HT754CRD Lockup override question -- SOLVED

Brian Diehl · June 12, 2006, 05:00:19 PM

Okay, Last summer I asked about the ability to "force" lockup of the torque converter while in 1st or 2nd. The reason was while climbing mountains when the transmission was in 1st or 2nd the torque converter unlocks and when that happens the engine temperature rapidly climbs. I had several hills I had to climb in 1st gear not because I didn't have enough power in 2nd, but because the temp guage was climbing too fast from all the extra transmission heat. Several on the board and locally recommended hooking up the main pump pressure port to the torque converter lockup port externally by way of a hydraulic solenoid valve. I thought great, no problem and even got the diagram of the underneath side of the ht754 so I could make sure I knew what the main and torque converter lockup ports were. Everything is flowing smoothly at this point. Now on to the problem... As you guessed it doesn't work.

So, what I did was hookup the solenoid to the dash by way of an electric switch (standard stuff) and mounted the solenoid to the tranny and connected the solenoid to the main and lockup ports using 1/4" copper. Well, after setting it up and making sure the solenoid worked (by sound) I took it for a drive and nothing happens when I turn on the switch. This evening I set about troubleshooting this and have completed the following steps in trying to troubleshoot the problem:
1) Made sure Solenoid works (by sound) (it sounded like it did, so moved on to next step)
2) Tore apart solenoid and cleaned out every passage and tested on shop air (no problems found)
3) Remounted on bus and left output port (lockup side) of solenoid undone. Started motor and tested (works GREAT!)
4) Reconnected lockup side and tested again... Still nothing.

So what I'd like to know is if anyone has actually done this "mod" and if so what tranny did you do it on?

One last piece of information... The ports on the tranny are 1/8 npt and the solenoid and lines are all 1/4" ports and according the the diagram I got from C&J Bus (JD Dickenson) the ports I connected to are correct.

Any ideas?

-- PS. I should also add that the transmission was overhauled by a competent local builder (not DD) less than 10,000 miles ago

Don Fairchild · June 12, 2006, 05:14:29 PM

Brian;

Put a pressure guage in the main pressure port and verify that you have pressure there if not look for another port.

Don

Brian Diehl · June 12, 2006, 07:38:40 PM

Thanks for the idea Don. I guess I just assumed I had plenty of pressure since the running test I made caused an impressive display of tranny fluid to shoot out at high velocity. However, doesn't hurt to test the true pressure. I'll see if I can find a pressure gauge to test with. I seem to remember that the main output pressure runs in the 180-190 lbs when in forward. Does this number sound right?

TomC · June 12, 2006, 08:22:30 PM

Brian- couple of things. I've heard that only an electronic transmission can get lockup in 1st gear. I assume you had pulled the shift lever to first when you were climbing and didn't have lockup. Another way can work, but only if you have an air throttle with an air operated throttle sensor to the transmission. You can get lock up if you put a solenoid with a dump valve so when you activate it, it dumps the air to the throttle sensor and the transmission thinks you're in no fuel position, where the transmission will lock up when in 1st position.
I have a V730 with an air throttle sensor, and when I get into 2nd torque converter, just slow a bit and pull it to 1st with my foot on the floor. When I'm just about up against the governor, I pull up on the throttle about an inch and you can feel the torque converter bump into engagement. Then just go up the hill at the 34mph without the tranny overheating. You are right that if you can keep in torque converter lockup mode, the trans won't heat everything up. Good Luck, TomC

JackConrad · June 13, 2006, 04:57:17 AM

Brian,
There has to be a valve inside the transmission that allows the pressure in the lock-up chamber to release when the transmission comes out of lock-up mode. If this valve is open (transmission in non lock-up mode), wouldn't this let the pressure you are adding simply flow through the lock-up chamber? Jack

Brian Diehl · June 13, 2006, 06:38:58 AM

Hi Jack,
I had assumed the same thing initially, but was told the port opening to release lockup was smaller than what could be put back in by this external means and therefore would keep the torque converter in lockup. This may not be true given my experience so far, but I'm hoping I can overcome this problem to simplify my life a little. If I can't get this to work then I'm moving on to radiator misters. Of course, I REALLY want this to work since it will help me significantly on the downgrades. -- We tend to stray off the beaten path and some of those 6% grades with 20 mph curves are a beast when the tranny drops lockup!

