Hi
1st post so please be gentle.... After buying a Motorhome on a Fiat Ducato X250 base, 5000Kg 3 litre Auto, the cruise control has me and my auto electrician stumped.
The cruise control works fine upto and around 52mph, and after that, the cruise will not set, but just keeps accelerating. Ive searched everywhere, and not found a similar problem, so coming to you for help,
Auto Electrican has run his software, and no faults were found, when we were testing it,

Does anyone have an idea what it could be.... Some say, because there could of been a tacho fitted, then maybe to do with speed restrictor ?

Any help would be appreciated.

I have considered your problem for several days, but have been reluctant to reply, as I can only make a guess which could well be incorrect.

You do not state whether your cruise control is OEM or aftermarket. On my x244 2.8JTD the ECU monitors engine speed against a set value, while some aftermarket units seem to use a road speed signal obtained from the OBD2 port. Perhaps others could confirm this.

Since problems only occur above 52 mph, it would appear that all connections are OK. Unfortunately this then suggests a fault in the appropriate ECU. However everything tests OK.

What MAY be possible is an obscure ECU fault, but I have not been able to match it with the 52 mph onset of the fault.

Computers often work in 8 bit binary arithmetic. The binary equivalent of 63 is 00111111, but 64 is 01000000. If for whatever reason the processor does not receive or read the isolated 1 in the second example it would read the value as zero.

On my x244 I have read that I have to be above 40 kph say 30 mph when engaging cruise control. Further legally you would should not be using it above 110 kph or 70 mph. This means that the ECU may only be designed to work over the 40 to 110 kph speed range.

Depending on the speed increments adopted and starting point, 52 mph could perhaps coincide with the binary equivalent of 64 in computer, and as this speed is reached, it acts as though the speed had decreased severely. (The vehicle/engine speed signal starts as a pulse rate signal, and I would expect it to be converted to a digital form before being used in the cruise control.)


The above is pure speculation, offered as a hypothetical explanation of the facts. It is quite likely that the final explanation could be very different.


It is worth noting that in critical systems a different code "gray code" is often used for data transfer. In this code only one bit changes with each step, so sudden changes as illustrated above, do not occur.


I am sure that Empi5 would appreciate any alternative, and simpler explanations,

Hmm. Sounds like it is commanding itself an icrease over 52, for some reason. It is likely seeing the set speed as not being reached, maybe due to an ill functioning input from a sensor. Does your speedo/trip meter work OK, any discrepancies experienced, maybe timed over a measured mile (not on vehicle odo in case it's misbehaving-what about a standalone gps/satnav?). If it's not been yours long, and was from a dealer, take them for a ride and show them, could be a warranty issue.

What software did your sparky use? It needs to speak fluent Fiat, not just generic OBD. I use Multiecuscan, and the free trial version is quite good as it doesn't have too many restrictions as to functions. Check it out. If you do, don't rely on a bluetooth connection, opinion seems to prefer wired, and beware cheap leads from ebay.

Check out www.gendan.co.uk they are a tree of knowledge and support, as well as agents for, Multiecuscan. The registered version will allow more than 4 parameters to be monitored/graphed, and no 10 minute (maybe it's 20?) restriction. You would need someone with good interpretive skills for the live data or recordings. Good luck!