Minor but annoying transmission issue

I

Ieharris

Member
This is my first Lightning and I got it in February of this year. I have looked through several forums and have not found an answer to the issue I am having with my truck.
A little information on the truck. 1993 with 157,000 miles. Stock except for under drive pulleys (done by previous owner), smog pump deleted, and Bassani exhaust system.
Since I have owned it I have replace the spark plugs and wires, distributor cap, ignition coil, upper and lower intake manifold gaskets, oil pan gasket, water pump, rear main seal, fan blade and clutch, belt, all hoses, master cylinder, brake lines, brake pads, changed all transmission fluid (including torque converter), transmission filter, installed larger transmission drain pan with drain plug, changed fuel filter, fuel pressure regulator, MAP sensor, TPS, cleaned throttle body and IAC, changed thermostat, replaced vapor canister solenoid, set timing, rebuilt the A/C system and upgraded the alternator to 130 amp.

I was having some transmission issues with very hard shifting, going into neutral while driving, weird double shifts into each gear and things like that. I checked ECU and found all 3 capacitors were leaking so sent it to ECU Exchange and they repaired and sent it back. Installed it and the transmission was MUCH better. The only two issues I have after the ECU was rebuilt is when the truck shifts into 4th gear, it feels like a "double shift". Two distinct shifts less than a second apart and the other issue is while driving on the interstate over 55 MPH (after everything is warm) the transmission feels like it slightly shifts to lower gear and back again but it is not shifting to 3rd. The RPM drop by 200 RPM and then will go back up. It is not a gradual change, feels like something engages and then disengages. Sometimes it will stay 200 RPM higher for 2 seconds or can stay for 20 seconds. Doesn't seem to matter if I am going up an incline or down a decline. It may not do it for 3-4 miles straight and then wont stop doing it for the next 3-4 miles.

The last time I drove it, it seemed like if I stayed under 55 MPH it didn't happen but I only tried it for a few miles. I have checked the electrical connections at the transmission and everything is very clean and the wires are in great shape. Everything is tight and sealed. I traced the wires up and over the transmission and everything looks good all the way back to the main harness. Fluid level is correct. I also sent the PSOM to Module Mechanics to have it rebuilt and see if that helped but it did not change anything.

Any suggestions on what to check next? I dont know much about how the solenoids work, the valve body or the torque converter clutch but I am hoping someone has some experience with this transmission and can point me in the right direction.

Oh, and I did at the very beginning of everything, and have several times since, pulled the codes and there are no codes now and have never had any that have any relation to the transmission. Only had one for the MAP sensor before I replaced it and the vapor canister solenoid before I replace that.

Any help would be appreciated.
 
Sounds like the converter is locking and unlocking. Does the speedometer bounce any? Do you have any LED lights in the truck?
 
Raymond_B said:
Sounds like the converter is locking and unlocking. Does the speedometer bounce any? Do you have any LED lights in the truck?
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The speedometer does not bounce at all and is accurate. No LEDs anywhere.
I was thinking the torque converter could be locking and unlocking but I don't know exactly what controls that and if the 200 RPM would go along with that.
 
Ieharris said:
The speedometer does not bounce at all and is accurate. No LEDs anywhere.
I was thinking the torque converter could be locking and unlocking but I don't know exactly what controls that and if the 200 RPM would go along with that.
Click to expand...
The computer, engine speed, RPM, and even brake lights control it. As a test you can go down the freeway and either lightly tap the brake pedal or turn on the hazards and it will unlock.
 
Raymond_B said:
The computer, engine speed, RPM, and even brake lights control it. As a test you can go down the freeway and either lightly tap the brake pedal or turn on the hazards and it will unlock.
Click to expand...
Okay. So the 200 RPM rise sounds about right for the torque converter unlocking?
I will try the hazards and see if I get the same rise.
 
The double-shift feeling is definitely the torque converter locking and that's normal. If you've put a higher stall converter in the truck, it is very noticable.

It shouldn't be locking and unlocking going down the highway, though. Have you made any modifications to any of the brake lights? Can you have someone follow you and see if the brake lights are doing anything weird?
 
FMOS Racing said:
The double-shift feeling is definitely the torque converter locking and that's normal. If you've put a higher stall converter in the truck, it is very noticable.