DrivingMissLazy · June 13, 2006, 07:53:20 AM

Well, if you want to know about misters, there sure is lots of information here about them. LOL

BTW I really missed the first gear lockup going down some of the passes in California. Even the Jake would not work.
Richard

Quote from: Brian Diehl on June 13, 2006, 06:38:58 AM
Hi Jack,
I had assumed the same thing initially, but was told the port opening to release lockup was smaller than what could be put back in by this external means and therefore would keep the torque converter in lockup. This may not be true given my experience so far, but I'm hoping I can overcome this problem to simplify my life a little. If I can't get this to work then I'm moving on to radiator misters. Of course, I REALLY want this to work since it will help me significantly on the downgrades. -- We tend to stray off the beaten path and some of those 6% grades with 20 mph curves are a beast when the tranny drops lockup!

TomC · June 13, 2006, 08:21:15 AM

Whether or not you have an electronic transmission, when you pull the gear selector down to 1st gear you should get lock up at least on deceleration so that the transmission would be locked up as low as about 15 mph. On my V730 it does that, and since it is the same transmission family, should also work with the HT754CR (it is either a CR-Close Ratio, or a DR-Deep Ratio. Since you have the 754, that was only availabe as the CR. The DR was a straight HT750). Good Luck, TomC

Brian Diehl · June 13, 2006, 12:34:30 PM

I don't get lockup while having the gear selector in 1st with no throttle. If I had that half my problems would be resolved.

Anyone buy the HT740-HT750 mechanics manual? If so, was it good enough I should buy it to try and help figure out this problem?

DrivingMissLazy · June 13, 2006, 02:58:20 PM

Tom my 740 would stay locked up till I got down to about 15 mph. However that was really too fast for some of the curves and the grades.
Richard

TomC · June 13, 2006, 03:04:15 PM

Richard- I admit, that for speeds under 15mph, a 10 speed transmission will give you the most control. But what percentage of the time are we down at that speed in an off road or severe road condition? I'll still stick with an Allison.
All I know is that when the gear shift lever is pulled down to first you should get lock up on no accelerator pedal position. If not, you'll have to consult Allison. Good Luck, TomC

Brian Diehl · June 13, 2006, 06:56:21 PM

I just tested the pressure and I have 160psi in neutral at cold startup on the "main" port.

I'm going to post some pictures of my setup so you can see how I have it configured.

Brian Diehl · June 13, 2006, 06:57:48 PM

Sorry the pictures are a little fuzzy. I couldn't get my camera to focus in the semi-darkness under the bus.

JackConrad · June 14, 2006, 04:14:29 AM

Brian,
First, please don't take this as being cynical, but are you sure you are connected to the lock-up port? Also, I see you used 1/4" copper tubing and fittings, but what is the diameter of the orfice in the solenoid. Perhaps the orifice is to small and not allowing enough fluid to override the vale in the transmission. You might try opening the line from the solenoid, start the engine, and engergize the solenoid. See how much fluid come out. Hope this helps, Jack

Brian Diehl · June 14, 2006, 05:52:29 AM

Thanks for the reply Jack. I have confirmed a very high volume of oil at very high pressure comes out the solenoid when energized with the engine started and the tranny in neutral. However, I am not 100% convinced I'm connected to the lockup port on the tranny. Though, from my initial post you can see that I think I'm on the right port. The solenoid uses 1/4" passages.

Does anyone have access to a manual that would show the lockup port?