LT1 Over-Rev (by cruise control this time)
11-08-2016, 02:35 PM
#1
Melting Slicks
Thread Starter
LT1 Over-Rev (by cruise control this time)
About a year ago I posted another thread because of an over-rev due to a missed shift. The car runs fine and haven't had any problems because of that event.
This over-rev event was a little different, not as concerning...
Not sure why but I was pretty sure that the cruise system monitored the transmission and wanted to find out... so while on the highway with cruise control engaged, I gave the gas pedal a little nudge and during the following brief decel I slide the tranny out of 6th into neutral (no clutch). I was expecting the rpms to drop to idle speed. What happened instead, the engine revs up and bumps off the limiter 2-3 times before I can turn off the cruise control switch. Yes I should have touched the brakes but that was not my first thought.
Everything is fine, I'm not worried about the engine. I just wanted to say that I learned the cruise control is not wired to the transmission, at least not in the 6-speed cars. And perhaps this should not be considered an over-rev since the limiter did its job...
This over-rev event was a little different, not as concerning...
Not sure why but I was pretty sure that the cruise system monitored the transmission and wanted to find out... so while on the highway with cruise control engaged, I gave the gas pedal a little nudge and during the following brief decel I slide the tranny out of 6th into neutral (no clutch). I was expecting the rpms to drop to idle speed. What happened instead, the engine revs up and bumps off the limiter 2-3 times before I can turn off the cruise control switch. Yes I should have touched the brakes but that was not my first thought.
Everything is fine, I'm not worried about the engine. I just wanted to say that I learned the cruise control is not wired to the transmission, at least not in the 6-speed cars. And perhaps this should not be considered an over-rev since the limiter did its job...
11-08-2016, 02:56 PM
#4
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The clutch switch should disengage the CC. I'd start by checking this operation first.
Not sure if the PCM will sense the trans being shifted to neutral, to disengage CC.
I'll have to try this on mine.
11-08-2016, 03:05 PM
#5
Melting Slicks
Thread Starter
There was no indication that the ECM/PCM (or any other part of the cruise control system) was aware that the transmission was shifted out of gear. The CC did not disengage. So the result of my little experiment was a few bumps off the rev limiter, before I manually turned off the CC with the left stalk switch.
11-08-2016, 06:57 PM
#6
Team Owner
Pro Mechanic
There was no indication that the ECM/PCM (or any other part of the cruise control system) was aware that the transmission was shifted out of gear. The CC did not disengage. So the result of my little experiment was a few bumps off the rev limiter, before I manually turned off the CC with the left stalk switch.
11-10-2016, 08:31 AM
#7
Le Mans Master
Yes, the cruise control is not set up to deal with what you did. At least most early computer controlled cars aren't. Most manual transmission vehicles utilize a switch on both the clutch and the brake to disengage the cruise control.
Regarding the rev-limiter... I have autocrossed my Miata for 10 years and my C5 a half dozen times. I can't even guess the number of times I have hit the rev limiter on the Miata. For that matter many of the participants in autocrossing do as well. I try to avoid it because it usually means my run will not be as fast as it could have been, but not a hint of damage.
I'm a bit more careful with the C5, but have bounced off the limiter at least a few times. The rev limiters on most of the newer computer controlled cars are what are referred to as soft limiters in that they cut fuel randomly to the cylinders. Back in the 60's & 70's most of them would simply cut power to the ignition until the engine rpm dropped below a preset level and usually resulted in a lot of popping and banging from the exhaust.
About the only way to over rev a modern (fully functioning), computer controlled engine is to do it on a down shift or hitting a lower gear as you are up-shifting, a mechanical over-rev. Don't do it!
Good luck... GUSTO
Regarding the rev-limiter... I have autocrossed my Miata for 10 years and my C5 a half dozen times. I can't even guess the number of times I have hit the rev limiter on the Miata. For that matter many of the participants in autocrossing do as well. I try to avoid it because it usually means my run will not be as fast as it could have been, but not a hint of damage.
I'm a bit more careful with the C5, but have bounced off the limiter at least a few times. The rev limiters on most of the newer computer controlled cars are what are referred to as soft limiters in that they cut fuel randomly to the cylinders. Back in the 60's & 70's most of them would simply cut power to the ignition until the engine rpm dropped below a preset level and usually resulted in a lot of popping and banging from the exhaust.
About the only way to over rev a modern (fully functioning), computer controlled engine is to do it on a down shift or hitting a lower gear as you are up-shifting, a mechanical over-rev. Don't do it!
Good luck... GUSTO
11-10-2016, 11:39 AM
#8
Melting Slicks
Thread Starter
Yes that's what happened the first time, about a year ago. Shifting from 2-3, accidentally went 2-1 instead and dumped the clutch (at about 50 mph) before I realized what happened. 1st gear synchro got a real workout, and the engine made a strange humming noise as it went way beyond its RPM range. I knew it was not good that this happened... but the engine and tranny have still not missed a beat, ever. Pretty tough drivetrains in these cars. But now I am a bit more careful and deliberate in my shifting.
11-10-2016, 04:23 PM
#9
Race Director
Good Ol' rev limiter They protect our stuff when we do dumb things
. The only way to release the cruise control is either turn it OFF or touch the brake or clutch pedal to either break vacuum or electrical signal to release it. When you glided it out of gear, the computer had no idea that the car was not still in gear. When it slowed below the set speed it tried to speed the car up again. No connection to the rear wheels, so when it didn't respond; more gas
!
Did that in my old 4cyl Mustang once and it scared the hell out of me. Don't know if that thing had a rev limiter or not, because the tach went WAY off scale! The numbers only went to 6 but the needle went to where a 9 should have been!!!
They protect our stuff when we do dumb things. The only way to release the cruise control is either turn it OFF or touch the brake or clutch pedal to either break vacuum or electrical signal to release it. When you glided it out of gear, the computer had no idea that the car was not still in gear. When it slowed below the set speed it tried to speed the car up again. No connection to the rear wheels, so when it didn't respond; more gas!Did that in my old 4cyl Mustang once and it scared the hell out of me. Don't know if that thing had a rev limiter or not, because the tach went WAY off scale! The numbers only went to 6 but the needle went to where a 9 should have been!!!