V70 Brakes: - Wiring

StephenCHorsman · Apr 25th, 2018, 16:33

Could re-wiring the outer brake lights to the centre brake light of the volvo V70 cause over loading to the control module?

petey · Apr 25th, 2018, 16:57

If you ran the CHMSL (Central high mounted stop light) off the outer lamps, you would be 'unlikely' to overload anything although strictly speaking you would be above the designed current limit. If you did it the other way around, as the outer lamps are incandescent they will draw much more current and almost certainly make something hot that shouldn't be. From memory the CHMSL is either 4 or 6W depending on model and the brake lights are going to be 21W, so you could be over 6 times the design power and hence current and there's no way that that is going to go unnoticed by the REM (rear electric module)

Depending on the model year, you either have current shunts or shunt chips to detect the current flow; in either case you will get a worded warning that the current through one or more of the brake lights is other than normal as a result of rewiring.

My '05 V70 D5 SE has a failed current sensing chip supplying the CHMSL which was leaking current and causing the lamp to always be on at about 60% intensity without pedal application and 100% when the pedal was down. After a lot of faff, I've got a replacement chip, but as the faulty one is buried under 10mm of conformal coating, the risk of damage is far too high, when instead, I simply remove the tailgate trim at MOT time, as a tester cannot fail for a failed CHMSL if it is shown to be disconnected. Once I get my MOT pass, I put the trim back for another 12 months. Should the REM fail in some other way, I'll get it repaired, but life is too short to be investing time into thankless tasks.

(I do however have to look at my brake lights when there's a truck up close to the back of me as I always have a worded warning telling me of the failure.)

StephenCHorsman · Apr 25th, 2018, 20:19

Many thanks, Peter,

Your reply is most helpful.

Cheers

Stephen

Missing Lincs · Apr 26th, 2018, 19:49

Failure of the brake light relay is a common problem for some model years, the relay is replaceable and readily available from Volvo for a few pounds. SOme models have a double relay which is replaced by two single relays doing exactly the same job. Either way, if that's the reason for wanting to change the wiring it's an easy fix.

As petey says. the current for the centre brake light is low, I think the driver circuit may be a solid state relay so it won't like any over-current at all.

Apr 25th, 2018, 16:33	#1
StephenCHorsman New Member Last Online: Oct 16th, 2018 09:36 Join Date: Apr 2018 Location: Crawley	Wiring Could re-wiring the outer brake lights to the centre brake light of the volvo V70 cause over loading to the control module?

Apr 25th, 2018, 16:57	#2
petey Master Member Last Online: Sep 11th, 2023 13:10 Join Date: May 2009 Location: Chippenham	If you ran the CHMSL (Central high mounted stop light) off the outer lamps, you would be 'unlikely' to overload anything although strictly speaking you would be above the designed current limit. If you did it the other way around, as the outer lamps are incandescent they will draw much more current and almost certainly make something hot that shouldn't be. From memory the CHMSL is either 4 or 6W depending on model and the brake lights are going to be 21W, so you could be over 6 times the design power and hence current and there's no way that that is going to go unnoticed by the REM (rear electric module) Depending on the model year, you either have current shunts or shunt chips to detect the current flow; in either case you will get a worded warning that the current through one or more of the brake lights is other than normal as a result of rewiring. My '05 V70 D5 SE has a failed current sensing chip supplying the CHMSL which was leaking current and causing the lamp to always be on at about 60% intensity without pedal application and 100% when the pedal was down. After a lot of faff, I've got a replacement chip, but as the faulty one is buried under 10mm of conformal coating, the risk of damage is far too high, when instead, I simply remove the tailgate trim at MOT time, as a tester cannot fail for a failed CHMSL if it is shown to be disconnected. Once I get my MOT pass, I put the trim back for another 12 months. Should the REM fail in some other way, I'll get it repaired, but life is too short to be investing time into thankless tasks. (I do however have to look at my brake lights when there's a truck up close to the back of me as I always have a worded warning telling me of the failure.) Last edited by petey; Apr 25th, 2018 at 17:01.

Apr 25th, 2018, 20:19	#3
StephenCHorsman New Member Last Online: Oct 16th, 2018 09:36 Join Date: Apr 2018 Location: Crawley	Thanks Many thanks, Peter, Your reply is most helpful. Cheers Stephen

Apr 26th, 2018, 19:49	#4
Missing Lincs Premier Member Last Online: Yesterday 08:55 Join Date: Jan 2010 Location: Louth	Failure of the brake light relay is a common problem for some model years, the relay is replaceable and readily available from Volvo for a few pounds. SOme models have a double relay which is replaced by two single relays doing exactly the same job. Either way, if that's the reason for wanting to change the wiring it's an easy fix. As petey says. the current for the centre brake light is low, I think the driver circuit may be a solid state relay so it won't like any over-current at all.

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