Subarumor Posted April 11, 2013 Share Posted April 11, 2013 I have a 1993 Loyale, 5speed, 2wd, wagon. The speedometer broke inside the tranny and it will be awhile before I can repair it. The Engine Check light remains on with code '33' all the time as I'm driving so I assume that the cpu is obtaining speed sensor readings to make adjustments to the engine. What kind of adjustments does it make based upon this input signal? Leaner/richer fuel mix? Timing advance? Or . . . ? The manual doesn't explain such detail. I just wanto know what it's doing to my car when it thinks the car is going zero mph. Quote Link to comment Share on other sites More sharing options...
l75eya Posted April 11, 2013 Share Posted April 11, 2013 Pretty sure the speedometer you read to gauge the speed of the vehicle, and the way the car's computer reads the speed of the vehicle are two different entities. Your speedo gauge is just a cable that runs to the transmission. There's probably a separate sensor entirely for the ECU to know vehicle speed. Don't quote me on that, but I'm pretty sure that this is how it is. Quote Link to comment Share on other sites More sharing options...
NorthWet Posted April 12, 2013 Share Posted April 12, 2013 The vehicle speed sensor is a set of points in the speedometer head. I do not know offhand if the ECU does anything other than pass this on to the cruise control and other options. Quote Link to comment Share on other sites More sharing options...
l75eya Posted April 12, 2013 Share Posted April 12, 2013 Northwet, are you saying that if your speedometer is not functioning then your vehicle is also not receiving any type of VSS signal?If so, I stand corrected. Quote Link to comment Share on other sites More sharing options...
NorthWet Posted April 12, 2013 Share Posted April 12, 2013 If the speedo cable or the contacts at the base of the speedo gauge are not working, then there is no vehicle speed signal. BUT, I do not think that this signal is used by the ECU. It is used by cruise control, trip computer, and as one of the VSS inputs to the TCU for 4EAT models. Maybe some other little things. Quote Link to comment Share on other sites More sharing options...
WoodsWagon Posted April 12, 2013 Share Posted April 12, 2013 The ECU most definitely uses the VSS signal from the reed switch in the speedometer head. That's why it's setting a code when it's not receiving the signal it expects. As to what the ECU adjusts as far a fuel ratio and timing advance in response to the VSS signal, I do not know. Subaru was pretty good about the limp-home programming on those cars, they will run OK with most of the sensors unhooked. Quote Link to comment Share on other sites More sharing options...
zukiru Posted April 12, 2013 Share Posted April 12, 2013 Title is WAY more interesting than the thread. Does the car run okay? Get decent gas mileage? 1 Quote Link to comment Share on other sites More sharing options...
86 Wonder Wedge Posted April 12, 2013 Share Posted April 12, 2013 Title is WAY more interesting than the thread. Does the car run okay? Get decent gas mileage? I was too! Like an "Unsafe at any speed" speech or something.. lol Anyway, the ECU uses the VSS in conjunction with the Neutral Switch to "make sure" the car is in motion. The main feature is for the ECU to cut the injector(s) upon decel (throttle in "idle" position) when the car is in gear to save gas and prevent backfiring/misfiring. The computer has a check-and-balance system in place so the speed has to be greater than 0 and the transmission in gear in order to cut injector pulse. While driving, rev up to about 4-5K, then let off the throttle while in gear and you'll probably hear lots of popping from the exhaust. The ECU won't cut the injector because it doesn't want to stall if you happen to be at idle (since it doesn't know if you're moving or not) Quote Link to comment Share on other sites More sharing options...
man on the moon Posted April 13, 2013 Share Posted April 13, 2013 I drove for a few weeks with the Speedo completely disconnected due to it simply being a PITA to screw into the transmission, not to mention it was freezing cold the day I had it apart. The motor does fine, and I didn't notice much difference, if any, in my fuel mileage. At that point I was commuting a moderate distance and would fill up every payday--and I didn't have to fill up any more gas or any more often during the period the ECU was out. Speed may be a minor factor but it's not significant.If I had to guess, the ECU reads engine RPMs and engine load rather than wheel speed to adjust the fuel mixture. The car won't run at all if the crank-angle sensor is disconnected, for example. Quote Link to comment Share on other sites More sharing options...
Gloyale Posted April 13, 2013 Share Posted April 13, 2013 I have a 1993 Loyale, 5speed, 2wd, wagon. The speedometer broke inside the tranny and it will be awhile before I can repair it. Are you sure it's not just the cable? Quote Link to comment Share on other sites More sharing options...
Subarumor Posted April 17, 2013 Author Share Posted April 17, 2013 Thanks for all the 'speedy' responses. To Zukiru: Yes it still runs fine. Nothing unusual to report. Can't figure the mileage because the odometer doesn't move either. To 86 Wonder Wedge: It doesn't pop at all when decel under load so your theory is incorrect? To Gloyale: It's not the cable because I lifted the drive wheels and ran it to see the speedo pinion wasn't turning inside the tranny. So I guess only the insiders at Subaru know what relevance the speedo input has to the computer. Quote Link to comment Share on other sites More sharing options...
robm Posted April 18, 2013 Share Posted April 18, 2013 I suspect the VSS tells the ECU when the car is moving. I notice that when coasting in neutral the engine idles at 1500 RPM or so, it doesn't drop to 700 until the car is stopped. Try coasting in neutral, and see what your idle is. Rob. Quote Link to comment Share on other sites More sharing options...
86 Wonder Wedge Posted April 20, 2013 Share Posted April 20, 2013 I SWEAR I read it in one of the manuals (Subaru written, not haynes/others) that specifically says thats what the ECU does foe the injector pulse.. Gotta find it.. Quote Link to comment Share on other sites More sharing options...
NorthWet Posted April 20, 2013 Share Posted April 20, 2013 I suspect the VSS tells the ECU when the car is moving. I notice that when coasting in neutral the engine idles at 1500 RPM or so, it doesn't drop to 700 until the car is stopped. Try coasting in neutral, and see what your idle is. Rob. No effect on my 88 MPFI. Can't remember for sure if it has happened on my subaru, but I know a couple of my vehicles do the fuel-cut even standing still: If I had set the idle too high, the rpms would roller-coaster at a traffic light as the ECU does a fuel-cut and then restarts it once RPM drop below its lower limit. My personal belief is that, although the ECU processes the signal from the speedo, it does not use the signal. Quote Link to comment Share on other sites More sharing options...
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