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Discussion summary
YOUR RESEARCHVoltage/Rev Drop at Stop [VIDEO]
Leading reported evidence
Check notes, the sai pump draws ~20a
Performed · outcome unconfirmed
No successful repair reported for this finding.
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Discussions
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Leading evidence: 'Check notes, the sai pump draws ~20a' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Check cables and terminals' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Suggestion
Definitely check your cables and terminals for corrosion..
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Suggestion · Original poster
I think the voltage is fine when I crank it in the morning, will check tomorrow.
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Suggestion
I'd check your voltage regulator on the alternator and make sure your battery has the proper charge.
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Suggestion
If the OP decides to remove the SAI pump for testing, note that it may trigger the CEL, which should go away by itself once the SAI pump is plugged back in.
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Suggestion
You can activate it using Durametric and put a dc current clamp on the black cable that connects to the "+" jump start terminal in the engine compartment above the a/c compressor.
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Suggestion
If you don't have Durametric, you can just unplug the SAI pump and supply power to its two pin terminal directly using a jump start cable from the battery.
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Outcome without a verified action link
highway) and coming to a stop, I depress the clutch and the voltage drops a couple seconds later and then goes back to normal.
Hey guys,
After driving at speed (i.e. highway) and coming to a stop, I depress the clutch and the voltage drops a couple seconds later and then goes back to normal. I can also feel the engine stutter/revs drop a bit. Anyone have any idea what might be causing this? Anything to worry about?
Thanks,
Chris
2002 Targa, 82k miles
Porsche 996 Voltage/Rev Drop at Stop - YouTube
I made sure to have the A/C off, but I did hear a faint pump sound come on at the same time and lasted for a few seconds, never noticed it before tonight (probably because I had my radio off for the video). What's an SAI pump?
SAI pump is the secondary air injection pump that's next to the coolant tank for emission. When the engine is not hot enough like after a highway drive, it may come on for a few seconds. It sounds like a handheld vacuum. The pump also comes on durung cold start for 30s to a minute or so.
Btw, do you have a 160F low temp thermostat.
Your temp should be fine with or without the low temp thermostat. With the low temp thermostat though, the engine will spend a little more time at the lower temp, say 170 or so hence the SAI pump would turn on a little more frequently.
With a/c and low beam on, if the voltage is 13v or higher, that means your cables and alternator are fine. If the vdrop only happens when the SAI kicks in, the SAI may be drawing too much current, which you can verify with a dc current clamp meter. The SAI draws about 20~30A if Memory serves.
This is the first I've heard of SAI trouble, is it a common fault? If it's drawing too much power, would that mean it needs to be replaced? Or ok to live with it like it is?
Too early to condemn it though. It's just my speculation so far until someone goes to measure the actual current drawn and also confirm the vdrop is indeed due to the pump. I would say just monitor and wait for now if you don't plan to diagnose it further.
The vdrop is not common but a noisy pump is.
Found a DIY page for replacing the SAI, looks like it's fairly involved to take out. Do you know if I can test the current without removing anything? If so, how do I trick the SAI into turning on?
You can activate it using Durametric and put a dc current clamp on the black cable that connects to the "+" jump start terminal in the engine compartment above the a/c compressor. That cable connects the alternator to the starter then to the "+" jump start point then to the battery.
If you don't have Durametric, you can just unplug the SAI pump and supply power to its two pin terminal directly using a jump start cable from the battery. Make sure you get the polarity right though before applying power.
You can also hear if the pump is struggling to start, in which case a big initial current will be drawn.
Btw, removing the SAI pump is very simple. Just two screws and one nut then one hose, not even a hose clamp. Photo in step 4 here
http://www.georgeco.org/?p=934
What makes you think it's the SAI? I'd check your voltage regulator on the alternator and make sure your battery has the proper charge.
Is the engine at operating temp when this occurs?
I checked my notes, the SAI pump draws ~20A. Next time when it happens again,
after
the voltage swings back to 14v and the SAI pump has stopped, can you turn on the low beam and rear defroster
simultaneously
and note any effect on the voltage? low beam + defroster draw about 25A total so that would be similar to the SAI pump load.
If you see a similar dip in voltage, that means you have some probles in the alternator/regulator/cable. If not, the SAI pump is more suspicious.
When cranking in the morning, does the voltage drop BELOW 10v? That will tell us how healthy your battery is.
I get a significant drop (to about 13v) when I turn on the low beams and rear defroster at the same time, and drops a little lower (about 12.5v) when I come off the highway. I think the voltage is fine when I crank it in the morning, will check tomorrow.
Ok, sounds like you have some fundamental problem in your alternator/regulator/cables. After you make sure your battery is fine, you can further diagnose it based on posts #2-9 here
http://www.renntech.org/forums/topic...er#entry240001
I had the same symptoms before and my alternator was on its way out... Definitely check your cables and terminals for corrosion.. Also just disconnect your SAI and test.. This could be a quick way to test if the SAI is the culprit. It only happens when the clutch is pushed, correct? I'm thinking the alternator/ regulator is having difficult time managing changes in load and rpm from the engine
Totally agree. If the OP decides to remove the SAI pump for testing, note that it may trigger the CEL, which should go away by itself once the SAI pump is plugged back in. If not, you can always clear that with a generic OBDII scanner.
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