tcm to control SSR correctly

[Dazza92VP]

hi guys
just needing a bit of help from the circuitry gods on here i have a 2006 isuzu npr with an AMT in it this thing has cooked about 12 -13 TCMs in its life. we have checked everything had the wiring harness out stripped new grounds terminals and refitted its had a compete trans brand new assembly swapped with new shift stroke and actuators fitted it was over charging but has since had new batteries and alternator fitted and another 3 TCMs since everytime it dies it cooks the driver that activates the gear select solenoids either 1,3,5 or R,2,4,6. now what ive done in an attempt to stop it at least cooking TCMs ive fitted SSRs to it instead so the TCM activates the SSRs then the SSRs take the load of the actuator solenoids ive also fitted flyback diodes to the SSRs for any back spike that may happen.

this is all good but the problem is that the tcm out puts a few volts constantly to the SSRs and then starts to pwm above that but they only require 1v i think to start to activate so what i need to do is remove that initial voltage to the SSRs so they turn off and only the pwm square wave is getting to them using an opamp circuit or resistors or something but im not sure how to build it once i know components i can assemble it.

Output fromTCM

these are the SSRs https://www.jaycar.com.au/solid-state-r ... g/p/SY4086

[picvrss]

Shouldn't need anything as elaborate as an opamp.

A low-voltage zener diode, say 3.3v, connected between each driver output and its respective SSR should block the residual DC but still allow the now slightly reduced PWM to switch the SSR correctly.

[Dazza92VP]

Awesome I’ll give that ago this thing has been haunting me thanks for the heads up I’ll fit them and report back I’ll owe you a beer or two if it starts working

[Dazza92VP]

Any particular wattage zener and I’m just connecting it in series with it?

[picvrss]

The SSR's input current should be around the 10 - 15mA mark with a 12V supply. For a 3.3v zener, power dissipation works out at around 45mW, so a standard 400mW rated device will be more than enough.

Yes, just in series. Be careful of polarity; positive current flow is into cathode of zener. You won't hurt anything if you get them the wrong way around, but you then won't get the desired result. Good luck!

[Dazza92VP]

Ok ended up with two 3.3v 1watt zeners fitted them but it seems to latch it to the 3.3v and not allow anything above the 3.3 to pass through and activate the relay. I tried around the opposite way but obviously it’s a diode and just acted like it wasn’t there any other ideas?

Just need what’s circled in red to be dropped to 0v or as close to as not to activate the relays oh and this is 24v to forgot to say

[picvrss]

I'm not familiar with the specific output driver of the Izuzu TCM, but relay or solenoid drivers are typically open-collector transistors, or open-drain MOSFETs. The driver acts as a switch, i.e. it's either fully on or fully off. When ON, the full supply voltage is developed ACROSS the load. Conversely, when the driver is OFF, there should be zero volts ACROSS the load because no current can flow through it.

The fact that you appear to have a residual DC voltage across the load (the load being either the original solenoid, or now, the SSR input) in the driver's OFF state suggests there's some form of additional, possibly load-current monitoring, circuitry within the driver. Even so, the zener should block the residual but then conduct as soon as the zener voltage is exceeded, i.e. when the driver is ON. Whether a 12v or 24v supply is being used is not important.

I feel there's something connected incorrectly. You've explained your situation well, so I'm sorry if I've missed something. The original solenoids would have had one of their terminals connected either to +24V or to ground. Can you tell me which?

[Dazza92VP]

i think its load sensing like you said but it is operating in emergency mode as i cant get the trans to learn positions until i sort this out and after deep frying so many tcms im petrified of hooking another to it and smoking this one with out this setup in it to protect it.
although it does have this voltage being outputted it doesnt set a fault code at all for the shift solenoids its only after a failed shift that it does so beats me as to why like you said its actually there in the first place
this is the diagram for it as it is supposed to be but ive added the relays between the shift solenoids and the tcm after the solder joint from pins d21 and d31 to 1,3,5 solenoid and d8 and d9 to 2,4,R solenoid

Screenshot 2020-12-09 201638.jpg (114.25 KiB) Viewed 2715 times

sorry for the pick circling the issue area i wasnt sure i had explained it correctly i posted once in one of the electrical engineering forums once and got flamed for not providing clear and detailed instructions of what i was trying to do so trying not to anger the electrical engineering gods around here to much

[Dazza92VP]

as another thought also i did add temp mechanical relays into the circuit between the SSRs and the TCM just to see if eliminating that small amount of constant voltage that the TCM is output made it work and it does shift it completely misses where is suppose to stop and shoots pretty much through to the next gate but it shifts and the mechanical relays go absolutely mental due to the pwm so i should work perfectly fine if we can filter that low voltage out

[picvrss]

OK, things are a bit clearer now that you've provided a schematic; unfortunately, it doesn't make the solution any easier. Sorry I gave you such a simple "solution" first: using a zener. I had my suspicion about the source of that offset, but thought "why complicate things to start with?".

The drive method shown is an emitter-follower configuration into a ground-referenced load, and not open-collector. As you're probably aware, solenoids in automatic transmissions control hydraulic line pressure, and this is done by varying the PWM duty cycle. But it's not enough for the control circuitry to simply drive with a certain PWM and hope this results in a correct positioning of the solenoid armature. By monitoring the solenoid's current in a feedback loop, the controller adjust's the PWM to get the desired result. It's this monitoring circuitry that's complicating things. By using an SSR - or any device between the TCM and the solenoid - you're depriving the controiler of this feedback.

Logic tells me that the solenoid driver devices in the TCM should be sufficiently rated to do their job because (I'm guessing here) not every owner of an Izuzu AT has cooked 12 - 13 TCMs! So, this is justification to wonder whether something else outside the TCM has been overlooked in diagnosing your specific situation. From your description though, it appears everything has been checked.

If the driver devices are in fact dying because they're simply underrated or prematurely deteriorating, then I'd be substituting alternative devices.

An obviously more complicated solution would be to modify the circuitry to separate the present situation where both load drive and load sense for a given solenoid are on the same node, e.g D21 and D31 in your circuit.

Other than my suggestions above, I'm sorry I haven't given a conclusive fix here. Perhaps someone else has more experience with known problems of these Izuzu ATs.