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Load or no-load? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 9 Jul 2001 13:14:25
In Reply to: low voltage to APC solenoid???, denis, Mon, 9 Jul 2001 12:15:28 Members do not see ads below this line. - Help Keep This Site Online - Signup |
When you took the measurements, did you do it with the APC solenoid still attached (loaded) or with the solenoid not in the circuit (no-load)? Did you measure between the pins on the solenoid connector, or from the pins to some point on the chassis for ground?
With the connector disconnected (no load) from the solenoid, you should get around 12 volts or so with the engine just idling. That should drop to around zero with the cruise control enaged, and the same with the brake pedal pressed. But that depends on what year - before around 86 (earlier?) that interlock wasn't in there. But if you have an 86 or newer, the solenoid voltage should be zero with CC engaged (not just on), or brake lights on. Make this measurement between the pins on the connector.
If you don't get 12 volts or so like this (no solenoid connected), measure between the 'live' pin on the connector and a known good ground. If you get 12 volts, then start checking the ground pin on the connector. Less than 12 volts? Then turn the car off, then turn it to ON but don't start it. Do you get 12 volts now?
If you get 12 volts with the car ON but not running, but less than 12 with the car running, then you know the APC is switching the APC solenoid on and off quickly - it's a proportioning valve. That's where the year of the car comes in. Newer cars pulsed the APC valve to control it - so you'd get an apparent lower voltage. What's the difference between ON and not running, and engine running? Well, other than the obvious, with the system just ON, nothing is moving - no noise, no vibration, no electrical noise. With the engine running, things are moving, sparks are jumping. The APC may be detecting knock, or THINK it's detecting knock, and pulsing the APC solenoid to reduce boost. A loose knock sensor or connector, or really loud lifters can cause the knock sensor to think the engine is knocking, even at idle. Worn spark plug wires or a bad APC box ground can induce electrical noise, making the APC think the engine is knocking.
However, if the connector shows 12 volts with it disconnected from the solenoid, but it drops down to 7 volts when you connect it to the solenoid, then there is a voltage drop somewhere. It may be a bad or dirty connector, or it could be the ground for the solenoid. With the connector connected (?!), measure from the 'hot' pin to a known good ground - if it's less than 12 volts, there is a bad connection between the high side and the APC box. Measure voltage from the low side to a known good. If the low on the solenoid is more than about 0.1 volts above ground, then clean up the ground wire for the APC solenoid.
The switch for the APC solenoid inside the APC box is a transistor. Transistors usually fail open or closed, and rarely fail 'part way'. There could be a problem in the circuit driving the transistor so it isn't saturated (fully on), but unlikely.
So - check you connections, and measure everything. Also, depending on the year of your car, the solenoid may be pulsed. A bunch of fast pulses will look like a lower voltage to a meter.
Good luck!
, Mon, 9 Jul 2001 13:14:25 <-- Viewing This Message, Mon, 9 Jul 2001 14:26:09
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