Engine/Transmission (1994 - 1998) TPS myth buster

[Howard Durand]

There is no kickdown nor automatic downshift under load. The TC stays locked when I lift off of the accelerator. It will unlock when I touch the brakes or when RPM gets down around 1200.

I like it because I can use the exhaust brake without manually locking the TC. If I find I am going to have to use the foot brakes, I lock the TC manually.



I am thinking about using a foot switch to lock the TC instead of the toggle switch. Don't have to take eyes off of the road that way. The plan is to lock it up with a tap of the switch and return to normal with a second tap on the switch.



As stated earlier, with my setup, maximum signal voltage is 2. 5 volts to the PCM. Ridesamule is able to apply up to the maximum of 5 volts.

I am thinking that somewhere between my 2. 5 and his 5 volts is a point where the TC will unlock and remain so until the voltage is reduced. This gives him a way of controlling TC lockup without a switch. My knob is located in a panel that I have in place of the drink holder and I don't want to have to reach over there to do that so I am going to go the foot switch route instead. My TC control switch that I am using now does have capablility to lock, unlock or leave it normal. The foot switch will only be used to lock it or return it to normal.

 

[Howard Durand]

Here's a diagram of what I am using now.

 

[Willys]

Does the TCC lock in at 1200 rpm as well as unlock at that rpm? What size tire are you running? The 47 mph highest lockup/OD setting seems a little slow as well, if you're towing. I would think with you're gearing a 42mph to 60 mph lockup in OD would be more preferable? I'm not being critical here, just comparing in my mind, your truck to my '96. (I plan to do this on my '95, not the '96. )



What are you using for a foot switch, a dimmer switch like old fords used to have?

 

[Joe G.]

I use a dimmer switch from a Chevy. At one time I had a '63 Chevy. I remember how the switch was made. It's ideal for this kind of application. I think it was used until the early '70s in Chevy pickups. I connected the center terminal to ground. One of the outside terminals to my trans lockup grounding wire. The other to an LED that has 12 volts on it. This way the LED is lit when the switch is not grounding the trans lockup wire. This behaviour is consistant with the way the OD lockout light works.

 

[Howard Durand]

Willys, the TC does not lock up again until after the shift back into OD. I am not sure what the TC would do with this setup if I kept the OD manually locked out. I haven't tried that yet.



The 47 mph OD shift is satisfactory for my purposes but is easy to change.

Looking at the diagram, you can just swap the fixed resistors around so that the 470 ohms is between the potentiometer and the 5 volt supply with the 1500 ohm resistor on the ground end. That should give about 4. 2 volts at the upper end with 2. 5 at the lower end.

I don't know where that would put your upper shift point though as I haven't tried it. It would certainly be higher.

Maybe Ridesamule could give us a better answer to that question since he can run his all the way up to 5 volts.

The resistor combination that I show in my diagram is not something that has to be. Ridesamule uses a potentiometer without any fixed resistors and can adjust the signal voltage continuously between ground and 5 volts. That may be more suitable for what you want.



As for the foot switch, I am thinking of doing what Joe G suggests. It would be a rugged switch and would not be likely to get operated inadvertently.

 

[Jim Fulmer]

I use a dimmer switch from a Chevy. At one time I had a '63 Chevy. I remember how the switch was made. It's ideal for this kind of application. I think it was used until the early '70s in Chevy pickups. I connected the center terminal to ground. One of the outside terminals to my trans lockup grounding wire. The other to an LED that has 12 volts on it. This way the LED is lit when the switch is not grounding the trans lockup wire. This behaviour is consistant with the way the OD lockout light works.

Joe I should still be able to get one of these at the auto parts store right? Then when wiring it I can tap it into the ground wire for the converter lock(manual) ? Then either use it or not, not being when I want to lock the converter for a blast.



Jim

 

[Ridesamule]

Howard Nice to see the depth and detail you have gone to while trying this. I had a chat with the transmission shop when ordering my new transmission and they had no concerns. I was told the transmission does not in any way see the voltage from the TPS.



