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1998 PCM - replaced - still not fixed?

Old 04-12-2019, 06:21 PM
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1998 PCM - replaced - still not fixed?

After giving up myself (Im working out of town) and having a dealership diagnose that the PCM was shot and Id have to find an aftermarket unit, I bought a new PCM from these guys:
www.fs1inc.com
It is supposed to be plug and play. I installed it and it didn't fix any of my issues.

What doesn't work:
- gauges do not move
- air heater grids do not activate
- alternator does not charge the battery
- odometer flashes same number
- (automatic transmission) Drive position = 3rd gear; positon 1 and 2 = 2nd gear (I think)
- fuel tank empty light comes on
-ABS and Park Brake light comes on

Gauge check/display 'C' codes:
920 - cluster not receiving vehicle speed from PCM
921 - cluster not receiving distance pulse from PCM
999 - error detected
So, basically, the PCM is not talking to the cluster

The grounds tested good.
On Plug C3 pins 28 and 30 (CCD BUS(-) and CCD BUS(+) have 2.46V

The dealership thinks I might have to sync the PCM to the truck, even though the unit is supposed to be plug and go. I think they just want to charge me to try it?

Has anybody had a similar experience?

What else could be wrong?
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Old 04-13-2019, 09:30 AM
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Years ago, on my '98, I thought my PCM was bad so bought a new one and had the dealer flash it. Mine would operate in reverse and 1st gear only, no speed readings. Flash didn't fix it. I spent several hours tracing the issue and finally found that the wire from the speed sensor into the PCM had been partially pulled out of the connector. Once discovered it was a very simple fix but it cost a lot of wasted time and money first. I even bought a new '06 before finding the problem.
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Old 04-13-2019, 05:17 PM
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I managed to get my truck hooked up to a scan tool today. Wasn’t convinced it would do any good but no harm in more information.

This from the mechanic:

“Attempt to communicate with scan tool but PCM will not respond, other module responding except PCM.”

So same issue as the old PCM.

I’m strategizing my trouble shooting protocol for tomorrow; lots of wires to check. Thanks for the feedback Danavilla.
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Old 04-14-2019, 01:44 AM
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Call the supplier of your PCM and ask them- maybe they sent a dud.
If you still have your old PCM you can use it for finding faults.
I had a PCM play dead because there were 3 sensor supply lines shorted to ground. As long as this was not fixed the PCM did not boot.
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Old 04-14-2019, 07:53 PM
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I figured it out! The Engine Speed Sensor (RPM sensor) must be shorting out internally.

I started trouble shooting by unplugging each of the 5V sensors related to the PCM and seeing if fixed the problem. When I unplug the RPM Sensor, all of the gauges work except the speedometer.

The transmission isn’t working correctly but that seems to make sense as RPM’s and the transmission talk through the PCM.

Thank you everyone for pointing me in the right direction.
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Old 04-15-2019, 04:51 AM
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I hope it's the tachometer that does not work with the engine speed sensor disconnected-
The trans is controlled by the PCM, and uses Vehicle speed sensor (rear diff), engine speed and throttle position to do so.
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Old 04-16-2019, 05:04 PM
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I replaced the sensor and everything works perfect now.

To be optimistic, I now know a lot more about this truck and own a spare PCM in case mine ever fails.
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