I would like to fit a tow bar to my 97 850 r estate....The mechanical bit is
not a problem as I can get a bar from a scrap yard. The bit that concerns me
is the electrics (sparks) . With these new fangled ECU gizmos and things on
modern cars I assume wiring in extra lights is not so easy as it used to be
in the happy days when you could splice straight into the wiring loom and
keep your fingers crossed.

Do any of you wizz's out there know how to do this??

ttfn....Alistair

In an earlier contribution to this discussion,
Alistair Ross <asr@eng.cam.ac.uk> wrote:

> I would like to fit a tow bar to my 97 850 r estate....The mechanical
> bit is not a problem as I can get a bar from a scrap yard. The bit
> that concerns me is the electrics (sparks) . With these new fangled
> ECU gizmos and things on modern cars I assume wiring in extra lights
> is not so easy as it used to be in the happy days when you could
> splice straight into the wiring loom and keep your fingers crossed.
>
> Do any of you wizz's out there know how to do this??
>
> ttfn....Alistair

The only thing you need to worry about is the bulb failure warning unit
which will get upset if it senses extra current taken by running the trailer
lights in parallel with the car lights. There are two possible solutions:

(a) make the connections on the battery side of the warning unit - which is
what happens if you buy the official (expensive) Volvo trailer wiring kit.
You can do it without the kit if you locate the unit[1] and splice into the
relevant wires on the input side.

(b) Buy a trailer lighting relay unit such as a PCT ZR1328A
http://www.pct.eu.com/automotive/ele...h_v2_frame.htm
This takes only a miniscule current from each lighting circuit - which
doesn't upset the bulb failure unit, and requires a heavy-duty feed from the
battery to *actually* power the trailer lights. If you're towing a caravan
which requires a feed via a 12S socket for fridge, battery charging etc.,
there is another relay unit (ZR1220) which will handle that.

[1] ISTR that on my 1999 V70 (which is basically the same car) the unit is
under the luggage compartment floor on the LHS (when facing forward). I
don't have the car any more so I can't go and look at it, but I think it is
a grey box about as big as a 20 pack of cigarettes, with lots of wires going
in and out - with a flat multi-pin plug on the input side (possibly another
on the output - can't remember). the Volvo kit has a plug and socket which
sandwiches between the input harness and the unit.

HTH!
--
Cheers,
Roger
(Warwick, UK)
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