'94 SL500 - SRS control module location?

[E55BOF]

I'm preparing the car for the MoT, which runs out in three weeks. The SRS warning light stays on after starting for anything from 30 seconds to two minutes or so before it goes out, which may be an MoT failure point. Do others have similar experiences? Also, STAR cannot communicate with the SRS module.

Plainly there's a fault, so first step is to replace the module and see what happens; they're pretty cheap on FleaBay. But where is it? I think it's probably on the transmission tunnel under the lidded cubbyhole below the radio, but I haven't been able either to confirm that definitely, or to get the cubbyhole out to take a look. Can anybody help with either problem?

 

[RichardF]

The SRS light will do this if you've turned on the ignition with an airbag disconnected. I.E. Working on the steering wheel or something.

Star should re-set it. The module is under the centre console ahead of the gear selector.

 

[E55BOF]

I thought the SRS light just stayed on in that situation? I haven't done anything to cause it, but the car had a pretty dead battery, and had been started more than once with a booster pack; the two over-voltage fuses on top of the base module were blown when I got it. Heaven knows what voltage spikes had been applied before they blew.

Star can't communicate with the unit at all, I'm afraid.

Thank you very much for confirming the location; now all I need to do is get at it...

 

[RichardF]

The light will stay on with an airbag disconnected. Once reconnected it'll exhibit the fault you describe. Low voltage will cause it as well. I've bought some modules back to life by re-seating them, might be worth trying this.

 

[E55BOF]

Thank you very much. There's hope, then. I'll try that tomorrow, if I can get at it. I presume one removes the cubby-hole by taking out the central screw in the lower edge of the liner, then removing the radio and pushing the cubby-hole out from behind? I can't see/feel any other fastening screws but that one.

 

[E55BOF]

Got at it; swapped it with one I had in a box of R129 bits I bought 'on spec'; the light stayed on all the time. Put the old one back in; the light still stays on all the time. Unless both units have the same fault, that suggests a change.

I'm hoping that the system will now be accessible with Star, so I'm off to WG M-B tomorrow (on the way back from having the exhaust sorted at Top Gear in Watford) to see what Star says. Fingers crossed it's something cheap...

Richard, thank you very much for your help.

 

[RichardF]

It's likely to something simple. I think that model yr only has driver and passenger airbags. No side airbags or seatbelt switches/wiring to worry about.

Could be something left unplugged behind the dash or the clock spring. Easy enough to check continuity, is the horn working?

What are you having done re the exhaust? Is yours just rusty or are you looking for an upgrade?

 

[E55BOF]

The warning light resumed going out after two-three minutes. Slight tearing of hair out (I haven't much to spare), but off to WG MB anyway. Star was unable to communicate with the SRS module before; this time it could, long enough to clear the codes. The SRS light now functions normally. Star showed lost comms again, twice, while plugged in to the car, so I suspect an intermittent fault in the wiring that allows the communication, but it's fixed - Hurrah! I'd not have got there so soon without your input, so thank you once again.

The exhaust was blowing at the flanged joint between centre and back boxes. The flanges and studs were too far gone to reclaim, and the exhaust was fairly rusty anyway. I bought a stainless SL500 back box 'on spec' from a forum member for £50 a few months ago, so off to Top Gear with it; throw away both standard boxes, fit a Y-piece behind the cat, and connect it directly to the back box (all stainless, of course). (Doodle has an AMG back box on his, and no centre box, and it's not anti-socially loud, so I thought it was worth trying - suck it and see). Nice job, good neat welding. I'd go there again.

As a bonus, the slight misfire at idle has virtually gone as well, but the sound; oh, the sound... It now sounds like a big n/a V8 should; a deep, solid, meaty rumble. It's not much louder at idle, but it is now unmistakably a V8. On light throttle around town it just sounds pleasantly promising, but tread on the pedal, and it takes on a much harder edge. When I was 20 I'd have killed for a car that sounded like that!

I worried that it would be too noisy on the motorway, but at a steady cruising speed of 80-85 (where legally permitted of course, officer...) on neutral throttle there's not much more sound than before; I can hear it, but what with wind and tyre noise, it's far from obtrusive.

Pleased? Dog with two tails? Oooooh Yes... :bannana:

 

[jonnyboy]

Nice one Dennis

 

[E55BOF]

Thanks John. I hope the op went well; get well soon.