Starter relay coil PCM ground

RIP · 09-28-2008, 02:53 AM

In order for the starter relay to pass voltage to the starter, the coil side of the relay has to get a ground from the PCM at contact 85. My question is:

What signal is the PCM looking for to allow the ground to be turned on? I'm thinking it's either the security system start immobilze system (if equipped) or something from the Transaxle Range Sensor (TRS) saying the gear shift is in park or neutral. Just can't find a schematic that shows it. I'm not thinking it's the cam/crank sensor ASD circuit. Anyone have no doubts about the answer? This question pops up in no start threads from time to time. I haven't seen a clear answer.

drum4Him · 11-10-2008, 12:28 PM

I'm at the same point in my troubleshooting my no start problem, so was wondering the same thing.

In the mean time, I was going to try and hard wire ground pin 85 of the relay to the chassis (battery) ground. I have not looked at the signal yet with a DVM between 86 and 85. 85 wither just floats until the PCM sends out a ground signal or maybe it is normally 12 volts.

I'm looking at a chiltons schematic. It does not show where the ignition switch is in the schematic. ?

Rich

Airjer_ · 11-10-2008, 01:52 PM

Year and engine?

drum4Him · 11-10-2008, 11:50 PM

98 Grand Caravan. 3.8 L

jpb53 · 11-11-2008, 11:02 AM

The park/neutral switch when in park or neutral will send a ground signal to the PCM on pin 76. The PCM also gets an ignition switch voltage signal on pin 20 at the PCM (start position). Once the PCM gets these 2 signals and the input from the VTSS (vehicle theft security system) then the PCM will ground the starter relay through pin 8 at the PCM. ASD relay is not energized by PCM until a crank and cam signal is received during cranking.

drum4Him · 11-11-2008, 03:04 PM

jpb53,

Thank YOU for the details. That should keep me busy for awhile. My original problem was intermittent speedometer cluster (I have resoldered the connector) AND a easy to crank, but hard to keep going engine.

It slowly degraded to the current no start problem. I followed some instructions last night about doing a self diagnostic using the 2 odomenter buttons on the gauge cluster. If a valid test, that failed, because I got no display readout from the digitial odometer.

Maybe BCM is the problem? If so, I'm thingking right now BCM from a wrecking yard, with the same van configuration as mine. If so, should I pull out both the Body Controle Module and Powertrain Control Module? I read some post where they need to be matched up.

Thanks so much!

Rich

drum4Him · 11-12-2008, 12:12 AM

Looked at the starter relay pin activity, thanks to guidance from jpb53. Everything behaved as expected. Starter relay got 12 across the coil and sent 12v from pin 87 of the relay output to the starter. Pin 87 output had previously been zero volts all the time. Reconnected smaller starter wire back up and although engine would crank, it would not start. (This was my original problem) almost acting like no fuel being delivered.

As I was turning the engine over several times, I pounded on the dash, and now it did not even turn over.

Did this a couple of times and this would cause the start to sometimes work and not work. Not sure what to try next. I have already soldered the back of the speedometer connector. Maybe look again? BCM? PCM?

Again, originally could start but would not always runs. Dash gauges not working.

Rich

RIP · 11-12-2008, 04:40 AM

Quote:

Originally Posted by jpb53

The park/neutral switch when in park or neutral will send a ground signal to the PCM on pin 76. The PCM also gets an ignition switch voltage signal on pin 20 at the PCM (start position). Once the PCM gets these 2 signals and the input from the VTSS (vehicle theft security system) then the PCM will ground the starter relay through pin 8 at the PCM. ASD relay is not energized by PCM until a crank and cam signal is received during cranking.

3 things needed to enable the ground. Thanks for clearing up that mystery. You are a king!

RIP · 11-12-2008, 04:43 AM

Quote:

Originally Posted by drum4Him

Looked at the starter relay pin activity, thanks to guidance from jpb53. Everything behaved as expected. Starter relay got 12 across the coil and sent 12v from pin 87 of the relay output to the starter. Pin 87 output had previously been zero volts all the time. Reconnected smaller starter wire back up and although engine would crank, it would not start. (This was my original problem) almost acting like no fuel being delivered.

As I was turning the engine over several times, I pounded on the dash, and now it did not even turn over.

Did this a couple of times and this would cause the start to sometimes work and not work. Not sure what to try next. I have already soldered the back of the speedometer connector. Maybe look again? BCM? PCM?

Again, originally could start but would not always runs. Dash gauges not working.

Rich

Ya, not what you want to hear but, I'd look over the cluster connector again.

drum4Him · 11-16-2008, 09:25 AM

I did look at the cluster connector again and it was ok. Everything else in the cluster looked visually ok. I just removed the Body Control Module and Powertrain control module. I plan on going to the junkyard and finding repacements. I know I have to find the same configuration as my van. There are 4 digits printed in bold letters on my BCM.. 62AC. Are these the numbers to match?

Rich

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