I realize that you do have the RH transmission, but on my RE I have adjusted the TV cable quite a bit and if I stay on the throttle or keep the load on I can delay lockup until around 68 mph. With my stock converter it takes a while and usualy let off for the lockup. but can control it with my foot from 48~68.



Just wondering if you tried any TV adjustments.

 

[Joe G.]

Jim,



NAPA should have it. I found a classic Chevy parts store on line (didn't keep the URL, sorry) and ordered it. The local NAPA store is pretty good, but I doubt they would have in it stock. Have to order it so I did it on line instead.



Don't forget, you will need an indicator light to keep track of the push button switch position.



There is a minor problem. The parking brake cable is exactly where you want to mount the dimmer switch. I mounted mine kind of high on the floor board (less likely to accidently step on it) and to the right of the parking brake cable.



The switch is pretty simple. The center terminal is connected to one of the outer terminals. Step on the button and it switches to the other outer terminal.

 

[Howard Durand]

There is one of the switches on eBay. It is an old one and he wants $5 for it but shipping is $8 or more. Would have to drill a hole in the floor or build up a bracket for it.

I think I found the GM site Joe is talking about. They have them for a little over $10 but have a minimum order of $25. They are surface mounted so no need to drill a hole in the floor.



I ran the truck a ways yesterday with OD switched off. The TC locks up at about 30 mph and behaves normally. I didn't try a kickdown though but, with the TPS missing, I would guess that it won't. I didn't change the potentiometer during that time, it was set for about 2 volts.

In one of my test runs, I found that with the potentiometer at it's lowest setting of 0. 8 volts, the OD engaged at 32 mph but the TC would not lockup at all. When I set the pot to 1. 2 volts, OD shifted at 32 mph and the TC would lockup at 44 mph.

With the pot at it's highest setting, 2. 5 volts, OD shifts at 47 mph and the TC locks up at 51 mph.

I have not done any adjusting of the TV cable. The 1-2 and 2-3 shifts are where I like them so have left it alone.



Normally, I am not too concerned about the timing of TC lockup, it is always shortly after the OD shifts. For towing, I have a speedometer circuit that I use to control the TC.

With it turned on, I can dial in the speed that I want It to lock up. It has worked real well for me when it shifts into OD at 45 mph and TC locks at 55 or 60 mph.

 

[Willys]

Is this the right dimmer switch?



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Rockauto has them($5. 99), this link might work:

http://www.rockauto.com/catalog/x,carcode,1324314,parttype,4340



if not, go to rockauto.com and select 1963 Chevy C10, electrical switches, then dimmer switch.



There is a 5% TDR discount, I think the code is 'turbodiesel'. There is a thread, I think started by Holeshotholset, that has the code. Hope this helps...



Edit: After looking, I doubt there is much difference between that one and an '85 F150 dimmer- and I'll bet the Ford one is more commonly available, locally.



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[Joe G.]

Yep, that's the one. Surface mount. Three lugs you can use for spade lug connectors.

 

[Howard Durand]

Look on "eBay Motors" for "headlight dimmer switch" and you will find a bunch of them listed by Van's Auto Salvage LLC.

Any one of them for $5 plus $3 shipping. H

He has them listed for a lot of different vehicles but all appear to be the same.

 

[Jim Fulmer]

I already have the dimmer switch that I can use for lockup. I wired mine so I can use it for lockup, valet or water turn on, just depends on which one you plug into it.



I need to know is were to get that variable switch so I can bypass that stupid TPS that gives me so much crap.



Jim

 

[Howard Durand]

Jim,



The potentiometer you need is usually not available at Radio Shack but most good electronic supply stores have them. The size is not real critical. I think the one Ridesamule uses is about 3000 ohms. The one I am using is 1000 ohms combined with a couple of fixed resistors. You can see a diagram of that on page 2 of this thread.

A total resistance of about 3000 ohms is close to that of the stock TPS.

