Cruise Control Max Speed

[pizzaman09]

Been driving corner to corner in Pennsylvania today, using the cruise control we've discovered the max speed it wants to set to is about 73-74 mph.  If we accelerate beyond that and try to set it, it will simply coast down to that speed before engaging.  Any idea why this might be?  It's not a big problem as the fastest speed limits are 70 mph, but the truck can easily cruise faster.

 

Side point, I have an REM and reading out TPS shows that the cruise can manage to use up to about 45 on the TPS signal.  WOT on mine tends to read 90 so that's a lot of throttle it will not use.

[eaglescout526]

Huh. I didn’t know nor think such a limit was there. Though I do suppose that would be a clever way to curb speeding when the national limit used to be 55. If almost wonder ifs either a limitation of the servo or if there’s something wrong on the module. 

[Ωhm]

Maybe something with manifold vacuum. "T" in a vacuum gauge near the Vacuum Release Valve on the Brake Switch. Check how vacuum is holding.

[llhat]

not seeing anything in the wiring diagram,  though  there is an input from the speed sensor...   nothing in the electrical book trouble shooting either.

 

is there not a 'minimum' speed for engagement,  like 35mph?

[eaglescout526]

There is a minimum. It’s 30mph. And the minimum is used for testing purposes to ensure cruise control and it’s components are operating in a basic manner. Like the module and the servo. 

[Tex06]

On the HO engines I set mine at 75 and it holds it. 

 

[pizzaman09]

I'll have to check the vacuum connections, the truck seems to hold good vacuum according to my REM.  It does seem like the servo shtbe able to pull more than it does.  Sometimes it loses speed on hills because the servo loses too much vacuum due to the engine being at a fairly open throttle level.

[llhat]

^^^   and that is where the 'football' under the front bumper 'kicks' in with vacuum supply....  if low vacuum your hvac would divert to the defrost ducts

 

noted also there is an 'adjustment' on the control box (multiple), 

"A" is the centering adjustment

"B" is low speed adjustment

"C" is sensitivity  (located on opposite side of box from A and B

 

one of those should help I'd think....  suggested to make fine incremental adjustments

[pizzaman09]

14 hours ago, llhat said:

^^^   and that is where the 'football' under the front bumper 'kicks' in with vacuum supply....  if low vacuum your hvac would divert to the defrost ducts

 

noted also there is an 'adjustment' on the control box (multiple), 

"A" is the centering adjustment

"B" is low speed adjustment

"C" is sensitivity  (located on opposite side of box from A and B

 

one of those should help I'd think....  suggested to make fine incremental adjustments

I'll have to play with it, I did a lot of adjustments when I first installed the cruise control and it operates nicely otherwise.

 

I did make one modification to the vacuum system on the truck to make it work better, I put a one way vacuum valve in the line connecting to the intake manifold.  It must leak down somewhere.

[eaglescout526]

Check the dump valve above the brake pedal, make sure that is good and sealed with the brake pedal in the neutral position. I literally test it by blowing into the dump valve. You want no air to get by, I can see it still letting some air pass when at those high of speeds and releasing it just a small amount.