown at Summit Racing. 

Proper alternator selection is important, not just in terms of physical fit, but also in terms of capacity. Modern cars, especially with fuel injected engines, draw much more power than older cars, and alternators have evolved to handle those additional loads. As a general rule of thumb, you should consider that a 130-amp alternator is the minimum for a street car with AC. A race car may be able to get by with less, depending on the load the fuel pumps draw. A wide variety of alternators in various sizes and capacities is available from Quick Start.
Here is a picture of an engine with a custom front dress with a low-mounted CS-144 style alternator and Sanden compressor.

The chassis wiring harness connects to the steering column and does not need additional attention. 

For builders using the traditional wiring method, the relevant pinouts for the 40-pin connector on the original XLR-derived column are here, courtesy of Wayne Marov:

The wiper stalk on column will not operate multiple wiper speeds or implement an intermittent function with the old-style harness. This is because the wiper switch is not a traditional switch in the sense that it opens and closes a circuit but instead provides a different resistance when the lever is moved. This approach is common in modern cars, but does not work in the old-style harness. You can remove the wiper switch from the column and use a separate switch for low and high speed.

Likewise, the Infinitybox system handles the switches by using only the high speed switch position to actuate the wipers, meaning that it turns them into a one-speed (high) wiper. This is possibly a reasonable solution as the low speed is pretty slow and is more of an intermittent position for some.

Another option is to use an aftermarket automatic wiper system that has a rain sensor; these are typically designed to understand the existing wiper controls, and you may be able to use such a system to restore the full low-high intermittent functionality, as well as add the rain-sensing function.

The SL-C now uses the standard KOSO RX2N gauge cluster, which has a large analog tachometer, all necessary warning lights, a shift light, speedometer and odometer, a fuel gauge, and temp warnings and displays. The connections to this gauge are described in the Koso manual.

The speed sensor for the gauge cluster can be mounted on any wheel, and is shown here mounted on the upright on a front wheel. The left front is preferable as it involves the shortest run of wire to the instrument from the sender; other wheel positions will likely involve lengthening the sensor harness.

The back of the rotor for the 4-piston Brembo-equipped cars will usually not return a reliable speed because the rotor has wheels studs (which are usually counted by the sensor) but also an irregularly-spaced number of holes drilled in the rotor. The speed sensor is normally confused by the three different surfaces (back of wheel stud, back of rotor and holes in rotor) combined with the irregular spacing, and thus cannot deliver a reliable signal to the gauge. The use of aftermarket two-piece rotors may alleviate this problem, depending on their design. Another solution is a GPS module like this one. Some builders have found that using this module and sending the speed output to the GMPP controllers also solves erratic idle problems.

Earlier SL-Cs were shipped with the DigiDash gauge. Whatever chassis harness you are using, only small changes are needed to use it. One of the changes is the speed sensor, details of which are shown below. The photos below show the DigiDash speed sensor mounted through the upright. It can be triggered by fitting flat washers to the rear of the wheel studs. This example does not use the supplied DigiDash magnets. If you are using 3rd party magnets please ensure that they are mounted in the same magnetic orientation as is labeled for the DigiDash supplied units.

If you are using the Koso gauge and the standard Centroid fuel level sensor, wire them according to the instructions supplied with each part, except that you should interrupt the Send signal from the sender to the gauge with a relay whose coil is powered by switched 12V. Not doing so will frequently work for some time, but failure to follow this setup will eventually result is a Koso that reads Full when it isn’t, particularly if you isolate the battery when not in use (which you should be doing…). You’ll know when you run out of fuel!

You can use an adaptor to install the oil temp sender & oil pressure sender, and to also accommodate the GM factory oil pressure sender that is relocated if the intake manifold is reversed. The photo above shows you one example of how you can mount the DigiDash senders. The GM factory sender is on the left, the DigiDash senders are at the bottom.
<???Need pics of the Koso senders? >
The top outlets that are temporarily blanked off are for connection to oil cooler. If you are not going to use an oil cooler, you must connect those top connections together to maintain oil flow. 

This picture also shows the transmission mount that is poly-isolated, instead of being solid-mounted.

The water temperature sender can be fitted to the water pump housing just behind the thermostat. If you choose this option, you will need to drill and tap a 1/8” NPT thread for the DigiDash setup or a similar metric hole for the Koso gauge cluster. Many builders use the existing, pre-drilled and tapped holes in the head to obtain water temp. This is arguably better, as the temp will be at its highest. It's also where the GMPP controllers typically get temp, so readings will be consistent between the gauge and the car's engine computer.

If you are using the Infinitybox, an almost unlimited number of switches can be used in the SL-C, since that system doesn’t actually switch the load, but uses a very low current at about 2 volts to sense switch state.

Other chassis harnesses usually switch the actual load (a light, motor, etc) and as such, the switches you select need to reflect the current demanded by the load. Choose your switches carefully.

For most functions with Infinitybox, a simple single-pole, single-throw (SPST) switch is adequate. Some applications need only a momentary switch, and some need a latching switch. But the Infinitybox system can be reprogrammed to allow a momentary contact switch even for latching applications. The other wiring systems typically require you to select a switch that natively latches, or not, etc., based on the requirements for your specific load.

For the hydraulic lift kit, it is easiest to use the switch provided. However, any DPDT switch that fits your interior scheme and can support the load requirements of the lift kit pump motor can be substituted. The air lift system uses the switch provided with the kit, and must be wired according to the instructions in the kit.

Since the switches for the turn signals and wipers are on the steering column, the typical builder has only to supply a horn switch and an engine start switch or traditional keyed off/accessory/ignition/start switch.

The AC/heat system has its own controls, with provided knobs.

The builder must supply a horn if one is required. Track day and race cars don’t usually have horns. The chassis harness expects the horns to be located on the passenger side radiator support brackets. The photo shows an installation on the other side of the car; reverse this for LHD cars.