TC lockup circuit question
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Junior User
Joined: Dec 2007
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From: D'Hanis, TX
TC lockup circuit question
I’m looking at the TC lock circuit from:
http://guzzle.rbmicro.com/tclock.html
Upon examination of this circuit and discussion, I have a question about the PCM and the TC.
Basically (when activated), the circuit separates the TC from the PCM and grounds the lead to the TC and puts a 22K ohm resistor on the lead going back to the PCM (to stop the OD light from flashing). In order to accomplish this, the author uses a DPDT relay. One pole to do the grounding and the other pole to insert the resistor.
Consider the following:
Assume that the supply voltage (with the alternator working) is 14 volts. The current through the 22K ohm resistor is less than 1 ma. I suspect (but don’t know for sure) that the output circuit of the PCM is capable of sinking a lot more extra current than that. (The circuit also inserts a LED to indicate when the TC is locked – a typical LED current draw is 35 ma).
Therefore, it seems to me that the circuit could be modified to use a simple SPDT relay, by connecting the resistor to the wire going to the PCM. (In the schematic – move the resistor from pin 3 to pin 2)
Does anyone have a electrical diagram of the contents of the PCM output circuit re: the TC lock part?
Thanks, Bob
http://guzzle.rbmicro.com/tclock.html
Upon examination of this circuit and discussion, I have a question about the PCM and the TC.
Basically (when activated), the circuit separates the TC from the PCM and grounds the lead to the TC and puts a 22K ohm resistor on the lead going back to the PCM (to stop the OD light from flashing). In order to accomplish this, the author uses a DPDT relay. One pole to do the grounding and the other pole to insert the resistor.
Consider the following:
Assume that the supply voltage (with the alternator working) is 14 volts. The current through the 22K ohm resistor is less than 1 ma. I suspect (but don’t know for sure) that the output circuit of the PCM is capable of sinking a lot more extra current than that. (The circuit also inserts a LED to indicate when the TC is locked – a typical LED current draw is 35 ma).
Therefore, it seems to me that the circuit could be modified to use a simple SPDT relay, by connecting the resistor to the wire going to the PCM. (In the schematic – move the resistor from pin 3 to pin 2)
Does anyone have a electrical diagram of the contents of the PCM output circuit re: the TC lock part?
Thanks, Bob
Lead Driver
Joined: Jan 2006
Posts: 6,267
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Bob, bob bob bob bob, bob bob.
You obviously know what youre talking about, but you might have to try it to prove this, I doubt anybody has an electronic schematic of the PCM, but even if they do, I'd just use the original plans, relay switches are cheap, PCM's are not.
You obviously know what youre talking about, but you might have to try it to prove this, I doubt anybody has an electronic schematic of the PCM, but even if they do, I'd just use the original plans, relay switches are cheap, PCM's are not.
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Junior User
Joined: Dec 2007
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From: D'Hanis, TX
I wasn't clear as to which resistor I was refereing to. I agree that the "optional Dimmer Resistor" can be installed to dim the LED (and to further limit the current to the LED). Note the parts list - "Red LED - 276-084 - 12 Volt" Thus the LED has its own builtin current limiting resistor.
The resistor that I'm talking about is "Resistor 1". Its purpose is, according to the orginal designer, to prevent the OverDrive light from being flashed by the PCM. In the deactivated mode, Resistor 1 is not connected to anything except the +12 supply. In the Activated mode, Resistor 1 is connected directly to the PCM. This would supply +12 volts (at very low current) to the PCM. I suspect that the TC lock is some type of solenoid valve, thus will require several ma's of current to activate.
Obviously, the PCM has some sensing circuit that causes it to flash the OD when it can't properly sence that the TC lock has been released.
At no time is Resistor 1 ever connected to the LED.
Bob
The resistor that I'm talking about is "Resistor 1". Its purpose is, according to the orginal designer, to prevent the OverDrive light from being flashed by the PCM. In the deactivated mode, Resistor 1 is not connected to anything except the +12 supply. In the Activated mode, Resistor 1 is connected directly to the PCM. This would supply +12 volts (at very low current) to the PCM. I suspect that the TC lock is some type of solenoid valve, thus will require several ma's of current to activate.
Obviously, the PCM has some sensing circuit that causes it to flash the OD when it can't properly sence that the TC lock has been released.
At no time is Resistor 1 ever connected to the LED.
Bob
More Turbo
Joined: Apr 2008
Posts: 639
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From: Indiana
I missed the other one, happens when at work and trying to check FTE. You are right about the insignificant current though I think it may be a sensing circuit more than an activation circuit. I thought I have seen a simpler version floating around as well that may not have even had the resistor.
Dang, this 7 x 12 schedule is starting to cloud the mind.
Dang, this 7 x 12 schedule is starting to cloud the mind.
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Joined: May 2008
Posts: 860
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From: Liberty Lake, WA
Thread Starter
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Junior User
Joined: Dec 2007
Posts: 86
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From: D'Hanis, TX
TC Lock
You are correct - much simpler & can even be done without a relay - just a switch. However, it doesn't obvertly address the issue of the OD light flashing. (there might be enough voltage across the green LED to trick the PCM. One would have to test to see.
My ultimate goal is to add a safety protection circuit (in case an old f... like me forgets to turn the lock off when needed) to release the lock when the RPM drops down below what other threads consider the 'acceptable' RPM.
Thanks,
Bob
My ultimate goal is to add a safety protection circuit (in case an old f... like me forgets to turn the lock off when needed) to release the lock when the RPM drops down below what other threads consider the 'acceptable' RPM.
Thanks,
Bob
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Fleet Mechanic
Joined: Feb 2005
Posts: 1,797
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From: Northern Colorado
To lock the TC, the PCM simply grounds the purple w/yellow trace to ground. A simple TC switch only requires a single switch to perform this function, ground the purple w/yellow trace.... simple.
The problem arises with some of the newer PCMs is when the PCM tries to unlock the TC and senses that it can not. A transmission code is thrown and the OD light begins to flash.
With the schematic shown, the relay isolates the purple w/yellow trace from both the transmission and the PCM when engaged.
As far as Resistor 1 goes, it simply is there to present a load to the PCM to make it believe that it is still controlling the TC when it attempts to unlock it, the same as if it were still connected to the TC solnoid in the transmission.
The problem arises with some of the newer PCMs is when the PCM tries to unlock the TC and senses that it can not. A transmission code is thrown and the OD light begins to flash.
With the schematic shown, the relay isolates the purple w/yellow trace from both the transmission and the PCM when engaged.
As far as Resistor 1 goes, it simply is there to present a load to the PCM to make it believe that it is still controlling the TC when it attempts to unlock it, the same as if it were still connected to the TC solnoid in the transmission.
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Elder User
Joined: May 2008
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From: Liberty Lake, WA
You are correct - much simpler & can even be done without a relay - just a switch. However, it doesn't obvertly address the issue of the OD light flashing. (there might be enough voltage across the green LED to trick the PCM. One would have to test to see.
My ultimate goal is to add a safety protection circuit (in case an old f... like me forgets to turn the lock off when needed) to release the lock when the RPM drops down below what other threads consider the 'acceptable' RPM.
Thanks,
Bob
My ultimate goal is to add a safety protection circuit (in case an old f... like me forgets to turn the lock off when needed) to release the lock when the RPM drops down below what other threads consider the 'acceptable' RPM.
Thanks,
Bob
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