No ignition on hot days

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biggles
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No ignition on hot days

Post by biggles »

Hello all

I have a 1981 e12 based car and it has an intermittant/ semi predicable issue. On a hot day +30C and it stands in the sun for a bit I often have a no start. As in engine does nothing, no turn whats so ever. I get clicks from relays but beyond that nothing. If I bump start it takes straight away. In winter I no issues.

I am thinking the temperature of the day will not impact engine compartment temperatures... so it must be something in the cabin. But given I do get some response from the key turning on I don't think it is the ignition switch.
81 BMW 635csi RHD Euro manual.
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emac
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Re: No ignition on hot days

Post by emac »

I think I would try to mark off the obvious culprits first. Battery/starter. When it happens, get someone to turn the key and start jiggling wires at the battery and starter to see if you can locate the problem. Tap relays, and starter. Then check power at starter to see if you are getting power to the start terminal wire. If not work backwards.
TimU
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Re: No ignition on hot days

Post by TimU »

Have a look at the "No Spark" troubleshooting guide.
viewtopic.php?p=236244#p236244

I know you mentioned the engine doesn't even turn over, but many of the tests will be universal.

For a complete "no start" troubleshooting guide, refer to HPSI Motorsport’s excellent Bosch Motronic Fuel Injection Tech Article.
https://hpsimotorsports.com/blogs/tech- ... ch-article
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biggles
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Re: No ignition on hot days

Post by biggles »

Thanks guys. Will start working through the tests.
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Pod
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Re: No ignition on hot days

Post by Pod »

Try changing the coil.
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86_6series
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Re: No ignition on hot days

Post by 86_6series »

Are you saying that on hot days you get clicking in the start mode? Then, if you jump
the battery the car starts? But not hot days the engine turns over and starts ok?

What temps are hot & not hot?
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Re: No ignition on hot days

Post by GRNSHRK »

biggles, you mention that you have an '81 that is E12 based, but that was the changeover period for Euro 6ers and you may have Motronic injection and not L-jet :-k

You should check this as it would help with diagnosing your hot no start problem :-"
:mrgreen:
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biggles
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Re: No ignition on hot days

Post by biggles »

86_6series wrote: Thu Feb 02, 2023 11:31 am Are you saying that on hot days you get clicking in the start mode? Then, if you jump
the battery the car starts? But not hot days the engine turns over and starts ok?

What temps are hot & not hot?
I am back with the car an trouble shooting... ofcourse problem never manifests when I am troubleshooting.

On a hot day, the engine will not overheat but just hot cabin or unerbonnet. Summers push 30C thats usually when it happens. I have not tried jump starting when it is not starting, I always bump start (rolling down a hill and dropping the clutch) will get it going everytime. In one instance it happened at the shops around the corner so I walked home an went back an hour later (it was staning in the sun) and it still wouldn't start. So seems to be ambient temperature related. In winter I rarely have the issue.

I sometimes feel that it is the ignition key unit that is the issue. Maybe hot cabin temprature makes a worn switch not make contact?

Battery fuses, relays all seem good, I have swapped out most the crucial relays.

The car is Motronic
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Ralph in Socal
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Re: No ignition on hot days

Post by Ralph in Socal »

Try connecting a jumper cable (just the negative cable of a jumper cable set) from the battery negative to the starter body or ground strap at the starter. Do this when you're in the no start condition. If the car cranks and starts then your problem is in the.negative side of the circuit

I had the same issue a few years back and found that the end of the negative battery cable was corroded against the engine block mount. Cleaned up that connection and car has been fine since

It also doesn't hurt to spray contact cleaner into the ignition switch. Put a multimeter at the Blk/Ylw wire at the starter solenoid and make sure you're getting +12V there when the ignition switch is turned to start position. No +12V means the issue is in the positive side of the start circuit.

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86_6series
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Re: No ignition on hot days

Post by 86_6series »

I think Ralph is on the right track. My thought was the battery terminals, make sure they're
cleaned and making good contact and it has 12+ volts.I had a no starter engagement when engine was @ operating temp.
turned out to be the starter solenoid. The starting circuit did not have enough to overcome the solenoid
magnet. I replaced the battery and starter and never looked back. I think there are about 11 connections in the starter circuit that the battery has to fight through.
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marc79euro645
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Re: No ignition on hot days

Post by marc79euro645 »

Are you trying with a/c turned off?
biggles
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Re: No ignition on hot days

Post by biggles »

So managed to get it in a no start condition.

The starter solenoid IS getting 12V. I jumpered the negative back to the battery and no change. I was interrupted before I could get any further.

So looks like a faulty solenoid?
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Ralph in Socal
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Re: No ignition on hot days

Post by Ralph in Socal »

Consider applying +12v directly from the battery to the solenoid. Just as a final confirmation step as the starter is actually a lot of work to remove.

Secondly, consider the B35 starter replacement instead of the regular and larger B34 starter. Much easier to install as hardware location is accessible with sockets instead of just a box wrench

Ralph
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84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
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biggles
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Re: No ignition on hot days

Post by biggles »

Will try that.

Away for another month and will tackle it when back.
81 BMW 635csi RHD Euro manual.
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biggles
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Re: No ignition on hot days

Post by biggles »

So manage to get some more diagnostics done.

The solenoid is getting an ignition signal, the motor turns when the solenoid is bridged (bendix stays off though).

So, it's pretty much a faulty solenoid. I will start looking into the solenoids but how servicable are they? Can they come off without removing the starter?
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hornhospital
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Re: No ignition on hot days

Post by hornhospital »

They are serviceable, but the starter will have to come off. I replaced the solenoid and bendix on my 633 several years ago. I got the replacement from WrongAuto (RockAuto), and it was wrong. There are two types of forked levers that go from the solenoid to the bendix. One has the fork ends that fit in a slot on the bendix. The other type has slotted fork ends that fit over a raised ring on the bendix. RockAuto sent what was supposed to be a matched set. NOPE. The solenoid had the same fork type (ends fit in a slot) as the old one, but the bendix had the raised ring. I couldn't get the fork off, so I disassembled the old bendix and the new one and swapped the fork engagement part. It's been working for 6 years.

By the way, when removing the starter, the flat 17mm box end wrench in the trunk tool kit is your friend.
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biggles
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Re: No ignition on hot days

Post by biggles »

Thanks.
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