Rostra Cruise Control - running off the HEI distributor Tach signal

[Kelvin]

This is a great post! I am replacing my OEM cruise with a Rostra once I switch out the steering column. The existing OEM Cruise is missing parts anyway and likely leaking vacuum.

I believe there is a post on this Forum about correcting the +- 2 mph creep - it’s a dip switch setting on Rostra. If I find those instructions in my notes, I will share here.

Ed

UPDATE: You may have already seen this dip switch setting but it has to do with 1 mph drop kwhen setting cruise.

View attachment 11526

Make sure the slack in the cruise control cable is minimal. It's a chain at the throttle bracket, so maybe you can take out one "ball" in that chain. I use Rostra in the GMC and my Datsun towd. I removed the slack, and they both worked better.

 

[pilondav]

On the Rostra website I only see the GlobalCruise model. I don't see a WorldCruise and it doesn't come up in a Google search. Is that the name JimK has given the kit he sells?

I confused the GlobalCruise with the WorldCruise model. My mistake. I believe the WorldCruise is obsolete and given that, I assume Jim is going to ship the GlobalCruise model.

At first I was wondering how Todd set his controller to 10k PPM when the attached DIP switch documentation (which appears to apply to the WorldCruise model) only went up to 8k PPM. Then I pulled up the GlobalCruise manual and found Rostra added two more DIP switches for more PPM options.

I think I'm going to follow the tried-and-true method with the speedo transducer instead of the tach-as-VSS method after all. I thought about the downshift issue as well, but I couldn't figure out how the controller could prevent rapid deceleration. Apparently it doesn't. Even with the torque converter slip, I don't think it's possible to avoid that deceleration.

 

[pilondav]

I'm looking over Duane Simmons' (RIP) installation notes for the UltraCruise II controller. He says to ground out the tach input signal instead of connecting it to the distributor.

What's the current recommendation on this for the GlobalCruise controller? I like the idea of having some engine overspeed protection, but I think I can tell by ear if it's really overspeed.

 

[tmsnyder]

From the Rostra website:
* Rostra Global Cruise will not accept input from a tachometer to provide a vehicle speed signal.

Using the Tach signal as the VSS will effectively make the Cruise Control (CC) into an engine speed governor.

This would create some undesired effects. For instance, if you were climbing a hill at say 2,500 engine RPM and you decided to manually pull the transmission down into Second gear but forgot to turn OFF the CC first. The CC would attempt to regulate the engine to 2,500 RPM in second causing the speed of the coach to drop dramatically.

This is not what you wanted or expected causing an "Oh Shit" moment! Not to mention any vehicles behind you would need to brake as you suddenly slowed down without any brake light indication.

Just my Ex-farm boy shade tree mechanic way of seeing things.

Yes exactly, it turns it into a engine speed governor.

Hopefully you're not driving in hills steep enough to require downshifting with a vehicle speed cruise control on, are you? Good way to cook a motor.

With the engine speed governed it's actually safer for the engine if you get into hills. If you come up to a hill and decide drop down a gear, that wouldn't hurt a thing. I do it all the time. It climbs at a slower speed and partial throttle at an RPM the engine is running best at.

But even if you weren't happy with the reduction in speed (12K lb motorhome climbing a hill + Physics = slow climb rate) your right foot is still operative. The accelerator still works. Just step on the gas.

 

[tmsnyder]

I've been trying to figure out why the tach-output-as-VSS-signal works better than the speedometer transducer signal.

i'm assuming the tach signal pulse rate happens to be close enough to the expected VSS pulse rate (10k pulses per mile) for the controller to function as intended. There must be some other factors at work that help the changing ratio of engine speed to vehicle speed become an accurate-enough vehicle speed input to the controller.

There might be some FM involved, but hey, if it works, I'm not going to question it.

That is a good question, I definitely did those calculations. But I can't remember if I changed the dip switches to match, or if the rate was close enough for the Rostra to accommodate. The Rostra really doesn't care if the number of pulses per mile is accurate btw.

I know I definitely did make the dip switch change to square wave in order to input the tach signal.

 

[RF_Burns]

Say 2500 RPM / 2 (cylinder fires every-other revolution x 8 cylinders = 10,000 pluses per mile!

Standard VSS for GM ECM is 2,000 ppm

 

[tmsnyder]

Say 2500 RPM / 2 (cylinder fires every-other revolution x 8 cylinders = 10,000 pluses per mile!

Standard VSS for GM ECM is 2,000 ppm

There are dip switch settings for that, from 2000 to 38,600 per mile.