Quote:
Originally Posted by 97 SL320
This is normal for a hydraulic shift trans. When the engine has low power, manifold vacuum is low / throttle is opened farther. This leads to higher line pressures / later / harder shifts as the trans thinks the engine is under load but isn't.
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Ok, thanks. Makes sense. I'm not exactly sure which particular trans is in this vehicle, being full time 4WD. There is a TCU under the left front seat, though I've no idea if that can be interrogated separately, at least not with HFMSCAN. The (former) Merc mechanic that rebuilt the head told me he'd looked at the trans and it seemed fine. He's been very helpful actually, he was able to tell me exactly what had and hadn't been done and a lot about the overall condition of the engine even the owner didn't know.
Quote:
Originally Posted by 97 SL320
This is generally ignition related, at higher engine loads voltage required to jump the gap increases. This can lead to not establishing a spark / spark leaking out through a boot or coil. Once engine load is reduced, the spark returns to the plug tip.
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Yeah, that was my thought too. I have replaced all three coil packs (admittedly with s/h) and one boot that was definitely dodgy. It
feels like weak spark to me as well.
Quote:
Originally Posted by 97 SL320
Coil or boot failure is pretty common.
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Yes, I thought the coil packs would fix it. Might just change the entire ignition system (coils, HT leads and coil boot to plug). Plugs are brand new and seem to be fine.
Quote:
Originally Posted by 97 SL320
Have you checked fuel pressure driving under load?
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No, no means to do so. It will cruise happily at high speed (120kmh plus) so I don't really think that's it, but I'll see if I can find someone with a fuel pressure gauge - any idea what size the fitting is?
Quote:
Originally Posted by 97 SL320
I'd hang a volt meter / scope on the throttle position sensor output to see if the throttle is opening enough.
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The throttle is physically opening, you can hear the intake note alter when you hit it - wondering about the Throttle Sensor again so might try that. It is a s/h part but only had 99,000k on it. The car had 213k when I got it.
Quote:
Originally Posted by 97 SL320
MB of this era had ASR ( Acceleration Slip Regulation ) tied into the ABS system. Perhaps the ASR is turning on due to a erratic rear wheel speed sensor. On a MB this can be completly shut off for dyno testing.
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This model Musso doesn't appear to have ASR - the throttle body appears to be a non ASR type.
Quote:
Originally Posted by 97 SL320
2 MV sounds low, measure it at the sensor and see if that matches the computer end.
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Seems to be about the same, it's what it's supposed to be according to a diagram I found somewhere of the waveform. I used an oscilloscope to look at it. It's a reluctor type passive sensor not a Hall Effect one.
Quote:
Originally Posted by 97 SL320
I'll counter the previous suggestion that the cam sensor causing the run problems. Cam sensors are to sync up the injector pulses for a slightly smoother idle. Once the engine gets up in revs, the injectors are open longer making the sync not an issue. This engine has waste spark where 2 plugs are fired from the same coil making any sort of compression / exhaust stroke ignition sync a non issue.
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Ok, as far as I knew the cam sensor was only there so the ECU could adjust the intake valve timing, there's a solenoid on the input camshaft to do this. The crank sensor supplies the TDC and without it, no spark. I'd mentally put the Cam Sensor issue on the back burner while chasing the missing/poor acceleration because I didn't think it would be that significant. My initial thought after seeing that there was a signal at Pin 8 on the ECU was that because it was so low (2mv is not a lot of signal but it seems to be what it's supposed to be) there was probably an opamp or possibly a Schmitt Trigger on the input to make the pulse a bit easier for the computer to handle and it had packed up. I was quite surprised when a replacement computer didn't fix anything, as I had assumed after changing coil packs that a (common) issue with the ECU driver transistors for the coils was the problem and fully expected that to fix it.
Quote:
Originally Posted by 97 SL320
For engines in general, the cam sensor is only looked at during cranking in order to syc things up so a come and go sensor won't be an issue.
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Well, that's not my understanding with this engine, the tech articles are quite specific that the ECU controls the input valve timing (over a limited range) using the difference between TDC (from the crank sensor) and the camshaft position in real time from the Camshaft sensor. That said, if I hadn't been able to get the data from the ECU I'd never have looked at it as it had been replaced when the head was rebuilt after the timing chain failed. There was also an issue with the Cam Solenoid, but this was due to an internal short in the plug, I simply attached the leads on their own by taking the plug apart and that error code went away and never returned.
Thanks for your help with this.
Regards
Geoff