I have just discovered, whilst cleaning out 1/2 litre of oil from my luggage compartment ( leaking spare oil bottle ), that the PDAS/ABS unit has the part number that ends '00', rather than '01'. I know that, according to Adrian's book, my MY92 C4 should have the '01' part, but I also know from my history file that it was replaced with a used part some time in its past. Adrain also states that the '01' unit cannot be downgraded to the '00' unit, but doesn't state why. The PDAS/ABS systems both work fine, so I am not sure if I should now be looking for a '01' unit or not. Any advice?

Thanks.

 

If its not broken ...

 

Very true, but I would still like to know why they are not supposed to be interchanged.

 

Right then... The 00 unit is fitted to C4's up to 91(ish). These cars had the speedo sensor in the side of the gearbox, which feeds the speedo directly. Later cars have the 01 unit, which, combined with R24, derives the speed signal from the front left ABS sensor. R24 isn't a Relay at all, it's an FCU (frequency conversion unit) which divides the pulses from the wheel sensor (45 per wheel revolution) to the original speed signal of 8 per wheel revolution. As with most things, it appears that the dividing line for the changes is fuzzy to say the least. Better explanation by Jason https://rennlist.com/forums/964-foru...-location.html
You can use an 01 in place of a 00 but not vice-versa - I've tried it!

 

My May production 91 C4 had an 01. The unit had gotten wet and did not function. I found several 00s for a reasonable price but not an 01.

 

Thanks guys, and especilly Sten for the info and link. Mind you, now even more confused and no convenient way to check if speed sensor fitted to gearbox, but assume it must be for the 00 unit to work; will have to check it next time on ramp at garage. Seems odd as mine is MY92 ( built in August ). Perhaps the people that fitted the 'new' unit changed everything else, but not billed for that and would be unlikely.

PS reading the link, it is interesting that the update was published in August 1991, refering to MY91 only, but that MY92 production started in August '91. As you said Sten, fuzzy dates!

 

Adam,
Do you have R24 fitted in the 'front row' of the fusebox (Central Electrics)?

 

Thought of that after I had posted; doh! Will be checking today, but assume it will not be there, otherwise my PDAS would not be working and it very much is.

 

OH dear, Barry! The R24 'relay' is present. Now I am confused?!? How can I have the older control unit and the R24 relay and still have a functioning system? Perhaps I should just listen to Indycam's advice and ignore it