I have bought several potentiometers of different sizes on eBay. If you like to mess with electronic stuff, it is a pretty easy setup to build and get connected.

If you don't like to mess with it, send me an e-mail.



Howard

-- email address removed --

 

[Willys]

I picked up a 5k ohm, 300 degree pot at RadioShack. I couldn't find any good electronic stores close around here. I figure a 5k should be ok? I will test it with my meter, find the range I want it to operate in, and either mark it or make a stop on the dial face so I can keep it within the correct range while I'm testing it.



I hope to find time to test it out this weekend. At the very least I want to see if I can bypass the tps, temporarily, to see if it is the culprit in my trans. randomly unlocking. If I like it, I might find a narrower range pot and add resistors to make it easier to fine tune... In the meantime, do y'all think a 5k will be ok?

 

[Jim Fulmer]

Cool, I'll look into it when I get home later this month. Funny when you run a simpler setup you have time to fix all the little stuff that you had to let go.



Jim

 

[Howard Durand]

I think the 5k will work OK. Just hook it between the 5 volt and ground. You should be able to get a signal voltage anywhere between 0 and +5.

The only thing affected by the extra resistance is the current through the pot, which will be lower. About 1 milliamp instead of 1. 7 milliamp with the 3K pot that I am using.



I disconnected my stock TPS but left it in place in case I want to go back to it.

When you go to buy a pot, make sure it is a linear taper and not an audio control.

 

[Willys]

Well I got a chance to install the pot and ran it a few days so far. It is going great! I like this far better than a TPS. I have found that there is some variance in the shift point even with the pot at the same setting- one time I was lightly accelerating, and had the pot set to lock up the T/C at 44 mph. At the same setting, in WOT, it would lock up at 50mph. I don't know what causes this, but it works great for being able to pull out in traffic or up a hill with a trailer and not get run over. I think I will find a better quality pot with a narrower range before I install it permanently in the dash- the range is a little too narrow right now. I can fine tune it ok, but it might be a little more difficult for a driver who is not used to the truck. We need to find a way to make a dial face that we can use to indicate lockup speeds.



The only problem I have experienced so far is that one time, the truck downshifted to 3rd. The lockup switch led on the dash lit up. I don't know how this happened, as my hand was not near it, and I don't think I had touched the pot either. I do have a resister in the TTS plug, maybe it got some oil or dirt in it for a second?(need to get under there and clean it up and seal it somehow... )

 

[Howard Durand]

Willys



You can get a 5K, 10 turn precision potentiometer for which you would need a turn counting knob. They are somewhat expensive from the electronics stores but I have picked up several of them on eBay at fairly reasonable prices.

That would give you much finer tuning of the pot and the turn counting knob would tell you exactly where it is set.



I find that the TC locks up shortly after the OD engages. Usually within about 5 mph.

The part I like is the control over OD engagement.



As for the OD mysteriously locking out, that seems to be a phenomenon of these beasts. Mine has done it several times over the years since it was new but don't necessarily attribute it to the TPS or the pot. It has only happened a couple of times in the last two or three years.

 

[Ridesamule]

Willys I have just upgraded my transmission and wanted to get comfortable before I put anything out. But a transmission change had no drastic effect on how my pot effects shifts and lockups. Verifies what I was told " an RE does not see anything from the TPS".



Any way here is how my shift points go. Disregarding voltage Min is the lowest I can adjust and still obtain lockup and Max is the highest. These results are taken real world Steady Half Throttle with the TV cable set tight at full throttle.



Min: OD locked out... into 3rd 32 mph, lockup at 36 mph

Max: OD locked out... into 3rd at 36 mph, lockup at 54 mph



Min: OD on... into 4th 42 mph, lockup at 47 mph

Max: OD on... into 4th at 52 mph, lockup at 64 mph



All shift numbers are slightly lower at light throttle except for the max #s come down to about the same levels as the min #s. And lock up never goes below 47 in OD.