WHT Z06 Modifications: Disable CAGS

• Engine coolant 171° F or greater
• Barometric pressure is greater than 76 kPa
• Speed between 15 and 19 MPH
• Throttle position is 13% or less

Note that the actual parameters can vary depending on application; these are the values for the 2007 Z06.

During the lockout, a dashboard light that says "1 → 4" is illuminated and you are supposed to shift from 1st directly to 4th. There is no timeout; that is, the lockout continues as long as the above conditions persist.

The good news is that it's easy to defeat CAGS. Prior to 1996 when OBD-II went into effect, one could simply unplug the CAGS solenoid connector. However, OBD-II is required to detect failures that impact emissions, so disconnecting the CAGS solenoid on a 1996 or newer manual transmission Corvette will throw a P0803 trouble code (Skip Shift Solenoid Control Circuit) and cause a service emissions system (SES) warning light. A number of companies (including Chevrolet) sell !CAGS kits. Early kits consisted of a connector with a 24 Ω potted resistor to simulate the electrical load of the solenoid. Unfortunately, at 13.7 volts this works out to 571 mA or 7.8 watts and the resistor used is typically only rated for ½ watt. This means the resistor gets pretty hot and eventually burns out. Recent versions of the kit use a 2.2K Ω resistor which results in a more reasonable 6 mA of current.

The CAGS resistor fuse

More recently, a clever person realized that the CAGS circuit on the C6 Corvette has a dedicated fuse and that it is possible to add a resistor to the fuse and leave the solenoid connected; thus was born the "CAGS resistor fuse". The first one I saw was in a post on corvetteforum.com and it used a ¼ watt resistor hung off the top of the fuse. Here's one I made using a 20K Ω resistor.

My first refinement was to use a 0805 SMD resistor which is 2 X 1.25mm and fits nicely inside the fuse.

How to make a CAGS resistor fuse

• Cut a square hole in the side
• Bend the fuse link out of the case (it is very soft so go easy here)
• Cut a 1mm section out of the fuse link
• Tin the fuse link ends with solder
• Solder the resistor in place (tweezers come in handy at this point)
• Bend the fuse link back into the case
• Test the completed assembly for proper resistance

Surface mount soldering isn't that difficult. Here's a really good video called Tangent Tutorial 3: Surface Mount Soldering Techniques:

This is my favorite skip shift solution. It's cheap to try; the fuse costs less than $1 and the resistor is about 5¢.

Installation

Note: Remember that ultimately you are responsible for modifications you make to your own car so if you have any reservations about the information contained on this webpage, don't modify your car!

Once you finish making your fuse, install it in position #10 of the underhood fuse block:

Underhood fuse block	Fuse assignments

Important: The underhood fuse box is labeled with numbers and letters to identify connector positions. Fuse #10 uses pins D12 and E12. The 10 A fuse next to the "10" (pins A10 and B10) is actually #8 and if you install your CAGS fuse there, you'll disable your parking lights! This is the mistake most people who have trouble make.

Testing

CAGS is one of those things that can be difficult to demonstrate on command. Here's a recipe that should help:

• Bring the car up to temperature and find a safe area
• Come to a complete stop
• Launch with constant and minimal throttle
• Wait for 15 MPH and the "1 → 4" instrument cluster indicator

As it turns out, you can actually launch any manual transmission car with no throttle. This is an actual exercise used at Spring Mountain to familiarize students with the exact point of clutch engagement.

The CAGS LED Fuse

I think the CAGS resistor fuse is the way to go; but for a variety of reasons, some folks didn't want to make their own so I kept searching for a solution that didn't require soldering.

I've always been intrigued by the Littelfuse SmartGlow Fuse which "glows when it blows." It seems pretty cool; it's got an LED (actually two) but also it also seems a waste that you only get to see a LED light up when you've blown the fuse. It's really kind of dumb! I mean, once it does its thing, you toss perfectly good LEDs into the trash. That's just not right.

But one day it occurred to me that this is the perfect off-the-shelf solution to the skip shift problem.

How to make a CAGS LED Fuse

Executive summary: Really, the hardest part of this mod is blowing the fuse. Here are a number of techniques.

The Old School Method

• Take all necessary safety precautions: glasses, gloves, lead apron, etc.
• Strip the ends of a 1-2 foot piece of wire (22 gauge or larger is recommended)
• Wrap one end around a fuse terminal
• Tape the other end of the wire to ground
• Touch the other fuse terminal to the positive terminal of the battery
• Watch for the small arc and then observe the nice LED glow

It's important for you to see that the LED lights up and at the same intensity; if it doesn't, you've done something wrong.

It's a good idea to flip the fuse around and test the other LED; see CAGS LED Fuse Notes for more information.

