Hot Starting Issue

1965 - 1972: 250S, 250SE, 300SEb, 280S, 280SE, 280SEL, 280SE 3.5, 280SEL 3.5, 280SE 4.5, 280SEL 4.5, 300SEL, 300SEL 6.3, 300SEL 3.5, 300SEL 4.5

Hot Starting Issue

Postby Rizz280s » Wed 17 May, 2017 1:01 pm

Hi all,
Im a long time lurker and first time poster.
I've had a 1970 280S for just over a year now and it has developed a fairly bad hot-start issue. It starts fine cold, first time every time, but after a short trip that gets up to operating temperature I can't just hop back in and start it up again. I'll turn the key and there is no response of any kind when I turn the key to position 2 (start) until the temperature returns to just below 40 on the temperature guage. At just above 40 it will crank intermittently (as if a connection is returning somewhere in the primary ignition circuit) but wont crank long enough to fire until the temperature is visibly below 40 on the temp guage.

From loads of reading I have a few suspects.
1. Immobilizer (installed 2005), gets quite hot
2. Neutral Safety Switch (has gone through phases of only starting in Neutral, now only starts in Park)
3. Ignition Switch
4. Pertronix Ignitor I electronic ignition (I only installed it last year December)
5. Coil? Looks original...(hoping to find some wisdom here)

My confusion though, is why it would only occur while the temperature is high.
**Bonus** - If I apply any accelerator while trying a hot start, it will crank but not fire, regardless of how long I crank it.

I've checked and cleaned as many electrical connections as I can without disassembling the dash (currently I dont have a good place to work on the car) and basically am looking for some of the vast wisdom you all have here before I go on to try anything major!


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Re: Hot Starting Issue

Postby TheMadRacoon » Wed 17 May, 2017 4:04 pm

The cranking / not cranking seems to be both temperature and accelerator dependent. So maybe look for something common there, like accelerator linkages moving / touching electrical wiring to immobiliser, that may also have a bad connection that is affected when engine bay is hot.

Is the immobiliser getting hot due to its electronics (wouldn't think so?) or could a heat shield help? The no start when hot may be due to the immobiliser not working properly when it's hot?
1975 350SLC Silver / Blue (2012, 284,900 km - present)
1988 190E Dark Blue / Cream (2006, 190,000 km - 2007, ~ 200,000 km)
1974 280E Olive Green / Bone (1993, 316,700 km - 2004, ~490,000 km and still A1)
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Re: Hot Starting Issue

Postby Rizz280s » Fri 19 May, 2017 4:55 pm

The connection with the accelerator is what got me thinking that it was something to do with the immobiliser. The immobiliser wasnt attached to any housing (it occasionally dangled down near my feet, so I tucked it up under the handbrake shaft). I thought perhaps there was some bad wiring to the immobiliser and that when it was being nudged by the accelerator, it was stopping something from shorting?
I followed the accelerator linkages all the way into the engine bay and there's 2 spots it touches wiring. I wrapped it in electrical tape at these points to try and prevent any touching of the bare metal linkages with any of the wiring. There does not seem to be any electrical connections between the accelerator at any point between the pedal and the carby. The only way it connects is indirectly through transmission, where they link at the carby for the transmission shift linkage (to modulate when the transmission shifts).
Perhaps I'll re-examine the accelerator linkages.

As for the immobiliser, it doesnt get heat from the engine bay or anything else. I think it's just carrying a lot of current through small wires and is getting hot on its own. Id love to just cut the damn thing out, but its tied in with some aftermarket remote central locking. Its a headache of wires down there and I am no auto electrician. Ill get it looked at by a pro...
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Re: Hot Starting Issue

Postby CraigB » Fri 19 May, 2017 8:57 pm

I hate immobilisers! And also the dodgy fitters of them.

I can't see how neutral safety switch, coil etc would be problem and then start working and still not start. Possible but unlikely i reckon.

I wonder if you can start eliminating things by say running a wire from battery to + on coil. I think that should be all it needs to run electrically (off the top of my head.) If you have that connected and as soon as it lets you turn over it starts, that would suggest to me the immobiliser is stopping one thing and then the other. The electrics are pretty simple for starting. If you jump a wire to the solenoid and the coil the car should just go. I think the immobiliser is cutting current to each of those things independently and one is reconnecting before the other. I wouldn't think it is hard to retro wire and just remove the immobilising wires but leave the other part of it working.
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Re: Hot Starting Issue

Postby Rizz280s » Tue 23 May, 2017 1:58 am

I think I've figured it out (with the help of a brilliant friend)!
I completely overlooked the starter motor (because it worked fine on cold starts right?) but when he bridged the starter motor it locked on and didnt stop til we disconnected the battery terminals.

I'm guessing that some part of the solenoid has gone bad or there's just too much resistance when it's hot for it to work properly.
I will report back afterI have a bit more of a play.
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