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Discussion Starter · #1 ·
Last week I had a costly incident at TMP while running my Chevy II. LS Turbo combo with holley dominator efi. T400 w/ and gear vendors. The car has been running the current configuration on the street and track for 7 years. This issue showed up as I let go of the trans brake button. The car left as per normal (1.25 ish) and just about at the shift point it started to rattle the tires (275 pro on reasonably decent prep) so I short shifted into second and instantly the rear end locked up solid. I brought the car to a sliding stop and shut it off. We ended up having to pull the car to the trailer with the rear tires on skates, and proceeded to drag the car into the trailer. Once I got the car home, and unstrapped it, I found whatever was locked, to be unlocked, and we rolled the car on to the hoist. We all assumed I had broken the rear gear, so I disassembled the rear, only to find it mint. We then pulled the trans and sent it off to Hutch for an inspection. When I got around to looking at the datalog, I didn't find anything abnormal. The green light for the TB being on shows on until I release the button, and ds rpm starts to show. I see where I started to loose the tire, shift, and then we broke. The TB light never comes on in the datalog after I released it. Got a mssg from Hutch, and he told be the TB had come on, and thats what locked up the driveline. Both Hutch, and Greg Powrie who tunes the car believe I have a flyback voltage issue, even though I have the holley FB voltage attachment wired into the trans brake circuit. I DO however have the reverse light switch tied into the TB button so I don't have to depress the button to back up. Is the flyback coming from there? I can't say, but this has never happened since the car was built, with not changes to the set up. Now, can I add a toggle switch inline between the reverse light wire, and the TB connection on the trans, to help protect against flyback voltage? Does anyone else have experience with this sort is scenario?
 

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Here's how I've had my Mustang wired since 2007. It has a reverse light switch in the tranny so for normal driving when I put it in reverse the power to the lights is routed to the reverse solenoid and the car backs up normally. When using the TB I use my cruise control switch to send a signal to either a common relay or in later years, to a SS relay. So the power to the reverse solenoid was always thru a relay. And yes, the reverse lights still shared the power circuit so activating the TB would turn on the reverse lights. Until 2018 I ran regular incandescent bulbs for reverse lights, in 2018 I changed those to LEDs. Never a problem with mine after 1300ish runs.

Just recently I changed to the Haltech NEXUS R5 ECU and the TB activation signal is processed by the ECU which internally controls the power in it's wiring harness to the reverse solenoid. It also have the circuitry internally to set the car up for bump/creep. Only have 30ish runs on this setup but hasn't been a problem either.

I'm trying to imagine what someone thinks is happening. I always thought flyback voltage was only a factor immediately after removing power from a circuit and was a short, transient pulse that would last milli-seconds..
 

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I wonder if you don't have a wiring issue, Seems odd that it worked fine for 7yrs and all of the sudden you have an issue, Maybe rattling the tires caused something to short and when the power was turned off it broke the connection.
 

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I don't know what happened but that seems fucking scary for something like that to happen on a pass. Guess it could have been worse and happened in high gear. If it were me, I would remove the reverse light switch and just use the transbrake switch to back up. Glad ya did not wad it up. I know ya got a lot of time and effort in your car.
 

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Discussion Starter · #8 ·
I wonder if you don't have a wiring issue, Seems odd that it worked fine for 7yrs and all of the sudden you have an issue, Maybe rattling the tires caused something to short and when the power was turned off it broke the connection.
I too am wondering if the cheesy little arm/spring on the microswitch on the shifter (B&M Stealth Pro Ratchet) may have vibrated hard enough to send a momentary signal. Hutch said it only takes a second of voltage to engage the band onto the drum, and once they make contact, the rotation with suck the band tight on the drum.
 

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Discussion Starter · #9 · (Edited)
Are you using a solid state or mechanical relay for the transbrake? If its mechanical and the tire shake was bad enough it's possible that the tire shake caused a momentary activation of the relay.
Solid state NOS Nitrous Solenoid. Power comes from a key on source (street car) that goes through a switch with a light, to let me know the SS relay has power.
 

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Discussion Starter · #10 ·
Have you tried to start the car since this even happened?
Yes. We fired it after finding the rear end ok, and before starting to remove the trans. No issues. Fired right up.
 

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Discussion Starter · #11 ·
So in my head, I'm thinking of adding a good quality toggle switch between the reverse light switch, and the TB connection on the trans. This way I can isolate the reverse switch from the TB after doing my burnout. Over the winter we will make some wiring changes. All the wiring is tucked under the factory console and is a huge pia to access. Greg also suggested pulling the led bulbs out of the reverse lights and going back to incandescent bulbs, which should suck up any flyback voltage in the reverse light circuit. Greg also wondered if we should power the SS relay from the TB button. Its a good quality Biondo button, so it should be ok with some momentary current. This way the relay only has 12V while I'm on the TB button. Right now the power for the relay comes from a key on source...which has its own switch with a light so that I am aware that I have turned power on for the SS relay circuit....street car. Thoughts on the added inline switch?
 

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Discussion Starter · #13 ·
Here's how I've had my Mustang wired since 2007. It has a reverse light switch in the tranny so for normal driving when I put it in reverse the power to the lights is routed to the reverse solenoid and the car backs up normally. When using the TB I use my cruise control switch to send a signal to either a common relay or in later years, to a SS relay. So the power to the reverse solenoid was always thru a relay. And yes, the reverse lights still shared the power circuit so activating the TB would turn on the reverse lights. Until 2018 I ran regular incandescent bulbs for reverse lights, in 2018 I changed those to LEDs. Never a problem with mine after 1300ish runs.

Just recently I changed to the Haltech NEXUS R5 ECU and the TB activation signal is processed by the ECU which internally controls the power in it's wiring harness to the reverse solenoid. It also have the circuitry internally to set the car up for bump/creep. Only have 30ish runs on this setup but hasn't been a problem either.

I'm trying to imagine what someone thinks is happening. I always thought flyback voltage was only a factor immediately after removing power from a circuit and was a short, transient pulse that would last milli-seconds..
That's what I discussed with Hutch, as the datalog shows the 12V blip that triggered the TB didn't come from the button. Hutch said inertia will suck the band onto the drum and hold it, so it only needs a small blip of power to trigger the TB solenoid and lock the band onto the drum.
 

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A clamping diode could be used too. Wiring a diode in reverse bias and parallel to the transbrake solenoid would take care of any flyback anytime the solenoid is activated (button or reverse switch). Where is the Holley unit that is supposed to take care of the flyback wired to?
 

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Can you explain what a clamping diode is?

Yeah sure. When the magnetic field collapses around a coil when a switch is turned off it induces a high voltage spike in reverse polarity of regular current flow. A diode only allows current in one direction. So you wire it in parallel and reverse bias with the coil (anode connected to the - side of the circuit and the cathode connected to the + side of the circuit in parallel).

When the current is flowing through the coil the diode is blocking on the parallel leg and not really doing anything. When the coil is turned off and the magnetic field collapses and the induction takes place, the induced electricity has a path to travel because the diode will be forward biased because of the polarity flip. In essence the flyback voltage will be clamped/eliminated.
 
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