Here are some pictures of the process. I used a scan tool (EFILive's FlashScan) to turn the CAGS circuit on for the last one:

Preparation	Blown SmartGlow Fuse	CAGS circuit hot

A small arc is created when you blow the SmartGlow fuse; this may leave a tiny divot in the fuse and whatever positive voltage source you're using. This can be avoided by arcing against sacrificial pieces of metal such as two coins. Wrap your wire around one terminal of the fuse, tape the other end of the wire to a ground source then hold a penny against the positive source with one hand and another penny against the other fuse terminal (be careful not to let the penny touch both fuse terminals!) Then touch the pennies together.

The Car Battery Charger Method

A corvetteforum.com member suggests using a car battery charger to blow the fuse. Connect the clips to the fuse and plug in; the fuse blows with no (external) sparks. This is one of the better methods I've heard of.

You do need to use a charger that can supply sufficient current. My charger is an antique that once belonged to my grandfather and is only rated for 6 amps but worked, as Grandpa would say, "Just dandy."

Important: Running too much current through a LED will shorten its life or even burn it out. Using a voltage source that outputs significantly more than 13.7 V will increase the current across the LED and can damage it. Some jump starters output 24 V and this higher voltage setting should not be used when blowing your fuse.

The Physical Break Method

Use a drill, Dremel tool, knife, etc to physically sever the fuse element. Note that you should still test that the fuse glows when 12 V is applied.

The Auto Parts Store Method

Several people have used another successful strategy: ask the store you buy your SmartGlow fuse from to hook it up to a battery charger and blow it for you. This has the advantage that you can install immediately the fuse before leaving the store parking lot. Plus it's worth it just to hear them say, "Wait, what now?"

After blowing the link in the fuse, install as described above.

When Things Go Wrong

A few people have complained that they install a CAGS LED Fuse and weeks or months later they get an engine check light. A few of them sent me their failed fuses so I could investigate.

Smart fuses contain two LEDs, one for each polarity (see CAGS LED Fuse Notes for the gory details). What I found is that one of the LEDS in failed fuses lights up but is very dim. Apparently something has damaged but not killed either the LED or the resistor and this causes a much lower current draw and this makes the ECM think the solenoid has failed. Flipping around or replacing with a new CAGS LED Fuse will fix this.

Note that my Z06 is my daily driver and I've been using the same Littelfuse SmartGlow Fuse for years. So if you have one fail, just replace it.

CAGS LED Fuse Notes

• The CAGS resistor and LED fuse mods only works on a C6 Corvette (2005 through 2013).

• These mods only disable the skip shift solenoid; the dashboard "1 → 4" indicator will still illuminate when the conditions for CAGS are met.

• The LED is only lit when the ECM energizes the skip shift circuit.

• The SmartGlow Fuse contains a 360 Ω resistor and two LEDs.

• An LED only emits light when its anode is connected to positive voltage and its cathode is connected to negative voltage (or ground). The two LEDs used in the SmartGlow Fuse are connected in opposite directions so that one or the other will light depending on which way the fuse is connected.

  If you somehow manage to blow one of the LEDs or the fuse is otherwise defective, it's possible that only one of them will light up. You can test for this by applying 12 V to the fuse and then flipping the fuse around and check the other LED. If you cannot get both to light up, there's something wrong with the fuse. If only one of the LEDs works, you have a 50/50 chance that when you install the fuse you'll get a engine check light.

• The schematic in the 2007 Y-Car factory service manual shows the skip shift solenoid to be the only device on the #10 (MAN TRANS) underhood fuse circuit.

  It also shows the following devices are fed by the #13 (MMRTD) underhood fuse circuit:

  • Reverse lockout solenoid
  • Electronic Suspension Control (ESC) Module (optional RPO F55: Magnetic Selective Ride Control)
  • Evaporative emissions canister vent solenoid

• The reverse inhibit solenoid blocks the reverse gear when power is not supplied; reverse is only available when the ECM energizes the circuit. If the fuse for this circuit is removed, you will not be able to engage reverse at all!

• The schematic in the 2004 Y-Car factory service manual shows CAGS fuse mods are not compatible with the C5 Corvette because, unfortunately, underhood fuse #19 (ENGIGN1) supplies power for:

  • Stop light switch
  • Mass air flow sensor
  • Clutch pedal position switch
  • TCC/Cruise release switch
  • Evaporative emissions canister purge solenoid
  • Evaporative emissions canister vent solenoid
  • Reverse inhibit solenoid
  • Skip shift solenoid

• The circuit description in the service manual for diagnostic trouble code (DTC) P0803 (Skip Shift Solenoid Control Circuit) concisely describes how the ECM can tell if the skip shift solenoid is working as designed:

  With the ignition ON battery voltage is supplied directly to the skip shift solenoid. The engine control module (ECM) controls the solenoid by grounding the control circuit via an internal switch called a driver. The driver supplies the ground for the component being controlled. Each driver has a fault line which the ECM monitors. When the ECM commands a component ON, the voltage of the control circuit should be low, near 0 volts. When the ECM commands the control circuit to a component OFF, the voltage potential of the circuit should be high, near battery voltage. If the internal fault detection circuit senses a voltage other than what is expected, the fault line status changes, causing DTC P0803 to be set. DTC P0803 is a type B DTC.

