FR Control Unit Repair Forums

davefr

Hi, I've been really impressed with the quality of answers in these forums, so here goes, hope all the technical minds are tuned in!

I have a Mercedes Vario panel van based vehicle, upon which the Exhaust Brake is playing up.

Merc dealers tell me the vacuum operation of the throttle flap in the exhaust manifold is working OK, but the problem is in the FR Control Unit.
A replacement for this would be around £1600 + fitting + VAT. Rather a lot to spend on a 10 year old vehicle, esp since everything else related to the FR unit is working fine.

But we'll be up & down Alpine passes this summer, and so the exhaust brake will be essential - I've experienced mild brake fade even with it in the past!

Merc themselves won't open the box & play (their policy), but my helpful & realistic workshop manager suggested I might be able to get a specialist (non-Merc) repairer - and the problem may well be a relatively simple component failure within the unit.

A google search for "FR control unit" comes back with precisely 7 returns, all unhelpful except for the following background info;
"Splitup of Telligentâ engine system
The control of the engine, the engine management system, is split over two subsystems each with its own control unit. The control unit of the subsystem Drive Control (FR) is attached to the vehicle and the control unit of the subsystem Engine Control (MR) is attached to the engine.
The FR control unit contains, among other things, vehicle-relevant data, determines the operating conditions of the vehicle and receives the input from the driver.
This can involve, for example, operating the accelerator pedal, the service or engine brake, the clutch and the power take-off control. The FR control unit determines the engine torque which is required for this and transmits it as an engine specified torque to the MR.
All the engine-specific data are stored in the control unit of the MR.
N54.00-2068-75
The operating conditions at the engine are determined by various senders and sensors. These conditions are analyzed and the fuel injection controlled in line with the FR request (engine specified torque), taking into account the maps stored in the control unit.
Communication between FR and MR makes use of an LS CAN bus, while a HS CAN bus is used for the communication between FR and other electronic systems in the vehicle.[size=Font Size] [/size]

That is enlightening for me, because my head was spinning with the terms FR, MR, & ECU. But I'm still no closer to a repairer!

I also searched for ECU repairers, & emailed all likely suspects, but without success.

So HELP!![size=Font Size] [/size]

Do any of you know anyone who could tackle an FR Control Unit repair?

Living in hope,
Dave

hilldweller

>> Do any of you know anyone who could tackle an FR Control Unit repair?

I do this sort of thing for a living but I couldn't touch it with a long bargepole, and that is assuming I could figure out what is wrong which on a CANBUS system is very very unlikely.

If you don't know, CANBUS is a computer network for machines. What goes on that network is known only to the makers and it's going to be very complex.

There is just too much risk, if the repair or anything near it failed and you squash a load of people the repercussions are unthinkable.

Not much help to you but it may explain why Mercedes can charge what they want and why you may fail to get a repair.

How about searching the breakers ?

JohnsCrossMotorHomes

Where are you Dave, I know a good Mercedes specialist in Reading.

01189-482-2953

Peter

davesport

By the way of a friendly bump, can someone tell me what an "exhaust brake" is & what function does it perform ? I'm an amateur petrolhead & spanner twirler so please dont spare the details Very Happy

Many thanks, Dave.

hilldweller

davefr

Well, thanks for your responses so far.

Brian, aka Hilldweller, your reply is certainly discouraging.
- "I do this sort of thing for a living but I couldn't touch it with a long bargepole"
Breakers are probably out of the question for me as my Vario is quite unusual for the UK. Although it was ordered new from a UK dealer, it was specified as LHD. The exhaust brake is also a rarely ordered optional extra. Consequently, the FR Control Unit is apparently different to the vast majority of UK Varios.
Your reply did prompt my brain cells into wondering about looking for the part from a breaker on mainland Europe, but where do you start? I tried Google, got a few returns of whole vehicles for sale (not my model) but no detail & certainly no part reference nos.
I don't know how complex the signal has to be from the unit to the flap. If 24v is applied, the flap closes just as it's supposed to, but I get an ECU warning light coming on on the dash. I had wondered about wiring from the footswitch direct to the flap, cutting out the middleman. Oh for the simplicity of purely mechanical systems just over a decade ago....
On that line, I should remind you that my vehicle is 10 years old, so I wouldn't have thought that the FR unit could be that complex - could it?
Any more wisdom would be welcome.
If I'm going to shell out £2000+ on a new unit, I'd like to more fully understand why I have to do it!

Sorry Peter, I haven't had time til now to read replies & I think it's too late to phone now. If your specialist is an approved agent, then they will not be allowed to help (Merc policy). If they're independent, then maybe there's a faint ray of hope.

To davesport,
I am certainly no expert, but going on from Brian's reply, my experience of an exhaust brake is that you have a butterfly valve, rather like the flap on a carburettor intake, but it's placed inline in the exhaust pipe. When it's in line with the pipe, there's no drag, but when it's rotated to block the pipe, you get the choking effect that Brian describes which means the engine cannot get rid of the gases & so the engine is slowed down. It's an incredibly effective form of braking that doesn't involve friction, so not only does it save on brake wear, it also avoids the loss of braking due to overheating. It really comes into its own with heavy vehicles on prolonged steep descents, hence it's rarely considered vital for the UK (at least not for this size of vehicle).

I would be very happy to hear from anyone else on the subject, please.