944 S2 Immobiliser

[HH1]

Hi everyone, my 91 S2 Cab hasthe factory immobiliser which has begun to malfunction. It will regularly cut all power to the car. It'll stay this way for about 30 minutes then all of a sudden I'll be able to start the car again. There is no way to know when the car will start or not. Even after driving for 1 hour when I switch the car off, I am unable to start it right back up. It's as good as dead. This is proving to be very difficult as I am unable to drive it normally. Has anyone had such issues before and if so, what's the fix? Thanks in advance, Sohan. 

[Buchanan Automotive]

First of all , more details needed , like }

A )  When you say " the car will not start " , does the engine crank over at the correct cranking speed during this episode of not starting ? , meaning does the starter motor crank OK and if so not too slow 
Note } on the 944/S2/951 etc , 9113.2> , 928 84> , if the cranking speed is just a bit too slow , then the engine speed reference sensor will not be able to supply the correct amplitude 2.5V signal to the engine management ECU/s & the engine will not "fire up " all because of slow cranking speed

B ) When you say " it will regularly cut all power to the car ", do you mean that the ignition lights ( red lights ) on the instrument panel do not illuminate when you turn the ignition key to ignition on position ?  and also do things like the headlights, interior fan, electric windows , horn , wipers etc  still can be turned on with ignition on position ?

The reason I asked is because the factory immobiliser only cuts power to the engine management ECU  & yes we have been deleting these factory immobilisers in the 944 , 911 , 928 series over the last 15 + years because the immobiliser ECU can & do get unreliable in old age ( BUT ) when they play up they ( immobiliser ECU ) will normally just stop the car as its driving along ( quite dangerous ) & very rarely in the engine start mode 

Note 2 }  When we delete the 25 year old + factory immobiliser unit we install new remote control system that means it talks to the factory central locking , so we end up with more reliability , but easier locking and unlocking of the car 

So first things first , more details needed on Questions  A & B 

Regards

Bruce Buchanan
Buchanan Automotive

[HH1]

Hi Bruce,

1) The car does not crank. No thing happens. Turn the key and is as if it's dead. 

2) No red light come on. No fan, no crank, no interior lights,  no headlights, nothing. It's almost like the battery is disconnected. But of course is connected. 

Everything will start up just fine after 30 minutes or so. Sometimes shorter sometimes longer. 

I've noticed that when this happens the clock on the radio starts up where it stopped, i.e. 30 minutes slow. The radio programmes on the preset stations are unaffected. 

Hope you can help.

Regards, 

Sohan

[Buchanan Automotive]

Ok , so its not the immobiliser , you may have one "or more" of the following }

A )  A cheap and nasty battery , meaning a battery that was inexpensive new ( cheap ) and is light in weight , meaning very little lead & very poor quality plates & its these poor quality plates that will disconnect( internally) by them selves & then reconnect , just like you have mentioned

We have seen plenty of the cheap and nasty batteries humans have fitted in Porsche's over the decades & its common with this syndrome , thats why for the last 20 years we only install high quality calcium battery/s

B )  A very poor / high resistance connection at the battery to body earth strap or its just loose 
      A very poor / high resistance connection at the engine to body earth strap ( engine bay ) or its just loose
      A very poor / high resistance 12v connection at the starter motor , or its just loose , remember the main 12v battery cable from the battery has to go directly to the starter motor solenoid & from there it branches off to the engine bay & to inside the cabin , so if the nut holding the 12v battery cable to the solenoid post is loose , you will get that symptom as well 

Note } at 20 + years , all Porsche's are usually in need of new main earth straps ( battery to body & engine to body ) because of their age , but mainly because of high internal resistance because these straps are mainly crimped from new & internal corrosion in old age ( unseen ) between the cable/ strap & its lugs/ terminals 

Regards
Bruce Buchanan
Buchanan Automotive

[HH1]

Hi Bruce, 

Thanks very much for your help. I'll replace the battery 1st and see how it goes then comes step 2.

It's so nice having sine info on this from an expert. 

Regards, 

Sohan 

[HH1]

Hi Bruce,

1 quick question, what sortof battery would you recommend? I'll be changing the battery first and then look at the connecting strap. I believe the battery's more than a few years old as I've had it for 2 and have not changed it. 

Regards, 

Sohan 

[Buchanan Automotive]

Hi Sohan , I generally do not do brand recommendations for good reasons , but I can recommend type , meaning what its made of , for the last 20 years + we have used / installed only Calcium batteries in Porsche cars we work on ( excellent shelf life , high Amperage output & low on venting nasty vapours/liquids ) , we have used in the last 20 years in the Calcium range , brands like Varta , Delco , Delkor etc etc , all have been excellent 

The size is important , often we see in a 944S2 or 968 models a battery that is too small ( these 3.0L high compression engines need all the cranking power available )

The size is DIN 66  , this also fits 928S 1986 > 1995GTS , 944 series 1986 - 1995 , 911 3.2 & 3.3 , 964 & 993 911 , Boxter & 996 911 ( Lots of Porsche's with this size ) , but I highly recommend Calcium & of that size , naturally the DIN 66 is an old spec , but battery guy's know what you mean when you mention DIN 66

Make sure , you or you get someone to check the amperage draw ( engine off & keys in you're pocket ) & the amperage & voltage input ( engine running ) , because you could fit a fantastic high quality battery but if you car ( 944 S2 ) has a amperage drain & you do not know about it you could damage the new battery within a week or two ( left parked )

So we know we need to see something to what most cars from last century exhibited when engine off & car locked  = around 0.02 of one amp draw ( consumption )  Note } 0.01 of an amp would be fantastic , but not common  , now its not unusual for us to see the odd last century Porsche with an average draw of 0.08 of one amp = 4 times higher than 0.02 ( NOT GOOD AT ALL )   or we see them with massive currant draws of 0.14 = 7 times higher than normal ( NOT GOOD on steroids ), that can flatten a battery in just a few days parked , meaning it may just crank over , but too slow a cranking speed to allow the engine to fire into life & if you are not aware of a amperage draw issue with you're car you will instantly blame the battery and not the car

Conversely we need to see the alternator working with the engine at idle ( multimeter ) and even with the headlights on and the radio on and the interior fan on these cars will normally be in positive amperage input & decent voltage input , now when the two large radiator cooling fans switch on ( with the headlights on ) this is the real test for the very high quality but now 25 year old alternator , because the longer you are at traffic lights / urban / city driving at night the less you may not be charging the battery 

Regards

Bruce Buchanan
Buchanan Automotive

[HH1]

Wow Bruce,

That's really comprehensive and extremely thorough. Thanks a bunch. You've been extraordinarily helpful.

Sohan.

[HH1]

Hi Bruce,

Changed the battery today with a 650 CCA unit. Checked earth connections at battery and engine bay - all ok. Starts all the time now. No issues so far. Seems to have solved the issue. Thanks so much for all your help. You're a real asset to the forum. 

Cheers, 

Sohan