|
The Haynes 940 manual I just got for my new acquisition shows that the exciter wire from the altenator pulls current through the 4 bulbs you listed:
• Parking Brake
• Brake Warning
• Bulb Failure Sensor
• Washer Fluid Level
plus:
• the Service warning light
It doesn't appear to drive the Overdrive indicator light. (It does additionally show a Turbo pressure warning light and another unlabelled light, but these are indicated as Not Used in my owners manual).
In addition to this, the following lights should also come on with the key ON and the engine not running, driven by other mechanisms:
• Check Engine
• Oil pressure warning
• ABS
• Seat belt warning
• Supplemental Restraint System
(The Seat belt warning and Supplemental Restraint System lights both go out after about 10 seconds).
Based on the fact that Herman is always losing all his "startup" lights, it's unlikely it's an exciter circuit problem. I'd like to suggest a totally different theory. Maybe instead of a break in the middle of the 12 volt line on the instrument panel printed circuit board, the problem is that the instrument panel is losing this 12 volt line altogether. (This doesn't meant there won't be illumination; that's fed from a different wire). If the 12 volts to all the warning lights is missing, and the Overdrive relay is energised (as it should be), the 12 volts from the relay would drive backwards through the Overdrive indicator bulb. It could then see ground by passing through the 5 "exicter-actuated" warning lights, their diodes, and on through the wire back to the alternator. These 5 bulbs may not be noticed as being illuminated, as they would only have about 2 volts across them.
One might argue that this current flow would be enough to kick-start the alternator, but I think that the voltage divider of the single Overdrive bulb, in series with a group of 5 additional parallel-connected bulbs, would mean that the voltage at the intermittently open-circuit 12 volt rail in the instrument panel would only be about 2 volts. This may not be enough to start the alternator charging.
If all this makes sense, it might explain why the Overdrive indicator goes out after a while - when the instrument panel 12 volts kicks in (ie. the intermittent contact feeding 12 volts to the panel starts working).
Answers to some questions may void this theory:
- do the fuel and temperature gauges work when the Overdrive light is on?
- does Overdrive seem to work correctly at all times?
- when the Overdrive light is on, does toggling the button on the shift knob reliably turn the light off?
If the theory does hold, the problem could be within the instrument panel itself, or in the wiring feeding 12 volts to the panel (it enters on connector 3, pin 4). By testing the voltage there when the problem exists, you'd be able to eliminate one of the two.
If the theory doesn't hold, and overdrive functionality does seem sporadic, probably the cheapest first step would be to replace the relay, in order to remove this factor from the mix.
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
|
