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Old 03-18-2010, 4:20pm   #5
Y2Kvert4me
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There are a few components that can cause random no crank issues...

The starter (or solenoid attached to it)
The clutch pedal switch
The ignition switch
The Theft-Deterrent Relay


If you have a voltmeter, you can troubleshoot this, but the kicker is, you can only do it when the car doesn't start. If the car starts, then you will not find the problem, because everything obviously worked fine. For that reason, it can be difficult to isolate until the problem happens consistently.

So, where to start? Well, everything related to starter operation meets at the TDR (Theft Deterrent Relay). This relay is located just above the BCM (the big silver box) under the closeout panel in the pass. floorboard. There are 4 wires connected to this relay, and they are as follows:

Red (thick wire) - direct feed from battery. Hot at all times. This is protected by a 60a fuse, but it won't be blown since the problem is intermittent .

Purple - this is the output wire of the relay, that connects directly to your starter solenoid. Simply put, anytime this wire sees +12v, your starter motor "should" be cranking.

Then, the TDR control wires...

Yellow - this is the +12v side of the relay coil wiring. This originates at the battery, through a fuse, through the ignition switch, through the clutch pedal switch, and then ends up here.

Yellow with black stripe - this is the -12v (ground) wire to the relay coil. This wire comes from the BCM, and when no VATS or theft (alarm) intrusions are present, the BCM grounds this wire, and coupled with the +12v yellow wire, completes the circuit, energizes the relay, and energizes the purple wire to the starter.



So, in a logical method of narrowing things down, take the panel out of the floorboard, and leave your meter there, so you can quickly test things when it fails to crank. Next time the car will not crank, check things in this order.

1. Check the thick red wire. It should have +12v to a good ground at all times. (seat mounting bolt works good....the metal BCM case is NOT a good ground). You will probably find voltage there, so do this first, as it verifies you found a good ground point for the negative probe of your meter.

2. Check the purple wire in the same fashion, while clutch is depressed, and key is turned to start. If you find it has 12v, then it's looking like time for a new starter.

3. If TDR is not energizing (clicking) during an attempted start, the next thing to check is the yellow wire. It too, should be +12v to a good ground when trying to start. If it does not, then it's ignition switch, or clutch pedal switch. The easiest next step here would be to test the clutch pedal switch (just 2 wires, one in, one out, and should have continuity with pedal depressed). If that checks out, next suspect is ignition switch.

4. Finally, checking continuity to ground of the yellow/black wire. Set your meter to check continuity (ohms), and one probe on the yellow/black, the other to ground. You should find NO continuity when NOT trying to start the car. When trying to crank, the BCM should switch this wire to ground, and if it isn't doing that, then your VATS theory may be good.

5. So what if both yellow wires are doing what they should, but still no relay action? Bad relay.



Good luck, this can be frustrating to troubleshoot when it happens only occasionally.


Attached Thumbnails
starter.jpg  


Last edited by Y2Kvert4me; 03-18-2010 at 4:22pm.
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