Mechanics

HOW IS THE CONVERTER LOCKUP SOLENOID ON A J2000 ACTIVATED?

1984 Pontiac Sunbird • 200,000 miles

I would like to know how the ECM activates the torque converter lockup. There are 2 wires going to the lockup solenoid at the front of the transmission, a purple and a brown/black. If I put power to one and ground the other there is no draw if the vehicle is not running, but if it is running at the speed the lockup would normally engage it blows a 20 amp fuse. This does seem to indicate that the circuit is somehow switched witin the transmission to allow lockup only at the correct time. What I don't understand is how the solenoid is activated through thses 2 wires unless there are wires to the transmission somewhere else that I am not a ware of. If these are the only 2, I should be able to activate the solenoid by duplicating what the ECM does. The lockup was taking a long time to attempt to engage and then was going in and out. It did not blow fuses however. I am trying to understand how this setup works so that by manually attempting to engage it I can determine if the problem is at the solenoid end(then I would get the same reaction as it has been doing when driven) or the signal end( then if I manually activated it it would work properly). Please tell me specifically how the solenoid is activated. Thank you so much for your help!
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Ponyshoe3
February 24, 2012.



You got the two wires, measure the resistance of the solenoid maybe you have a bad solenoid.

Blackop555
Feb 25, 2012.
Can someone explain why there is no draw from the solenoid if vehicle is shut off and power & ground are applied to the solenoid but if applied when driving at 45 MPH it blows the fuse. It appears the circuit is switched somewhere within the transmission. So if I try to take a resistance test it seems it could only be done while the vehicle is being driven. What am I missing? Also, why doesn't it blow the fuse when hooked up to the ECM?

Tiny
Ponyshoe3
Feb 25, 2012.
Probably because it's not the solenoid causing the draw. It can be something else in the circuit including possibly the PCM.

Wrenchtech
Feb 25, 2012.
Also, one of the wires inside the transmission could be grounded. The coil will still read normally between the two pins. Besides continuity, measure each pin to ground.

If you DO find one pin grounded, you might try switching the positions of the two wires in the connector. Instead of blowing fuses, I think the solenoid will engage all the time of it is switched on the ground side.

If it is switched on the positive side, which it sounds like since it doesn't blow the fuse until it's time to activate it, it might work fine that way. Logic would dictate if there was no switching done on the ground side, they would ground it inside the transmission and just have the one wire in the electrical connector, but they might have designed it this way for some other reason.

Wrenchtech will correct me if I'm wrong, but I don't THINK any switching is done inside the transmission. If that were the case, all they'd need outside would be to apply battery voltage when the ignition switch is on. The reason for getting the Engine Computer involved is to add control for other variables. Examples would be to instantly unlock it when you tap the brake pedal in preparation for slowing down. The transmission wouldn't know that. Lockup doesn't occur when the transmission fluid is cold. The Engine Computer goes by coolant temperature. When a computer switching circuit is used, it's easy to pulse the solenoid during engagement to ease it in without a thump or clunk. Can't do that with a simple switch.

These are just things going through my mind that might help. Also, is that 20 amp fuse you're blowing in the car's fuse box or in your jumper wire? If it's in the fuse box, the computer can shut the circuit down to protect its switching circuit from damage. That type of solenoid shouldn't draw more than probably 3 - 5 amps. There are similar fail-safes for some sensor circuits on many cars. If you even momentarily short the 5 volt or 8 volts supplies to ground, the voltage will stay at 0 volts until you cycle the ignition switch off and back on to reset it, but it won't blow any fuses. It would stand to reason the lockup circuit would be protected the same way. You're bypassing that safety circuit when you use jumper wires.

Just out of curiosity, what happens if you switch the wires around when you're using jumper wires? Does the fuse still blow?

Caradiodoc
Feb 25, 2012.
This solenoid works like all the others in the car. It gets a constant power feed and the PCM controls is by applying ground with a quad driver.
I suspect your short is something not even connected with this circuit. This circuit is protected with a 20amp fuse and there is no way that much is needed just for that solenoid so I suspect that fuse also powers other things and one of the other circuits is where the problem is.

If you give me the location and exact ID of the fuse, I will try to see what is on the circuit.

