Engine/Transmission (1998.5 - 2002) How can I retard cruise control

[PERMENTER]

How can I keep the cruise control from downshifting when the speed loss is minimal. In the flatlands of S. Louisiana, an occasional overpass is about the only hill we have. Yet when the cruise controls perceives any speed loss at all, it downshifts and petal to the metal. This is my second diesel, and the '95 would lose 3-4 MPH before downshifting, and this would get me over most bridges and overpasses, but the 2002 downshifts when there's only a shadow to pull over. I've read most of the related replies about restricting the vacuum line, but I didn't see anyone with my identical complaint.

Rick

 

[TowPro]

Add either bosch 275 injectors, or a VA/EZ box. More low end, less down shifting.

 

[Mundgyver]

I gather you have an automatic transmission. This might work for you and might not. I use this method to eliminate surging of the cruise control on my truck.



Find yourself an adjustable precision needle valve that can be locked to the setting with a jam nut on the adjustment shaft.



Install this into the vacuum circuit for the cruise control throttle boost module.



Close the needle all the way down. (Do not reef on it, just snug).



Open it back up about 1/2 turn and go drive the truck. You will find that the cruise will react much slower, more like you having your foot on the throttle. If it is too much then close the needle by 1/2 of what you opened up to or at the 1/4 open if you will. Drive the truck again and see how you like it. You should see a mileage improvement with this also.



Remember though that when you engage the cruise with this installed it might take as much as 40 seconds for it to respond at first. The best way is to pick your speed, set the cruise and then slowly allow the truck to drift down about 5 mph. The cruise will pick it up and should take you back with no problem.



If you have a manual transmission like I do and use the exhaust brake in the armed mode in conjunction with the cruise, it will hold you to within 1/2 mph no matter what.



Thats my method and it works very nice especially when towing.



Good luck

 

[Joe G.]

This behavior seems really strange to me. MY '95 freezes the speedo and tach regardless of hills or lack of them. I can have it set on 70 and go up Donner without it shifting or any of the needles moving. Even when I'm towing my 32' TT at 55 it won't shift unless the hill is pretty steep. I think it needs a trip to the shop.

 

[illflem]

His '02 doesn't have a vacuum cruise servo otherwise the valve or another restriction on the vacuum line would work. Some folks just cut the vacuum hose and insert a tight fitting piece of striped off electrical wire insulation in the hose.



With the '02s electronic cruise system I agree with Joe, time for the shop. Either a PCM or TPS problem. It shouldn't be doing what you describe.

 

[Mundgyver]

What signal does an 02 look at Are they using the Tach signal and Speed signal. PCM=Power Control Module I gather, what is TPS, Timing Position Sensor

 

[OTRPU]

Think TPS = Throttle Position Sensor. As for my 99 auto, when it runs is has to lose near 10 MPH affore she downshifts.



Cheers,

Steve J.

 

[Mundgyver]

Yep, I new I was thinking too deep. I hate it when I do that

 

[y-knot]

I have never had mine shift now while on cruise... . even with the stock trany. I have pulled the trailer several times with the cruise set and it just trucks on. I would think you have something going on there that isn't right.

 

[ntillm01]

Illflem... are you sure the 2002 doesn't have the vacuum servo? My 2001 does. It pulls on the cable going to the TPS and pretends it's my foot.



Regardless, if it is similar to my 2001, I can say I've only had mine down shift with cruise when going up really hilly stuff, pulling 5500 lb. It would hang on to 4th for a long time before it had to downshift. I think it need to be checked out too.

 

[illflem]

Yes, the way I remember it is only one year 24 valve has the electronic, the last year, '02



If the TPS voltage isn't correctly adjusted a partial increase in throttle can be interpreted by the PCM as floored which causes the auto to downshift. I know how to adjust the PCM on 12 valves but not 24s, maybe someone with a factory service manual can jump in.

