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Old 07-21-2021, 06:15 PM   #1
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Default 1991 D21 Slow Crank

I've been trying to find the possible culprit and can't seem to locate the problem it cranks roughly 4 to 5 times before firing. It doesn't click so I don't think it's the starter going bad. Could maybe be maf sensor or temp sensor? Just recently replaced the timing chain and water pump, alternator seems fine but has a little whine, currently looking into that but its supplying sufficient voltage, battery is good too. Any thoughts?
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Old 07-21-2021, 10:37 PM   #2
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Check and make sure the grounds are tight. Especially the one to the head.
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Old 07-21-2021, 10:49 PM   #3
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Quote:
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alternator seems fine but has a little whine, currently looking into that but its supplying sufficient voltage, battery is good too. Any thoughts?
It's not all in the voltage on the battery. If you don't know how old the battery is, take it to your local Autozone/O'Reilly's/ect and have it load tested.

Most/Many batteries only last 4-6 years, if you're lucky. Also note the CCA on the current battery. Helps to know these things.
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What he is saying is the aerodynamics of a D21 is a brick in the wind.
Probably at least as bad as a Jeep so worse than a cow.
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Old 07-21-2021, 11:33 PM   #4
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With electric motors voltage is RPM and current is torque.
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Old 07-22-2021, 01:27 AM   #5
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Could maybe be maf sensor or temp sensor? Just recently replaced the timing chain and water pump, alternator seems fine but has a little whine
A bad MAF or Temp Sensor would have a code. Pull the codes to find out which.

If you just replaced the timing chain, my guess is your ignition timing is off. When is the last time you check it? You have to use a Timing Light, and set the Ignition Timing to 10 BTDC.

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With electric motors voltage is RPM and current is torque.
Does this reply have to do with this thread? I don't understand how this fits in.
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Old 07-22-2021, 01:34 AM   #6
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Electric motors draw more current when they turn slowly, and run at a higher voltage when they are running fast. Kind of like how a really bad fuel pump or blower motor will occasionally blow the fuse when it goes, it's drawing more current than the circuit is rated for.

Mathematically, Watts = Volts * Amps, so assuming a constant wattage voltage goes down as current goes up and vice-versa. I am not an electrician, though. Nor do I have a background in electronics.

I am also a little confused by this thread. OP, is the starter actually turning more slowly or is it an extended crank time instead? If you just did the timing chain it might be wise to check your ignition timing. It should be 10 degrees BTDC.

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Old 07-22-2021, 02:30 AM   #7
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The quote button doesn't want to play nice on mobile for me but this is a replay to you, JP.





Let's assume it's turning over slowly. Let's also assume the battery voltage checks out OK at rest. When you turn the key you get a current inrush to the starter motor. This inrush will cause the voltage to dip. If the battery cannot supply enough current the motor will not make rated torque. The motor wants X amount of watts and it'll take it however you give it. When the voltage dips the current must spike to maintain the state wattage as SBJ showed mathematically.



If the battery is failing and cannot supply the required watts and the voltage dips the motor will not be able to spin at rated speed.



DC motors are different than AC motors. AC motors rely on the power frequency (60hz in the US) to denote their rated RPM and changing voltage will not change that RPM nor will messing with current.



DC motors are rated at an RPM at a voltage. I'm going to use simple brushed DC single phase as an example. Free spinning the motor will hit it's rated speed at, say, 12v. Let's say that is 5000RPM. Increasing that voltage (still unloaded) to 24v should yeild a roughly 100% increase inspeed (assuming the winding survive). It's wattage will still stay the same and it's current draw will decrease by half. Go the other way, 6v, and it's speed will roughly halve from it's rated speed and it's current draw will double.



We are doing that every time we start our vehicles. It pulls current from the battery causing the battery voltage to dip which in turn requires more current to be supplied. If the battery cannot supply the current (even if it's resting voltage is OK) you will get a slow crank. It all falls to the batteries ability to supply current. The more current it can supply the less cottage dip we will get. This is why diesels require 2 batteries. Their starters are enormous and require tons of current. They would suck the voltage of a single battery so low the motor would not have enough potential to spin and would be in a dangerous locked rotor state where current draw is the highest and the possibility of permanent damage to the motor is most likely.



This is overly simplified.



I'm not an electrical engineer by any means but work where I need to basically be an electrical engineer and understand the fundamentals of power, DC and AC circuits I know just enough to be dangerous and get myself into trouble. Not always enough to get back out.
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Old 07-22-2021, 05:39 PM   #8
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Quote:
Originally Posted by 91nissan94 View Post
Check and make sure the grounds are tight. Especially the one to the head.
The grounds are all tight.
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Old 07-22-2021, 05:40 PM   #9
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Quote:
Originally Posted by 89'HBV6 View Post
It's not all in the voltage on the battery. If you don't know how old the battery is, take it to your local Autozone/O'Reilly's/ect and have it load tested.

Most/Many batteries only last 4-6 years, if you're lucky. Also note the CCA on the current battery. Helps to know these things.
I will take it to oriley's today and have it tested.
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Old 07-22-2021, 05:42 PM   #10
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Quote:
Originally Posted by jp2code View Post
A bad MAF or Temp Sensor would have a code. Pull the codes to find out which.

If you just replaced the timing chain, my guess is your ignition timing is off. When is the last time you check it? You have to use a Timing Light, and set the Ignition Timing to 10 BTDC.



Does this reply have to do with this thread? I don't understand how this fits in.
Yes the engine is at 10 degrees btdc compression stroke. I will pull codes with the ecu in around an hour and let you know what comes up.
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