1989 TBI 350 5.7 Grumman Kurbmaster electrical issue runs rough, PULLING MY HAIR OUT

[JW454NOVA]

Working on a customers 1989 Grumman Kurbmaster with a 5.7 350 TBI. Came into the shop running rough and dying. Pulled up scanner data and found CTS showing -40 KOEO. CTS comes up to maybe 30-40 degrees when the truck is running and RPM'S are up. Goes slowly back to -40 and then dies at idle. TPS voltage does not change whatsoever (2.56 volts). Battery voltage showing 1.7 volts through scanner. Using the snap on brick. Truck keeps flooding out due to funky sensor readings. Unplugging the CTS sensor doesn't change anything.

Component's replaced and repaired ~
- Coolant temp sensor and pigtail replaced
- Thermostat ground eyelet replaced
- Ground cable (back of head to frame) replaced
- ECM and PROM chip replaced
- TPS replaced
- Fuses are good
- Checked wiring harness from sensors to ECM

New temp sensor ohms out at 1700 ohms. What am I missing? Is there another ground for the ECM somewhere? Truck is charging at 13.6 volts. Is there any possibility of a bad alternator diode causing the sensors to act up? Any input would be much appreciated. Thanks.

 

[GoToGuy]

Gauge coolant temp sensor one wire in left head , ecm coolant temp sensor two wire near thermostat.

 

[evilunclegrimace]

Gauge coolant temp sensor one wire in left head , ecm coolant temp sensor two wire near thermostat.

Coolant temp SENDER in the left head will not contribute to driveabilty issues

 

[GoToGuy]

Agreed. I was just looking for clarification. Not diagnosis.

 

[scott2093]

Truck is charging at 13.6 volts

Is this reading directly at the alternator output?
All fusible links look good?

 

[JW454NOVA]

Coolant temp SENDER in the left head will not contribute to driveabilty issues

I meant the Engine Coolant Temperature sensor. Two wire plug next to the thermostat. Replaced along with pigtail harness.

 

[Schurkey]

If the sensor has appropriate resistance with temperature, and the computer thinks it's -40, you clearly have a wiring problem between sensor and ECM.

It'd be worth checking the TPS as well. Either the TPS is toast, or the wiring is, since the voltage doesn't change as you open the throttle.

Given that the battery voltage is in the toilet via the scan tool, it seems like the wire harness may be a three-way fail; and who knows what the other sensors are showing.

Battery charging at 13.6 seems low. I'd have expected 14.2 or perhaps a bit higher. What is the battery voltage with the engine off and no load? Is the battery so depleted that it's sabotaging the rest of the electrical system?

 

[JW454NOVA]

If the sensor has appropriate resistance with temperature, and the computer thinks it's -40, you clearly have a wiring problem between sensor and ECM.

It'd be worth checking the TPS as well. Either the TPS is toast, or the wiring is, since the voltage doesn't change as you open the throttle.

Given that the battery voltage is in the toilet via the scan tool, it seems like the wire harness may be a three-way fail; and who knows what the other sensors are showing.

Battery charging at 13.6 seems low. I'd have expected 14.2 or perhaps a bit higher. What is the battery voltage with the engine off and no load? Is the battery so depleted that it's sabotaging the rest of the electrical system?

Voltage at Battery is 12.3 volts. Battery load tested fine. I put a battery charger on the battery while looking at data and the scanner data went to 0.00 Battery Volts.... WTF. Swapped back old ECM and then the scanner showed 24 volts KOEO. Re-installed new ECM then it showed 14 volts (KOEO). The voltage is all over the place. Im going to rename this rig "Christine".

Found bare wiring at one of the fusible links down below starter. I will investigate that today. Not sure if that would cause the issue, but needs to be addressed considering I jiggled the those wires and it started sparking when it touched the transmission.

 

[scott2093]

Voltage at Battery is 12.3 volts

That's over 25% discharged iirc..

 

[JW454NOVA]

That's over 25% discharged iirc..

Threw in temporary new battery, still no change. Repaired corroded positive cable end.