PeachParts Mercedes-Benz Forum - View Single Post

stevebfl · #2 02-18-2001, 04:03 PM

OK I'll stick my neck out.

Two factors make this rather difficult. One is that different test equipment bracket testing in different format/criteria. (I have no specific experience with that specific equiptment). Second I am not where I can look up exact numbers.

Third all diagnostics should start with verification of symptoms. I have no idea what are the symptoms. I presume there are symptoms.

After reading the whole sheet I only see one definite problem. That is the "High Speed secondary KV Test". Cyl #1 is lower in activation voltage (the amount of potential voltage necessary to ionize the molecules in the spark gap or whatever greatest gap. The possible causes for low voltage include shorts, rich mixtures, low relative compression and timing.

The even more important factor is the reduced "burn" time. Normal minimums here are around 1.0 to 1.3ms. The fact that it is so much lower than the third and fourth in the order is very important. All the spark energy is burning out in only 60% of the time on the first two cylinders in the order.

The fact that all the activation voltages dropped at "High Speed" is normal and a result of the lowered effective compression incurred by the advance in timing . The fact that the low speed test showed no difference is something to consider. It eliminates shorts as the reason for the reduced high speed voltage (the shorts would be more apparent to the higher KV of the low speed test).

The next test should be a snap KV test, but since it isn't here I will speculate that a reduction in compression would cause a reduced firing KV but it would probably lengthen the firing time. The relationship is higher activation voltage - lower time, lower activation voltage - higher time. The total spark energy is always the same (with a single coil).

The next possibility would be a rich mixture (that isn't there at low speed - tricky condition). I tend to see this as the condition although the condition of spark plug/plug gap could tell a lot. If I could draw it here I would. The normal firing condition involves a build-up of voltage as the coil primary field collapses. At the point where the greatest source of resistance is overcome the voltage is discharged through the new connection. In normal ignition systems this resistance is the plug gap and because the connection is bridged on ionized fuel molecules there is a large activation voltage (spike) followed by a steady firing voltage (much less) which uses up the the rest of the energy. In a good designed and running system the spark lasts long enough that the fuel gets thoroughly burned this means that at the end of the line a natural increase in voltage occurs before the spark dies out do to all the hydrocarbon molecules beeing used up. On a scope the whole thing is: a vertical line to 10kv a burn at .7kv for 1.7 milliseconds, a small curl up and then a drop and maybe oscillations depending on the type of electronic coil control.

In the case of the high speed kv pattern for this car I imagine the low firing spike is broadened and sloped to the end of energy with all the energy disapated to ground in a hump and a reduced time.

To try and make actual diagnostics from this single issue is only relavent for discussion. I would look at that plug first, check the compression second. Actually I would have also done a snap kv test and probably done a relative compression test with the scope by comparing cranking current with an inductive amp probe triggered off #1 cyl secondary kv (spark plug wire). In this test with a scope attached to the battery cable with an inductive amp probe the amps necessary to crank each cylinder through its compression stroke is graphed on the screen. The peaks are easily compared to get a simple non intrusive compression test.

After all that I'm not sure I would do anything if you have no SYMPTOMS.