Seat Belt interlock problem

[Robert S.]

HELP! My 4-speed '74 Stingray's seat belt interlock has been unplugged and car has always started with no problems. On a rainy day yesterday I turned the ignition -
seat belt buzzer sounding, fasten seat belt light on with ignition on (has never done either with interlock unplugged at both seats) NO START with clutch depressed or not. tried starting several times..no click, nothing...just the seat belt light and buzzer. Fastened both belts...still no start (neutral safety switch and backup lights have been inoperable..haven't replaced a possible defective neutral safety switch?) but car always started. Any help would be appreciated.

[Roger O.]

Re: Seat Belt interlock problem

Try pulling the interlock relay in the engine compartment and putting a jumper between the purple and green/black wires.
(Relay plugs into an orange receptacle.)

[Robert S.]

Re: Seat Belt interlock problem

you mean take it out? how could it cause this if its disconnected at the seats?

[Roger O.]

Re: Seat Belt interlock problem

Just so were on the same page. In the engine compartment,passenger side,upper firewall is a silver relay with a button that plugs into an orange connector that mounts to the firewall. Remove the silver relay with the button. On the firewall will be the orange connector,in the connector are 4 wires (if my memory is correct) , put a jumper between the purple and green.
Try to start your car, if it doesn't start my guess would be the clutch safety switch is permanently open (not working)or the connector fell off or your stater solenoid is bad.

Why the system is not operating as it has in the past is diagnosable but this would be part of the process AND will most likely allow you to start your car until you find out what happened.

[William C.]

Re: Seat Belt interlock problem

The relay is to bypass the Seatbelt interlock system. As Roger says, if that doesn't allow the car to start, look elsewhere just like the interlock system was never on the car. Another step would be to jumper bewtween the battery cable and the purple wire on the solenoid. That should activate the solenoid and engage the starter motor. If not, you likely have a bad solenoid.

[Roger O.]

Re: Seat Belt interlock problem

you mean take it out? how could it cause this if its disconnected at the seats?

There is more than 1 relay in the system,the relay I speak of is an emergency bypass relay for 1 start at a time,this relay will not cause the car not to start it is there only to start your car if another part of the system fails.
If you want push the button and try a 1 time start,if it starts that rules out the safety switch and solenoid.

[Jack H.]

Re: Seat Belt interlock problem

The interlock relay others are talking about looks like the one pictured. It's a silver can with black plastic push button on the top. It's supposed to over-ride the interlock system in an emergency to get you going...

[Robert S.]

Re: Seat Belt interlock problem

my relay is missing the button, I was planning on replacing it and the neutral safety swtch (backup lights not working) I think the switch is part of the backup light operation. do you think replacing both will solve the problem. the seat belt interlock connectors are unplugged at both seats so i don't understand why the light and buzzer are engaging. I did test the system when I bought the car in January...plugged the connectors in at both seats and the interlock worked properly then I unplugged the connectors again. has always always started with the connectors unplugged (with neutral safety and backup lights also not operable)

[William C.]

Re: Seat Belt interlock problem

I'd still try junping the terminals at the starter to see if it cranks.

[Robert S.]

Re: Seat Belt interlock problem

Before I try to do tests with a test light or a jumper.... this observation and question...I believe the trouble is from the interlock ....why is the buzzer and light now working (interlock working) with the connectors behind the seats disconnected? the buzzer/light/interlock was not operating before the other day and car always started. (I mentioned I had connected the connectors behind the seats several months ago just to see if the interlock would work for judging) It did (car did not start and light/ buzzer worked) and I then unplugged them and I never had trouble starting car and buzzer/light did nor come on for for six months of daily driving until the other day. (key buzzer has always worked)
neutral safety and back up lights have not worked and planned to replace the switch (located at the clutch pedal)

interlock relay under hood doesn't work-just checked it-not sure if it worked before
neutral safety/back up lights do not work- was always that way
seat belt connectors under seat disconnected-was always that way

buzzer/seat belt light NOW on and car doesn't start, no starter click-last two days
.
what is causing the buzzer/light and interlock to work with the connectors behind the seats disconnected when they did not operate before the other day? how can I bypass the interlock system other than discconnect the connectors at under the seats?

[William C.]

Re: Seat Belt interlock problem

Do you have an electrical diagram for the car?

[Jack H.]

Re: Seat Belt interlock problem

The interlock system is reasonably smart (INTENDED to prevent the car from starting IF appropriate safety pre-conditions weren't met). What makes you think that disconnecting the seat weight sensor and belt employed sensor (under the seat) is going to make the interlock system 'happy' and allow the engine to start?

Removing external inputs from a computer generally doesn't stop it from running...

[William C.]

Re: Seat Belt interlock problem

Actually, the system could be defeated by disconnecting the module from the system, but I don't know all the details other than the system simply went into a default mode and operated in the "uncontrolled" mode. There is an interlock relay that has to activate to allow the car to start in the 6 circuit (purple wire) I'd be lookinf for that realy, or try the old battery feed to the pin on the purple wire at the solenoid for starters, just to verify the solenoid did not die. If the warning lights are on. I'd be looking for a ground problem also. I found a wiring schematic for the interlock system, If you don't have a service manual, I'll copy it and post it for you. The seat switches tell the module what seatbelt switches to look at to verify the riders are belted and to ignore the others. (I thought I had forgotten all this stuff from ?? years ago!) The module also had a bypass feature that was intended to allow a mechanic to start the car from the outside. That is how the system operated if the inputs to the module were removed.

[Harmon C.]

Re: Seat Belt interlock problem

With a four speed the switch for the back up lights is on the transmission reverse linkage. The switch on the clutch needs to work to engage the starter. Is the interlock cable in place and working so the key can be removed only in reverse?

[William C.]

Re: Seat Belt interlock problem

Wiring print for MT application attached. Note that the first thing to do is to check all functions without the SBI system involved. The SBI is simply an overlay over the standard system If the relay button does not allow a start, then check out the conventional starting system first! That includes the clutch switch and all Grounds! If you can't read this print, email me and I'll send you a larger size file.