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Discussion Starter #1
I have read the posts around S4 Spiders with interest. The problem I have is subtley different so any advice would be welcome.

My car has just developed a poor starting habit. Starter motor turns the engine over well but even after 5-10 seconds of cranking it refuses to start. I don't get any Tach blip - suggesting that the eCU isnt . If I try again to start a second or third time, I will eventually get a tacho blip. Try once more and it will start and run smoothly after a drive it will usually restart for the remainder of the day...but back to square one if left overnight.

The car has had new plugs and leads 2-3 months ago and are not on the suspect list. I have just replaced the dizzy cap and rotor arm to no effect. I will replace the battery and finally the coil as my next step - even though the coil tests ok although not to spec.

The Crank Sensor has been replaced at some stage and looks to be gapped within tolerance. The original artice is still tie-wraped into the loom.

I suspect that there is no Tach blip point to battery issue rather than the coil. I guess I am talking to myself but if the voltage drop during cranking fails to switch the eCU up then further cranking will probably cause a bigger drop so it should never start ? Unless the starter starts drawing less current

I am stumped :confused:

Any suggestions ?


l
 

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I don't know the specifics of the S4's Motronic system but in the L-jet a marginal battery can easily drop below the ECU's 'wake-up' threshold. It may sound like it is cranking over at a decent speed but if the voltage drops below about 10.3V, the ECU never goes to work.

Try cleaning the battery cables & terminals, the ground connection on the floor of the trunk and (while the battery is disconnected) clean & tighten the connections on the starter motor. Also, on the L-jet cars there is a ground strap between the bellhousing and the body (a metal tab in the transmission tunnel). Assuming the Motronic has one - clean & tighten that, too

To test the low voltage theory, connect voltmeter to the battery and see what it reads before and during cranking. A good battery should read 12.3 or more volts with no loads (everything off). Then see what it reads when you turn the key to 'start'.

I'm guessing it is possible that when your car has sat for a few days the resistance to cranking is higher when you first try to start it. After a few attempts the oil gets circulated, it cranks just a little easier and the available voltage may be just enough to allow the ECU to get the 'go to work' signal'.

Or, your Spider is mad at you for not driving it and punishing you.
 

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Based on what you've told us, I got to agree. At this point, I'm betting you have a primary voltage issue (weak battery), or a corroded ground, or maybe both. On our S4's, anything less than 11.8 volts or so is very "iffy". Strong battery, clean terminals, solid grounds are what the Motronic unit likes.
 

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Yes, I would check that no accesories are draining the battery, and I would check that the battery is good after sitting for a day or two. 12.3 would be minimum voltage. Again, check your grounds - at the battery and at the engine.

If you suspect the battery is old and weak, just replace it with a good fresh one.

Try either one and report back, and we'll go from there.
 

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Discussion Starter #5
Started well today so nothing to report. I think I will replace the coil first as it is the only ignition component that has not been replaced in the last 12 months (part of preventative maintenance). If I buy a new battery and it works..it may just be because its fully charged etc and so will mask the problem without being the route cause.
Does anyone know what the spec of the coil is on a S4.
 

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Don't forget about ignition switches...one wire goes to the starter one drives electronics, accesories etc. You may have a switch going bad and that after a few attempts and a 'make', you are good to go for the day. Sitting overnight with things cooling, oxidizing etc make for no joy on the morning..

You can build a test ignition switch for about $5 that easily replaces your ignition switch. The test switch is easy to plug into your existing harness and is an good way to eliminate any concern you might have with your stock switch.
 

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Does the Motronic management system have a cold-start-enrichment system? If so, maybe that could be the problem.

I had similar symptoms on my L-Jetronic managed engine that turned out to be a malfunctioning Thermo Time Switch.
 

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Does the Motronic management system have a cold-start-enrichment system? If so, maybe that could be the problem
No, it does enrichment through the regular injectors via increased pulse duration.
 
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