86 engine shudder with voltage drop
04-22-2022, 07:34 PM
#1
86 engine shudder with voltage drop
Howdy ya'll and happy Friday
I'm a bit lost on this and would appreciate some input, long time lurker for this issue.
I have an 86 944 and when I got it I was told it's engine shudder at idle was caused by the fact it needed it's water pump and timing belt replaced. (I have both just haven't gotten to installing them). However I have recently come to the conclusion that it might be electrical related. The voltage sits very healthily at 13 volts for the most part but at idle as the RPM's drop down from 1000-900 to about 700-500 and does it's little shudder dance, the volts also drop down to about 12 and than jump back up. I also noticed as we moved into warmer months that when the AC is turned on the engine and whole car shudders much more frequently and violently, but the whole time the AC is engaged no matter what speed I'm going the voltage sits at 12 volts and the thing feels like it's barely hanging on. I have played around with different things (I've been away from my tools) and when the 2 window motors and the sunroof motor are engaged the volts drop to 12 and the car feels like a horse on its last leg. I should mention the engine shudder and the 12 volt symptom are synchronized to a T. Hence my belief that this is electrical vs mechanical or vacuum related. Do you guys think a new battery and alternator will do the trick? or is it time to bust out the multi meter and test wires and fuses.
I know the Idle valve under the intake manifold, vacuum line leaks, DME Relay, throttle body adjustment, and hall sender, are all things that could be causing this but I'm taken back by the fact that the electrical system is bogged down by these small things like lights and window motors and how much it struggles with the AC. It leads me to believe that maybe in fact the issue is in the electrical system. Though I'm not sure, hence why I posted here. Any help is appreciated.
Thanks for your time.
I'm attaching a video as well. The first few dips into 12 volts is when I brake and start slowing down. The last time it dips (about 0:22) and holds close to 12 volts is when I turn the AC on. I should note when I turn the AC on, even while driving in 5th gear, I feel a change in power and the car begins to shake slightly even at speed.
I'm a bit lost on this and would appreciate some input, long time lurker for this issue.
I have an 86 944 and when I got it I was told it's engine shudder at idle was caused by the fact it needed it's water pump and timing belt replaced. (I have both just haven't gotten to installing them). However I have recently come to the conclusion that it might be electrical related. The voltage sits very healthily at 13 volts for the most part but at idle as the RPM's drop down from 1000-900 to about 700-500 and does it's little shudder dance, the volts also drop down to about 12 and than jump back up. I also noticed as we moved into warmer months that when the AC is turned on the engine and whole car shudders much more frequently and violently, but the whole time the AC is engaged no matter what speed I'm going the voltage sits at 12 volts and the thing feels like it's barely hanging on. I have played around with different things (I've been away from my tools) and when the 2 window motors and the sunroof motor are engaged the volts drop to 12 and the car feels like a horse on its last leg. I should mention the engine shudder and the 12 volt symptom are synchronized to a T. Hence my belief that this is electrical vs mechanical or vacuum related. Do you guys think a new battery and alternator will do the trick? or is it time to bust out the multi meter and test wires and fuses.
I know the Idle valve under the intake manifold, vacuum line leaks, DME Relay, throttle body adjustment, and hall sender, are all things that could be causing this but I'm taken back by the fact that the electrical system is bogged down by these small things like lights and window motors and how much it struggles with the AC. It leads me to believe that maybe in fact the issue is in the electrical system. Though I'm not sure, hence why I posted here. Any help is appreciated.
Thanks for your time.
I'm attaching a video as well. The first few dips into 12 volts is when I brake and start slowing down. The last time it dips (about 0:22) and holds close to 12 volts is when I turn the AC on. I should note when I turn the AC on, even while driving in 5th gear, I feel a change in power and the car begins to shake slightly even at speed.
Last edited by Ellendejefferes; 04-22-2022 at 09:19 PM.
04-23-2022, 09:20 AM
#2
Rennlist Member
Use a voltohm meter and test the voltage at the battery and at the alternator output. With the engine running and for tests purposes, above 1000 rpm and a/c off , the reading at the + battery terminal and the alternator should read at least 14 v. Then retest after driving car to get the alternator hot. I found my alternator was ok at cold but my at hot.
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Ellendejefferes (04-25-2022)