  The ECM will post a DTC P0803 and throw a malfunction indicator light (MIL) during the second consecutive trip (aka run cycle) that the skip shift circuit is found to be in a state other than commanded for more than five seconds.

  The ECM will clear the MIL on the first run cycle the test succeeds but the code remains in the DTC history until 40 successful run cycles.

• Some say they have pulled their skip shift fuse and not have the engine check light come on. Based on my own testing, I do not believe these claims. Here's the test:

  • Pull the skip shift fuse
  • Bring the car up to temperature
  • Let the car run an additional 10 seconds
  • Shut the engine off for 10 seconds
  • Start the car a second time and observe that the MIL is immediately illuminated

  A scan tool will show that the DTC is a P0803.

• I did some testing to see how large a resistor you could use and not get an engine check light. 20K Ω worked ok; 100K Ω generated a MIL.

• There are five SmartGlow fuse values: 10, 15, 20, 25 and 30 A. It does not matter which value you use since the fusible link will be blown; all of the fuse values in the assorted 5 Pack use the same 360 Ω resistor. The 10 A is probably the best choice because it's the correct value for the specific fuse location and won't cause confusion if someone who doesn't know about the CAGS LED fuse mod comes by later and replaces it.

• A fuse is a device that protects component and wiring from excessive current. It's true that when you use a blown SmartGlow fuse the skip shift circuit is no longer protected by a fuse. Instead, it's protected by the current limiting characteristic of a resistor.

• With the skip shift circuit on, I measured the current and battery voltage to be 26 mA @ 11.8 V. This tells us that the voltage drop across the 360 Ω resistor and 24 Ω solenoid is 9.3 V. We can subtract that value from the battery voltage to find the voltage drop across the LED (which is somewhat independent of the current applied to it) is 2.5 V.

  This means with a typical run voltage of 13.7 V, the drop across the 384 Ω load of the resistor and solenoid will be 11.2 V and the drop across just the resistor is 10.5 V. The maximum current in the normal case (skip shift circuit on) would be 29 mA or 0.3 watts.

  While I was making "skip shift circuit on" measurements I noticed the LED glows very dimly when the engine is on and not running (and the skip shift circuit is off). I saw 150 µA @ 11.8 V. We can use this value to calculate the resistance of the measurement circuit inside the ECM. The battery (11.8 V) minus the voltage drop across the LED (2.5 V) is 9.3 V. Divide that by 150 µA and we have 62K Ω. This explains why using a 100K Ω resistor doesn't work; the voltage drop across the 100K Ω resistor would let the sensor only see about 38% of the battery voltage. Based on these calculations, I would think a good value for a CAGS resistor fuse is 6.9K Ω. This would let the ECM see 90% of battery voltage.

• Based on its physical size (3.2 X 1.6mm), the SMD resistor used in the SmartGlow fuse is a 1206 which has a power rating of ¼ watt. This seems like a potential problem until you consider that the resistor is embedded in an encapsulant specifically chosen for its thermal management characteristics. Think of it as a transparent heat sink.

  In the CAGS LED fuse application, we can add in the resistance of the skip shift solenoid which is 24 Ω. That lowers the current at 13.7 V to 29 mA or 0.32 watts.

Other LED fuses

• Harbor Freight: They sell a 30 pack of mini blade fuses for $10. These are interesting because they use lower power surface mount LEDs. I measured its current draw to be 1.5 mA @ 12.5 V. The value of the SMD resistor is a 682 or 6.8K Ω. Based on its physical size (1.6 X 0.8mm) it's a 0603 which has a power rating of 1/16 watt. The LEDs are the same size as the resistor.

• Velleman: The 10 A part number is AFUM10L and these pretty cheap, less than $0.50 each. Surprise! These use an incandescent lamp! I measured a blown fuse to draw 40 mA @ 12.6 V.

  

• Imperial: The 10 A ten pack is part number 72179-5. I don't have any of these yet; they seem to be quite expensive.

Other Skip Shift Disable Methods

• Radio Shack: Disconnect the skip shift solenoid connector at the transmission and jam a 2.2K Ω resistor into the wiring harness; optionally wrap with black electrical tape.

• Programming: Another way to disable CAGS is to use a programmer such as EFILive's FlashScan. There are several ways to do this; for example you can lower the maximum throttle position to 0.0% or raise the minimum temperature to 600° F.

• Denial: By far the cheapest and easiest method, all you need to do is claim that the way you drive your Corvette, CAGS never kicks in!

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Craig Leres