A4LD no computer lock up until 50 mph | Ford Explorer Forums - Serious Explorations

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A4LD no computer lock up until 50 mph


New Member
October 6, 2010
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City, State
Joplin, MO
Year, Model & Trim Level
Ranger Custom 1991 2WD
I'm a new member. I saw some of the older posts and thought some of you might have some ideas.

I have a long bed 1991 Ranger Custom 3.0L MFI. EEC-IV.
A4LD 2 solenoid automatic. Low miles 80,000 but old.

It quit locking the torque converter up at lower

It waits until ~50 mph. 3-4 shift is totally
normal. I cleaned the valve body and raised the line
pressure Very slightly and adjusted the bands. All shift points are the same. It works really well except for the late TCC lockup. The solenoids are 2nd generation and check good both off the valve body and through the EEC harness.

I put a meter and an LED on the purple and yellow wire that the PCM grounds the TCC override solenoid through. The computer just doesn't ground the wire until 50 mph.

Question: It is not the VSS [I think?] because the
3-4 shift is normal at the correct speeds. I replaced
the TPS and there was no change. [Ouch] I guess
it could be a bad new part but probably not.

MAF is cleaned and the engine runs like a 3.0L Ford.
enough power pretty normal. The ECT sensor could be bad but is not reported on the KOEO self test of the EEC-IV. [Pass 11]

What's wrong here I don't want to believe the computer just quit this one function. It actually does lock the TCC, just late.

Any ideas?

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A few thoughts:

1) It appears that this is something that has changed recently. When was it locking up before (how much has it changed)?

2) 4 Main inputs for locking the TCC: 1) BOO switch (open) 2) TPS, 3) ECT, 4) VSS. If it has changed when it locks up, I would first suspect something has changed in one of the sensors. BOO probably not, because it is only used to unlock the TCC when the brake is pressed. Unfortunately, in a case like this, you probably don't have readings from those sensors when it was locking earlier, so it will be hard to see what has changed.

3) Any chance you've changed tire size or anything that would throw off the VSS?

4) ECT is a basic thermistor that can be tested farily easily with an ohmmeter. I don't know how far off it would have to be to delay lockup that much, but basically you just want to see if the thermistor is reading something believable.

5) TPS is a simple potentiometer, so it, too, is pretty easy to test with an ohmmeter.

6) Make sure there are no bad connetions in those circuits that would unduly increase the resistance of that sensor as seen by the computer.

7) VSS is a hall effect sensor, so I would expect it to be either working or not. Without something to reference to, it would be hard to say if it is reading "slow".

8) I don't know if this will have significance, but does it lock up at the same speed in 3rd gear (D) as in 4th gear (OD)?

Thanks for the interest. No new tires or drive changes.

The only other changes recently other than checking connections
was repairing the fuel inlet hose [Ebay] replacing the fuel sender float, and engine coolant overflow hose. The radiator was down about 2" and I filled it up. The temperature gauge read higher? afterward. The truck shifted normally for a week after.

The TP sensor seems to have 'dead' spot measuring resistance but the voltage output
is smooth as glass with the key on. The new TP did not change anything. It's installed
reading was about .8 volts. the old one was about .7 v almost exactly in the center
adjustment range .6 < 1 v.

I did a KOER EEC4 test today Pass 11 1 1 no trouble codes. I flubbed the Dynamic Response test the first time and got 73 74 66 errors about brake pedal depression and
no throttle change so the computer seems OK again [the second time around] ;-)

The ECT sensor might be reading 216 degrees when the engine is 180 that would stop the TCC. [I think thats what the book said] I'll try to measure the ECT for realistic comparison to other sensors by its resistance at fully warmed up.

I will also check to see if the truck locks the TCC in 3rd. I don't recall that it ever has.
I know it is supposed to.


So is it doing the 3-4 shift and locking up the converter after that? Because it's not going to lock the converter until it's in overdrive, unless you just have it in D, in which case it will lock the converter when you are over 45mph or so.

Forgot to mention the lock up speeds before.
locked TCC at 44 mph unlocked at 40 mph [minimum throttle]
It was very consistent would unlock at heavy throttle, braking or coasting.

NOW it is 50-52 and unlock at 48
I know 6-9 mph difference seems slight but it really affects the truck's performance
mileage and driveablity.

HI Williard,

Yep does a 3-4 shift and then locks the TCC at about 50-52 mph.


You didn't change wheel or tire size? MAF could be or the computer coolant temperature sensor, not sure it uses one like the 5R55E does.

No change in the tires or wheels; they are the same. I put a meter on the MAF and the wires are connected. The MAF was cleaned with a 'special' MAF cleaner from the auto chemicals in a parts store. KOER test says Pass 11.

I tried the TCC lock up in 'D' and it works at about the same speed as 'OD' ~50mph.
everything works pretty well just happens a higher speeds than normal.

Still trying different solutions.
I added a manual switch ground to the TCC lock up solenoid. The truck shifts normally more or less. Locks the torque converter at 40 mph light throttle. If mid throttle it will immediately lock going into overdrive at about 45. A noticeable bump but not rough just not smooth like the computer. Unlock occurs at 35 mph. Works for me for now.

I also drilled out the bolt holes in the TPS slightly in order to adjust it. The metal spacers cast into the TPS tend to spin if the drill bit grabs the spacer. I used a thin set
of vise grips to hold it. I reamed it a bit on a drill press with a loose fit after that. I changed the adjustment from .66 volts to .81 volts. by rotating the sensor and then tightening it. The 'adjustment' on the throttle linkage I set for the best operation of the IAC motor.

BIG improvement in engine smoothness and a bit of a performance increase. Ford engines
all seem to be a little different and the newer ones [mine is a 1991 3.0L] seem to need to be higher. The spec on the old EEC-IV is > .6 < 1.0. Higher is better for most but try it and fly it to see.

Hey that's an interesting thought but no. I have not changed out the bulbs. Replaced a park lamp 194 incandescent but that is stock.

I know this post is old but I would like to know how to make the manual switch for the TCC?
mine rarely works and is quite annoying trying to get it lock up. I'd rather just have a switch like the later model explorers

I don't know how the later model Explorers do it. Manually forcing lockup on the 1st gens is pretty easy. The PCM acts as a ground side switch, so all you need is to wire your own switch in place of or in parallel with the PCM. Recognize that TCC lockup is hydraulically inhibited in 1st and 2nd, so you can only force lockup when the transmission is in 3rd or 4th.

Adding to that a converter that won't lock up under normal driving, still won't lock up with a switch. Also forcing lock up on a fully functioning trans will cause the OD light to flash and on a 2nd gen explorer it will go into limp mode. I have experimented much with this and this is what I found.

I don't know how the later model Explorers do it. Manually forcing lockup on the 1st gens is pretty easy. The PCM acts as a ground side switch, so all you need is to wire your own switch in place of or in parallel with the PCM. Recognize that TCC lockup is hydraulically inhibited in 1st and 2nd, so you can only force lockup when the transmission is in 3rd or 4th.

So that would be to the purple and yellow wire from the PCM right?

That's what the wiring diagram for my '92 shows. There's a good chance it is the same on your '91.