Starter motor won't crank engine

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monoi
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Starter motor won't crank engine

Post by monoi »

Hi,

I have had an intermittent issue with the starter motor not turning when the engine was hot. I could hear a relay in the dashboard but that was all. With the engine cooled down, the starter motor would work fine and so would the car. As it was happening more and more, and based on what I read on the forum, I decided to change the starter motor assuming it was just too old and replaced it with the new smaller model.

However, I can still hear the relay but no luck at all with the starter motor. I cannot even blame heat as the engine has not run for a while...

Has anybody experienced this?

There are 2 big wires connected which are permanently live, and a small wire which I assume gets live when I turn the key. If I wanted to test the starter, would it be a case of wiring directly the small wire to the battery positive?

Any ideas welcome...
635 Csi highline 3/89
monoi
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Re: Starter motor won't crank engine

Post by monoi »

I managed to start the engine by hot wiring the starter direct to the battery, and engine started immediately.

After shutting it down, I tried with the key and it worked fine. :-$

I'm going to keep that piece of wire in the glove box just in case.
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Ralph in Socal
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Re: Starter motor won't crank engine

Post by Ralph in Socal »

Make sure the ends of the large cables are free from corrosion. Where the black ground cable attaches to the block is also critical. You need excellent contact on both large cables to effectively turn the starter motor

Your battery is likely fine if the car starts without issue when it's cold

The small black and yellow wire going to the starter provides the +12V to engage the solenoid and fire the starter.
You can bypass the entire ignition switch process by applying +12V to that contact to fire the starter. Part of trouble shooting to see if your problem is the ignition switch circuit or the starter/battery cable circuit

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
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anchored
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Re: Starter motor won't crank engine

Post by anchored »

Ralph in Socal wrote: Wed Jun 15, 2022 5:48 pm Make sure the ends of the large cables are free from corrosion. Where the black ground cable attaches to the block is also critical. You need excellent contact on both large cables to effectively turn the starter motor

Your battery is likely fine if the car starts without issue when it's cold

The small black and yellow wire going to the starter provides the +12V to engage the solenoid and fire the starter.
You can bypass the entire ignition switch process by applying +12V to that contact to fire the starter. Part of trouble shooting to see if your problem is the ignition switch circuit or the starter/battery cable circuit

Ralph
This was my issue when I had a recent no start.

gave the cables a quick sand and I was back in business.
SHRKATK - Red 87' 635csi
monoi
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Re: Starter motor won't crank engine

Post by monoi »

The starter turns fine when I directly energises the solenoid, and with ignition on, the engine starts and runs fine.

I can also hear a loud relay noise under the dashboard, behind the central console when I turn the key so it's not the ignition switch.

Is there another relay in the circuit somewhere?
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86_6series
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Re: Starter motor won't crank engine

Post by 86_6series »

Page 1240-0 (27 pdf) manual trans.
Page 1240-1 (28 pdf) auto trans.

https://www.millerperformancecars.com/f ... Manual.pdf
86-635CSi
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monoi
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Re: Starter motor won't crank engine

Post by monoi »

Thank you for that. I have looked at that as well, although I can't really make sense of it.

Yesterday, the car started fine whether cold or hot so it's not a life and death situation.
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hornhospital
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Re: Starter motor won't crank engine

Post by hornhospital »

My 1984 633 developed a reluctance to starting, that is, the starter wouldn't spin the crank. The solenoid would engage, but the starter would spin slowly if at all. It had the symptom of a poor ground. I took the starter off and cleaned the contact area between the starter body and bellhousing. That temporarily fixed the slow/no spin, but it was back in a month even though the mounting bolts were
still tight. This time I ran a separate ground cable from the lower starter mounting bolt to the negative battery clamp. The problem went away and hasn't returned in 4 years.

An aside here: the upper mounting bolt is a right booger to get to to loosen or tighten. The best tool I've found for the job is right in your trunk-lid toolbox, the 17mm box end wrench. The angle is perfect to reach between the intake manifold #5 and #6 runners, with just enough room to move the bolt.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
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Ralph in Socal
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Re: Starter motor won't crank engine

Post by Ralph in Socal »

hornhospital wrote: Fri Jul 01, 2022 8:43 pm My 1984 633 developed a reluctance to starting, that is, the starter wouldn't spin the crank. The solenoid would engage, but the starter would spin slowly if at all. It had the symptom of a poor ground. I took the starter off and cleaned the contact area between the starter body and bellhousing. That temporarily fixed the slow/no spin, but it was back in a month even though the mounting bolts were
still tight. This time I ran a separate ground cable from the lower starter mounting bolt to the negative battery clamp. The problem went away and hasn't returned in 4 years.
Great solution to a typical bad ground problem. Also a good way to test for a ground problem is to run a battery jumper cable from the Battery Negative to the starter ground cable on the side of the starter motor.

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
monoi
Posts: 134
Joined: Mon Nov 02, 2009 10:15 pm
Location: London

Re: Starter motor won't crank engine

Post by monoi »

To close this chapter, the issue turned out to be bad contacts on one of the relay under the dash...
635 Csi highline 3/89
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