'99 seat belt problem
 

I'm a fairly new owner of a '99 SL500 that just developed a nagging problem with the seatbelts - once the seat belts are latched the reel locks and will not extend. (this happens on both seats) The roll bar/top mechanisms all work properly.

As I understand, the belts have a relay that must be energized for the reel to unlock. All fuses seem OK. The only anomaly I can see is the seal belt warning light comes on when the car starts whether the driver's belt is buckled or not, and if I remove the belt while driving the light does not light.

I've seen some posts that mentioned checking the diagnostic connector in the engine bay - however I understood the connector to be on the passenger side of the car close to the shock mount, but I can't find it - was it relocated in the '99 models with the introduction of the 113 motor?

Any suggestions on how to diagnose? Thanks a lot!

Just to clarify, I noted that the seat belt light blinks 4 times when the car starts, then goes out, whether the belt is buckled or not. If it's not bucked the warning tone sounds though.

The blionking action I think is all correct. I've had to replace some belts on 129's for problems like these.
On the passengers side, make sure the small switch on the side trim (the side towards the door) is in the correct position, switching that changes the way the seatbelt reel works for using a child seat.

Gilly

Thanks for your reply. I've flipped that switch a few times to no avail, it's strange because the driver's seat also exhibits the same problem. Driving today on a somewhat bumpy road the belts freed themselves for a short while, which leads me to believe the problem may be a loose connection somewhere. Just not quite sure where to start looking.

Any idea where the diagnostic connector is on a '99? BobTerry99 posted some excellent pics of this a while back but it must be a slightly older model, there's no electrical circuitry down by the passenger shock tower on mine. Thanks

The diagnostic connector on a '99 is under a round plastic cap close to the hinge for the hood on the passenger-side of the car. But you need fairly exotic diagnostic equipment to read most of the codes from it.

The circuit for the seat belt relays is fairly simple to describe. Fuse #3 in the trunk fuse box is wired to one side of each of the seat belt relays, and the other side of each relay is independently grounded by the roll bar controller.

Diagnosing the problem will require a basic multimeter and removal of the bottom seat cushions (easy). Post back if you wish to proceed.

Thanks, yes I'd be very interested in learning how to diagnose this issue, any help would be appreciated.

I suggest beginning with the driver's side. You'll need to remove the seat cushion. The cushion is secured by screws under its left and right front corners. With a 10mm socket and an extension, remove these screws and simply lift the cushion out. If you have heated seats, seperate the small electrical connector at the rear before withdrawing the cushion.

With the cushion removed, locate the "B" plug on the X55/3 connector shown in the photo below. The plug has two pairs of pins which are labeled: B1 and B2. You are interested B1.

I suggest making the following checks to begin with:

1. Remove the B plug and measure the resistance between the male terminals of B1. These wire to the coil of the seat belt relay, and if all is well, you should measure 60-90 ohms.

2. Measure the voltage between the two female sockets of B1 on X55/3. You should read battery voltage.

If you reply back with your findings, I will suggest further steps.

Thanks Bob, appreciate the photo, it sounds pretty straightforward. I'll pull the seat bottom off this weekend and check it out.

Hi Bob,

I removed the seat cushion as you mentioned, very easy btw. I measured 0 resistance between the male B1 terminals, and 0 voltage on the female side. This was with the key off and the belt unlatched.

Since this problem affects both seats, I am thinking that there's a problem in the wiring between the fuse box and the seats I still need to check the voltage coming across the #3 fuse, I just need a different type of probe on the meter to test that. The fuse itself is fine.

The problem contiunes to be intermittent also, twice this week the belts unlocked while driving, only to lock again a short time later. Any further guidance would be appreciated.

Thanks again

Are you sure the resistance across B1 is 0 ohms? If it is, you have found a problem.

You should try measuring the voltage across B1 again with the key on. If you don't read battery voltage, then measure from the rearward female socket on B1 to ground. To be perfectly clear, by rearward I mean the socket closest to the trunk, and for a ground, one of the mounting screws for the seat controller will suffice. If the 2nd measurement gives battery voltage but the first one does not, then the seat belt relay circuit is not being grounded through the controller.

Thanks again. I'll double-check that circuit, and check the passenger side also. I understand what you mean about checking to see if using an external ground completes the circuit. Will check it out next.

Again, thanks for all your help!

Steve

Bob,

Removed the seat bottom again and rechecked the readings. There is resistance across the male terminals, upgrading to a better multimeter I measured approx 85 ohms.

Your are also correct in grouding to the rearward B1 female terminal completes the circuit, I measured 12v when grounding to the seat mounting bolt. (this is on the driver's side with the key in #2 position and the belt unlatched.). So yes, it sounds like the belt circuit isn't being grounded properly.

Can you suggest where to look for a possible bad connection? Driving the car the belts lock/unlock randomly, making me suspect a loose connection or damaged wiring.

Also, just curious if the seats are wired "in series" or "in parallel". Since the problem occurs simultaneously on both seats, I'm assuming that both seats are grounded at the same point.

Thanks again, your help is greatly appreciated.

The relays are grounded independently by the controller, Steve. You can check the wiring as follows. Locate the controller under the false floor in the storage compartment behind the passenger seat. The circuits from the left seat B1 ground through pin 25 of the "2" connector, and the right seat grounds through pin 16. The "2" connector is the long one of the three connectors attached to the controller. You will probably find it much easier to remove from the controller for testing if you first unbolt the controller from the body.

One other thing to check. Verify the ground of the controller!

If the wiring proves to be good and you are confident the connection at the controller is good, then I think you may conclude the controller is bad. If the wiring is bad, well, making that repair could be prohibitively difficult.

I was in your very situation Steve with a problem belt on a car and simply decided to hot-wire the belt relay. It was easy to do, since a convenient ground is available on one of female sockets on the adjacent B2 connector. Depending on how often the car is used, however, a serious drawback may be the continuous drain on the battery. Normally, with a functioning roll bar controller this is not a problem since the controller interrupts the circuit if the ignition has been off for a period of time. A better way to do the mod would be to power the relay through a switched voltage source -- probably tapped from somewhere near the radio in the center of the dash.

Thanks Bob, your help has been invaluable. I am thinking that since the belts on both seats lock/unlock simultaneously, logically the problem must occur at a connection somehow common to both.

One more quick question, if the controller unit itself is defective, would that not cause the roll bar dashboard and/or switch warning lights to activate, or cause the roll bar itself to misfunction? I ask only because the roll bar itself seems to operate fine and no warning lights are present. I'm afraid to ask what a new unit would cost...

And if I do need to wire an alternate ground, is there a switched circuit back at the roll bar controller?

Thanks again!

Steve, one controller handles the seat belt relays, windows, roll bar, and the top on your car. The malfunction indicators only come on for problems with the roll bar. If you were to read codes from the controller, there might be one for the seat belt relays set, however.

Regarding a switched power source at the controller, I can't identify one. Good luck.