Wrenchtech
Feb 26, 2012.
Thanks for the responses fellows.I will try to clarify some points that you addressed. I have cut both wires from the computer to the solenoid, so any results I previously described and am describing are happening solely through the 2 wires that go to the solenoid. My thought was that if the solenoid worked as I believed and any problem was at the computer end, I should be able to add power to one wire and ground to the other and engage the solenoid. I have tried doing that with the connections both ways. I got my power fron the cigarette lighter, which explains the 20 amp fuse I refer to. That way I was able to easily rig up a temporary circuit and switch to power from inside while driving. If I take the car up to the speed at which the lockup could occur it immediately blows the fuse in that circuit. However, if I close the circuit without the car running, but with power, nothing happens.
Something one gentleman spoke to that would maybe explain the fact that when I started having lockup problems it would go in and out- in and out. That may have been the computer shutting down the circuit to protect itself then turning back on a then finding it still needed to shut down, etc, thus the on and off scenario. If that were the case it would mean the problem is at the solenoid end, but there is still one unanswered question. Why does it not blow the fuse when the car is not running and I close the circuit(the lighter circuit in this car is live when not running). If there is no switch within the transmission then my only conclusion is that there is a short that is not grounding when the vehicle is not running but is grounding due to vibration when it is running.
Final question is this. Am I correct in assuming that if everything was working fine on this vehicle and I were to cut the 2 wires to the lockup solenoid, apply power to one and ground the other, the solenoid would engage?
I know curiosity killed the cat, but I tend to want to know how and why things work rather than just whether they work or not.
The vehicle is old and not worth putting much into. I had hoped that by attempting this I might discover whether the problem was at the solonoid end or the control(computer) end. If it were the control end I would have attempted to rig up a setup with a switch inside the car to lock it up during cruising. That would possibly have included a switch to break the circuit when the brakes where applied. It appears my problem is at the solenoid end!
Thanks again for your input and would welcome any more based on the above clarifications.

Tiny
Ponyshoe3
Feb 26, 2012.
I hope Wrenchtech has an answer because I'm interested too. I have the same setup on my '88 Grand Caravan. I use it at a minimum of once a year to drag a tandem axle enclosed trailer to the nation's second largest old car show swap meet 55 miles from my house. That trailer is bigger and heavier than the van. Stopping is no problem without using the trailer brakes but the wind resistance makes it impossible to hit 70 mph going down a very steep 3/4 mile-long hill. By the time I'm half way there or half way home, the engine is running warmer than normal, (it's also the weekend after the 4th of July), and the lockup function kicks out at anything over 1/2 throttle. If I let up on the gas to keep it locked up, I lose speed and become a nuisance on the freeway. By the time I hit the city it won't lock up at all.

I've been watching this post because I've toyed with the idea of adding some isolation diodes and putting a manual switch on the dash to keep the solenoid energized. The reason to kick the lockup off is to get increased fluid flow through the transmission cooler, but it's assuming there's a problem based on engine coolant temperature. There is no temperature sensor in the transmission. '88 was the last year for the indestructible hydraulically-controlled transmission. In '89 they went to the miserable computer-controlled transmission and that one would explode pulling the trailer before I got to the end of my driveway.

Your '84 is also hydraulically-controlled but I don't know if there's a temperature sensor incorporated with the solenoid. My guess is the Engine Computer will kick the solenoid off, like mine, when the engine gets too warm, or when the throttle is nearly fully released. You might look for intermittent dropouts in your throttle position sensor as the cause of the lockup kicking in and out. When I release my gas pedal then press it again, the tach goes up another 200 rpm, then drops back down one or two seconds later when it re-engages. That's based on throttle position.

If there's a short with the solenoid that can blow a 20 amp fuse, I don't think the computer would ever be able to engage it. It might try but if you ever feel it lock up, there would have to be no short.

Something else you might consider is connecting a test light to the solenoid wires coming from the computer and watching what it does when lockup is supposed to occur. If it simply switches on and off, a digital voltmeter will work too, but if it modulates it for a smoother application, that will be hard to see with a voltmeter because the voltage will go from 0 to 12 volts on and off rapidly for a fraction of a second. The voltmeter won't display that average but a test light will flicker rapidly or just be dim during that time. My guess is it would be easier to build a small accumulator in the transmission to ease the clutch engagement and just put a simple on / off switching circuit in the transmission, but the engineers seem to want to complicate anything they can think of with more sophisticated electrical circuits, so it's hard to say.

Caradiodoc
Feb 26, 2012.
I have no idea where you're at now. We just don't cut wires and try to drive it. The computer will recognize that the solenoid is not connected and I have no idea what it's programmed to do at that point. That's just not the way a tech would approach this and to tell you the truth, I have no idea what you have done at this pointy so I'm going to back out of this and see if anyone else wants to get involved. Simply reading the resistance of the solenoid will tell you if it's shorted or not.

Wrenchtech
Feb 26, 2012.