From: Wally Plumley [wplumley@bellsouth.net] Sent: Sunday, January 13, 2002 9:14 AM To: 928 Subject: [928] Re: 1983 928S At 01:12 AM 1/13/02, you wrote: >Having the Central Warning Light and Brake pressure light come on, brakes >are all a-ok. Didn't come on for 2 days after battery was disconnected. >Any ideas, remedies? Perhaps a short tutorial on the warning system would help. Your car has two independent hydraulic brake circuits. You have two brake light switches, one in each circuit. Each switch is a double-throw switch - That is, there is one set of contacts that are made with the switch in the OFF position, and open in the ON position; and another set of contacts that are open in the OFF position, and made in the ON position. When you step on the brakes: If the hydraulic pressure is equal in the two systems, both switches move at the same time, breaking the NC (Normally Closed) contacts, and making the NO (Normally Open) contacts, disconnecting the warning system and connecting the brake lights to 12 vdc. If the hydraulic pressure is not equal in the two circuits, one switch will remain open for an instant, while the other will close. The closed switch will connect the brake lights to 12 vdc AND at the same time, the warning system will be connected to 12 vdc thru the NC contacts in the switch that remained open. This sets the warning system. Some possibilities: Hydraulic pressure is not quite equal in the two circuits, perhaps due to a sticky caliper, worn seal in the master cylinder, low brake fluid, etc. One of the brake light switches is faulty. If you replace one brake light switch, I strongly suggest that you replace both. One old switch and one new switch usually causes brake warnings. Wally Plumley 928 Specialists From: Wally Plumley [wplumley@bellsouth.net] Sent: Wednesday, January 23, 2002 7:11 AM To: 928 Subject: [928] Re: Urgent light flashing At 11:06 PM 1/22/02, James Wehl wrote: >My auto electrician needs to know what the flashing "Brake Press" lamp >means. >(Bottom right of dash cluster) >The problem is that this lamp never worked before as we just redid the whole >instrument cluster. > >It is a 79 4.5L 928 > >Also the "Stop lamp" light stays on every time I start the car until I push >the brake down really hard >(Top left of dash cluster) >But the Brakes are supper strong and work very well! Your problems stem from the same source. There are two brake light switches on the earlier cars, one in each of the split brake circuits. These brake light switches are single-pole, double-throw, and are mounted in parallel. If either switch works, you will get brake lights. If both switches are activated, and both work, the contact to the blue/brown wire to the warning computer is disconnected in both switches, so no signal goes to the warning computer. If there is brake pressure in only one side of the master cylinder, only one switch will be activated. The other switch will remain connected to the blue/brown wire to the warning computer. Voltage will be fed back from the brake light circuit to the warning computer, setting the warning. While it is possible that you actually have hydraulic pressure problems, it is more likely that you have brake light switch problems - but you need to find out for sure! One check is to jack the car, and spin each wheel by hand as someone gently applies the brakes - you should have even braking on each wheel. If you don't, you need to find out why. If you have even braking, it is probably a good bet to replace BOTH brake light switches, and carefully flush the brake system (and clutch, if it is a five-speed). There have been reports of wiring/connector problems at the brake light switches, so it might be wise to check this first - free fixes are always nice. Wally Plumley 928 Specialists From: Wally Plumley [wplumley@bellsouth.net] Sent: Sunday, March 16, 2003 4:18 PM To: 928 Subject: [928] Brake Light Switch Tutorial I have gotten a couple of question on the brake light switch setup on the earlier cars, and thought that I would post the info here as well. Up thru 1983, the brake lights are operated by twin pressure switches on the bottom of the brake master cylinder. From 1984 - up, the brake lights are operated by a simple plunger switch mounted on the brake pedal under the dash. The earlier brake light switches do double-duty, operating both the brake lights and the brake pressure warning signal. These switches can cause some problems. The switches are identical - one in each hydraulic circuit. They are single-pole, double-throw switches. This means that you have only one circuit (single pole) thru each switch, and that the switch chooses one of two outputs (double throw). Each switch has three connections:power, central warning computer, and brake light. Not Activated There is a power feed into each switch. When the switch is not activated (no pressure in the brake circuit) the power isn't hooked to anything. When the switch is not activated, the central warning computer and the brake lights are connected inside the switch. Nothing happens, since there is no power applied on either circuit. Activated When the switch is activated by brake hydraulic pressure in one of the circuits, the central warning computer connection is broken, and the brake lights are hooked to the power feed. This removes the central warning computer from the circuit, and puts power on the brake lights. (This might be easier to visualize if you think of three wires: one for 12 vdc, one to the brake lights and one to the warning computer. The lead that goes to the brake lights is connected to the one to the warning computer until brake pressure pushes it off of that wire and over to the one going to power.) All three leads go to both switches. When you apply the brakes, both switches should activate. Both switches disconnect the warning computer and connect the power to the brake lights. The brake lights come on, and no signal is sent to the warning computer, so it is happy. If only one switch activates (either no pressure on one side of the brakes, or a bad switch), then things change. On the switch that is not activated, the brake light circuit is still hooked to the warning computer. On the switch that activates, the brake light circuit is hooked to 12 vdc. The brake lights come on. In addition, power flows on the brake light circuit back to the non-activated switch. Since in this switch the brake light circuit and warning computer are hooked together, 12 vdc is applied to the warning computer circuit. This set the Brake Pressure Warning light. If you have a bad brake light switch, I strongly suggest that you replace both switches. (P/N 113 945 515 G, $23.60 each plus shipping) Replacement of the switches requires bleeding the brake system, so this is a good time to flush the brake and clutch systems. Wally Plumley 928 Specialists