It shouldn't be locking and unlocking going down the highway, though. Have you made any modifications to any of the brake lights? Can you have someone follow you and see if the brake lights are doing anything weird?
Click to expand...
I haven't made any modifications to any of the lights and I believe the torque converter is stock. I will get my wife to follow me this weekend and see if the lights are doing anything while driving.
I checked last night in the garage and the taillights, blinkers, and brake lights are working normally and the brake light switch seems fine. I had to move the brake pedal forward, just guessing maybe 1/2", before the lights came on. I still want to know if they do anything while driving so I will check that out.
I read somewhere that someone indicated that their trailer wiring harness was causing a similar issue. The previous owner did install a hitch and plug so I will check that wiring as well to see if something is going on there.
 
FMOS Racing said:
The double-shift feeling is definitely the torque converter locking and that's normal. If you've put a higher stall converter in the truck, it is very noticable.

It shouldn't be locking and unlocking going down the highway, though. Have you made any modifications to any of the brake lights? Can you have someone follow you and see if the brake lights are doing anything weird?
Click to expand...
Can LED's cause issues? Whats up with that?
 
Without some kind of resistor inline, yes. With bulbs that are designed for CANbus, or the full taillight assemblies, you typically won't have a problem. LEDs without resistors have such low current requirements that they can behave like a closed circuit.

I had originally used LEDs in mine for the blinkers and tails that didn't have the resistors and all kinds of odd things happened. I replaced the blinkers with CANbus compatible LEDs and used entirely different taillight assemblies, and no more problems.

Sent from my motorola edge plus 5G UW (2022) using Tapatalk
 
That's some good info here about LEDs. My friend is looking at this '94 with similar shifting issues. One of the recent mods was the LED tail lights. It is neat overall with new brake pads, nitto tires, and seat covers.
 
I thought I would give an update. I checked all of my bulbs and everything looked good. I checked the brake light switch and it was also good. Had my wife follow me and the taillights didn't do anything funny. I verified my TPS was set correctly and had even/smooth increase in voltage when moving the throttle.

I finally removed the trailer wiring harness since I don't intend to tow with it anyway and that was what fixed it. Have driven it about 100 miles since I removed it and have not had it do it once since then.

The harness looked fine and is the kind you plug in between the factory connectors, but there must have been something about it that the computer did not like.

Thank you very much for everyone's suggestions and help with this.
 
Ieharris said:
I finally removed the trailer wiring harness...and that was what fixed it. Have driven it about 100 miles since I removed it and have not had it do it once since then.
The harness looked fine and is the kind you plug in between the factory connectors, but there must have been something about it that the computer did not like.
Click to expand...
I doubt that could affect it. Did you try plugging it back in to see if the symptom re-appeared immediately?
93SVT said:
Can LED's cause issues? Whats up with that?
Click to expand...
The EEC monitors the voltage on the BOO circuit to determine if the brakes are applied. But the high resistance of LEDs that makes them use so little current also makes them behave like an open circuit (burned/removed bulb) which makes the voltage stay high, just like when the lights are on. So the EEC can't tell what's really happening, and so it doesn't control the transmission correctly. This page shows & explains what's involved in controlling an E4OD (which is similar to later electronic automatics):

(click this text)

FMOS Racing said:
...they can behave like a closed circuit.
Click to expand...
Open. ;)

To run LED brake bulbs without problems (or if you want to prepare the truck just in case you ever decide to swap to LEDs), simply add a ~200 Ohm ~1 Watt resistor between the BOO circuit (LG) and ground. On '94-up trucks, the trailer brake controller connector is a convenient place to add it. Adding it with normal (incandescent) bulbs will not cause any problems.

(click this text)
 
Nope, closed circuit. Because of the lack of resistance, it'll carry current all the time, which makes it seem like that the brake lights are on even when the brake pedal isn't depressed.

On mine, with the outer doors closed and the lights off in the garage, I could see a slight glow from the bulbs. This is why the ballast resistors open the circuit.
 
There isn't a lack of resistance - LEDs have very high resistance, approaching that of an open circuit (which has nearly infinite resistance). Adding any conductor (even a resistor) in parallel lowers the total resistance through the circuit, making it draw more current.
 
