Lights cut out on deceleration and randomly 1997 S70 T5

Fosgate · Post by **Fosgate** » 19 Sep 2018, 19:05

Hey guys,

So I have this strange issue with my 1997 S70 T5 ... and this happens usually after bit of heat soaking.
This happens if I press on the gas and let the engine decelerate or even randomly when driving. The radio will cut out, the ABS and TRACS lights would come on briefly along with the headlight cutting out momentarily. There is no impact on the engines operation.
Also when idling I can see the dash back-lights flickering slightly and generally goes away after you start accelerating.

I got a spare alternator just incase its the alternator however if it was the alternator I expect the light would dim but not cut out as the voltage would not drop more than the voltage of the battery itself. The battery has been replaced with Verda unit last year.

Looking at the diagram to see whats common between the radio, lights and ABS/TRACS circuit I see that they all somehow interlinked with 2/30 (105) relay which I suspect could be the issue... so going to replace this today before diving into alternator replacement which I dont believe is the issue here.

Has anyone seen this before?

Thanks

WhatAmIDoing · Post by **WhatAmIDoing** » 19 Sep 2018, 20:02

Check the B+ cable from battery to main fuse box and the grounds. Can cause all sorts of weird electrical issues if not making good connections.

Post by **abscate** » 20 Sep 2018, 05:00

Pull the relay and look at the contacts first. My 1999 developed dark oxidation on the posts on most of the relays over 20 years

Fosgate · Post by **Fosgate** » 20 Sep 2018, 23:38

replaced the relay with another one and it didn't make much difference ...the dash backlight is still flicking which tells me that wasn't what causing the issue.. so going to go back to the wiring diagram to see what else it could be, there are some junction points after the relay which I suspect could cause that issue ....

Fosgate · Post by **Fosgate** » 20 Sep 2018, 23:40

I guess another thing I could to to eliminate the alternator is to disconnect the power cable going to it and see if that makes any difference ... if everything becomes stable without any flickering then that would tell me the issue is indeed with the alternator.

Post by **mrbrian200** » 21 Sep 2018, 01:45

This sounds like a wire breaking continuity as the engine rocks back and fourth under torque. I've seen where all the individual copper strands inside broke from repeated flexing but the outer plastic insulation remains intact. At a glance the wire looks ok. Bend it back and fourth at the 'break' and continuity disconnects/reconnects. You're looking at any-all of the wires/looms that run between the chassis and the engine. If you have the schematic you can trace the power and ground circuits from the headlamps back through the wiring all the way to the battery and eventually find it.

Post by **abscate** » 21 Sep 2018, 04:46

New battery cables would be a cheap thing to “parts dart” , and a likelY contributing cause.

Post by **JimBee** » 22 Sep 2018, 20:25

A couple of weeks ago my 96 850 na base seemed to misfire just slightly going on an errand. I parked and when I tried to start it again it was very hard starting, which it never is. By this time, it's dark outside. I got it going, barely, with a lot of accelerator jiggling. There were many traffic lights on my way home and I struggled to keep it running. It's a manual so I slipped into neutral approaching red lights and stop signs while I jiggled the accelerator. Half mile from home the headlights and dash lights went out, tho I kept it running, barely. Turn signals worked and I think the brake lights and backup lights worked.

I parked it and it started alright the next day but with the hood up I was hearing an almost deafening screeching sound, like an bearing was seizing. Upon closer inspection, I discovered it was air I was hearing—the IAC valve hose to the vacuum tree had come off. Somebody :/ had left the clamp off that hose.

I'm not trying to hijack this thread, just adding a strange occurance that caused the lights to go out.

Did the erratic surging and dying rpms cause the voltage regulator to trip out a relay? And they didn't all go out, just the headlights and dash lights.

With the hose back on and clamped

it starts and runs fine with no issues with the lights, tho I do notice one thing different: the idle is around 1k to 1100 rpm. It has always been around 700.

Fosgate · Post by **Fosgate** » 25 Sep 2018, 00:17

Haven't had a chance to check anything but yesterday I drove something like 35kms and it was fine... not a single miss from the dash lights or the ABS/TRACS light ... Only thing I have done over the weekend was the change the pads and the shocks all around ... and that shouldn't effect any of the electrical bits obviously...
However the day before that it was so bad that every time I would let go of the gas and let the RPMs drop I would get the ABS/TRACS light coming on momentarily with the radio turning off and on ...
I run out of time over the weekend so haven't don't any trouble shooting then but Im planning to map out all the relays and connections points and go through them one by one ... so will keep you posted.

JimBee... thanks for your comments, the thing is that my engine runs perfectly no issues there.. looks like whatever it is it only effects the lights/radio and ABS/TRACS line .. I have also noticed this happening when the cooling fan kicks in so thats also related in some way .. perhaps puts more load on that same line.

Post by **abscate** » 25 Sep 2018, 04:46

Jim...clean out that IAC , it probably has dirt in it and is sticky.

Mr Spinner has a good video on it , I think.