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Old 12-25-2003, 04:07 PM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The actual correcting current (EHA) is not as much a problem as the variation. During similar steady state activities (idling..holding at 2000; for two examples) the EHA current range shouldn't change more than ten percent. In other words if your are idling one minute at -8ma +or- a couple ma for lambda closed loop correction it should stay in that same range anytime you check over both the long and short term. It should be the same in three weeks or months or years. Obviously over time it can wander some but not more than 10% without explanation.

This also applies to idle versus 2000 steady rpm versus 3000 etc. All steady state conditions should be within 10% or something is wrong.

Consider this example. The engine runs smothly at 700rpms at idle with a plus 4ma correction. Running at 30mph down the road it runs at a steady rpm but is now running at -4ma. A simple explanation is a fixed size airleak (vacuum leak). This makes the car lean at idle which is corrected by the 4ma additive corrective. When moved to road speed the relativeley small same size leak does affect mixture much so the mix is actually slightly rich and a subtractive correction is maintained.

In another post about the effect of the OVP in stalling a engine I mentioned it would be hard to imagine a car dieing on the road due to OVP failure. I said I could dream of a condition but it would be extremely out of adjustment. Let me explain such a senario.

As it turns out the total lambda correction is only a little over 20ma. THis changes the differentail pressure about .2bar from lean to rich. On decel cut-off the fuel is stopped by a -60ma subtraction. Given these relative values, a totally disconnected EHA can only change the lambda correction half of the 20 or 10ma. So the only way that can make a huge difference is if the car is massively set lean and lambda is using all its capability to keep it running (a better than 10ma + correction). This would probably not be enough to kill the motor when removed unless the correction at 10ma wasn't achieving closed loop and just was keeping it running. A real certainty for such a happening would be if the car was stuck in a 60ma starting enrichment. If such were the case then the car wouldn't run (that is how much it takes). But if the mixture was then leaned way out mechanically then the car could be running fine when the OVP dropped out removing the stuck on 60ma enrichment and leaving the car running at zero ma and way too lean.

This system works better when defeated unless purposely set wrong, except for cold starting/running and emissions control.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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