 

[Mundgyver]

Besides the adjustment, what actually does the pulling down of the throttle cable to activate the throttle position sensor on the 2002 model then



Does it have an electric stepping motor for that vice the vacuum module that my 2001 would have.



I can see the PCM electronically controlling the VP to achieve an increase in RPM to maintain the desired set point signal from the speed sensor.



But if the throttle is being pulle from under the foot, what is doing this on the 2002 model

 

[Hohn]

My '02 is electronic. When you set the cruise and lift your foot, the pedal comes up. When you use the steering buttons to fine tune speed, the pedal doesn't move at all.



With these drive-by-wire "throttles" on newer vehicles, you won't have the the pedal experience physical movement. It's all in the PCM and APPS.



Justin

 

[Hohn]

Originally posted by Mundgyver

Besides the adjustment, what actually does the pulling down of the throttle cable to activate the throttle position sensor on the 2002 model then



Does it have an electric stepping motor for that vice the vacuum module that my 2001 would have.



I can see the PCM electronically controlling the VP to achieve an increase in RPM to maintain the desired set point signal from the speed sensor.



But if the throttle is being pulle from under the foot, what is doing this on the 2002 model

The '02s don't require the pedal to move when the CC is operating. The PCM remembers the APPS and associates that with a certain speed and RPM. The PCM then takes over to maintain that speed within its defined parameters. Once the CC is set, the APPS is not a part of the equation.



Since the APPS is not involved, there's no pedal movement.



At least, I think so





Justin

 

[RustyJC]

Right. On the 2002, the PCM is looking at the speed signal and using any deviation in speed to change the fueling signal it sends the VP44 over the data bus.



Funny thing, though - mine won't downshift on cruise control!



Rusty

 

[Mundgyver]

The down shift is probably determined by engine loading, at least that would be my guess. My RoadRelay 4 gives me what % engine load I have when driving. It gets this from the engine control module. I would imagine that the cruise program would be looking at this as well to determine when to down shift.



You got to love all that torque

 

[RustyJC]

It was a smart-aleck comment. The reason mine won't downshift on cruise control is that I have a 6-speed.



Rusty

 

[Mundgyver]

Dog gone it I'm bieng slow today, but that ok cause I got a 6 speed too



But with that said I can tell you from personal experience that installing an EZ into a stock automatic equiped 2500 will allow it to pull a trailer more efficiently.



A buddy of mine wanted to see what improvement it would make for pulling is 9,000 lb 5ver. We did a test up a very steep grade from a standing start over a given distance. The 1st test was with out the EZ. He could only get to 28 mph and the transmission was constantly shifting in and out, hunting for the right gear to be in.



When we put the EZ on and repeated the same test at the same spot from the same standing start, we got to 41 mph and the transmission was rock solid locked in with no hunting.



That why I think the computer monitors the engine's load or torque output for proper shifting. If there is enough torque then it does not need to shift. Does this make sense

 

[PERMENTER]

Yes, my 2002 has a vacuum line to the server. In fact, I traced the vacuum line back to near the firewall to a junction, pulled the line off and plugged the fitting to make sure I had the right line.

The cruise would not engage, I stopped and reconnected the vacuum and things went back to normal. I think I will try the needle valve remedy since 3 shops have already said there was nothing that could be done--there was no way to adjust or retard the downshifting--in other words, learn to live with it. It only happens when I'm towing and a steep overpass comes up, so it is really only a nuisance.

Thanks for all the replies.

Rick

 

[Mundgyver]

PERMENTER,



Do you have any mods on your truck or is it completely stock?



I'm just curious. I put my needle valve into the vacuum circuit for the cruise to prevent the surging from the performance mods that I had installed.



My buddies truck that we tried the EZ on was all stock with no mods at all.



For a good adjustable precesison needle valve you will wind up laying out around $40.



The EZ is in the $400 range.



Hopfully the needle valve will work for you because it will slow the response of the throttle down which the transmission would be looking at as a load demand.



This should prove an interesting project/fix for you.