Diodes (including light-emitting ones) have almost no resistance in one direction, and almost infinite in the opposite. However, we want our LEDs to actually E some L, so the infinite resistance direction doesn't do us much good in that application.
 
...which is exactly why LED bulbs made for 12V vehicles must be built with internal inline resistors to keep them from drawing MORE current than the incandescent bulbs they replace (as we all know to be the case) in the forward (light-emitting) polarity. Without the resistor, the LED element would get so hot that it would burn out instantly from the short-circuit through it. Grab a digital multimeter, and measure the resistance through an LED bulb - it's on the order of 10~100x the resistance of the corresponding incandescent bulb that it replaces.
 
Ieharris said:
This is my first Lightning and I got it in February of this year. I have looked through several forums and have not found an answer to the issue I am having with my truck.
A little information on the truck. 1993 with 157,000 miles. Stock except for under drive pulleys (done by previous owner), smog pump deleted, and Bassani exhaust system.
Since I have owned it I have replace the spark plugs and wires, distributor cap, ignition coil, upper and lower intake manifold gaskets, oil pan gasket, water pump, rear main seal, fan blade and clutch, belt, all hoses, master cylinder, brake lines, brake pads, changed all transmission fluid (including torque converter), transmission filter, installed larger transmission drain pan with drain plug, changed fuel filter, fuel pressure regulator, MAP sensor, TPS, cleaned throttle body and IAC, changed thermostat, replaced vapor canister solenoid, set timing, rebuilt the A/C system and upgraded the alternator to 130 amp.

I was having some transmission issues with very hard shifting, going into neutral while driving, weird double shifts into each gear and things like that. I checked ECU and found all 3 capacitors were leaking so sent it to ECU Exchange and they repaired and sent it back. Installed it and the transmission was MUCH better. The only two issues I have after the ECU was rebuilt is when the truck shifts into 4th gear, it feels like a "double shift". Two distinct shifts less than a second apart and the other issue is while driving on the interstate over 55 MPH (after everything is warm) the transmission feels like it slightly shifts to lower gear and back again but it is not shifting to 3rd. The RPM drop by 200 RPM and then will go back up. It is not a gradual change, feels like something engages and then disengages. Sometimes it will stay 200 RPM higher for 2 seconds or can stay for 20 seconds. Doesn't seem to matter if I am going up an incline or down a decline. It may not do it for 3-4 miles straight and then wont stop doing it for the next 3-4 miles.

The last time I drove it, it seemed like if I stayed under 55 MPH it didn't happen but I only tried it for a few miles. I have checked the electrical connections at the transmission and everything is very clean and the wires are in great shape. Everything is tight and sealed. I traced the wires up and over the transmission and everything looks good all the way back to the main harness. Fluid level is correct. I also sent the PSOM to Module Mechanics to have it rebuilt and see if that helped but it did not change anything.

Any suggestions on what to check next? I dont know much about how the solenoids work, the valve body or the torque converter clutch but I am hoping someone has some experience with this transmission and can point me in the right direction.

Oh, and I did at the very beginning of everything, and have several times since, pulled the codes and there are no codes now and have never had any that have any relation to the transmission. Only had one for the MAP sensor before I replaced it and the vapor canister solenoid before I replace that.

Any help would be appreciated.
Click to expand...
I am having basically the exact same problem you're describing with the weird shifting which led me to ECM (I just went to it, didn't really do any diag) and I can see failures visually. How did you like the company you sent it off to for repair and about how much was it? What other options have any of you done when you have an ECM problem?

I found though one for sale, has anyone used this company before?

The ECM has a hypertech chip in it from previous owner. Do ECM's have a stock one that someone replaced with the hypertech? Can I just leave the hypertech out? Any best guesses on what that chip even does?
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Sent from my Pixel 7 Pro using Tapatalk
 
93SVT said:
The ECM has a hypertech chip in it...
Click to expand...
That's not really "in" the EEC. It just plugs onto the board externally.
93SVT said:
Do ECM's have a stock one that someone replaced with the hypertech?
Click to expand...
No. All the original chips are soldered to the board, and then coated in-place with that green plastic layer.
93SVT said:
Can I just leave the hypertech out?
Click to expand...
Yes.
93SVT said:
Any best guesses on what that chip even does?
Click to expand...
Very little.
 
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