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Battery Electric Vehicle (BEV) Cab Heater Diagnostics

NOTE: Cab heating function will only be active when the ambient temperature is below 15 °C (59

°F). If the air distribution knob is set to defrost and/or the AC button is on, the cab heating function
will only be active when the ambient temperature is below 20 °C (68 °F).

Diagnostic Steps:
1. Has the Cab Heater update been performed?
a. If No, please perform the update for improved cab heating.
b. If yes, continue to step2
2. Is the coolant system full and system been bled?
a. If No, fill coolant and perform bleeding procedures.
b. If Yes, continue to step 3.
3. Do you have any heat out of the vents when ambient temperature is below 59f/68f. (See note
above)
a. No Heat- See step 4
b. Some heat- See step 5
4. If no heat, please check the power to component M122, which is at the bottom of the coolant
piping.

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This is the Cab Heater Coolant Pump. It draws fluid through the cab heaters and pumps it into the
cab. Unplug the connector and check for power and ground on the harness side. You should have
24v and a good ground when the heater system is active.
a. No 24v- See step 6
b. No Ground- See step 7
c. 24v and Ground OK- See step 5
5. Checking the heaters - The truck is equipped with 3 heaters in the front for Cab Heat. Each
heater draws around 75 amps and has a temperature sensor mounted into the housing.
a. Check heater temperatures with Tech Tool Function 1700-08-03-06.

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You should see temperatures more than 140 degrees F and all 3 heaters should have similar
temperatures.
Note - the accuracy of these temperatures requires the pump to be functional.
No coolant flow will show lower temperatures.
i. If They are not equal, check the relay and amperage draw for whichever unit is not.
See step 4b for amperage draw test.
b. Check amperage to each heater. This can be done at the bundle where the 3 heater
cables are together. You will be looking for approximately 75 amps per wire, which would be
~225amps if measuring all three at once.
i. If amperage is equal but temperature is not, please replace the heater with low

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temps.

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ii. If amperage is low or zero on a heater, check its supply voltage at the relay and
make sure the relay is activating.
1. Relay inop - replace
2. Relay working but no 24 volts to the relay, inspect the wiring back to the
batteries.
6. No 24v to M122- first we need to check the fuse for being in good condition and having 24v.
The fuse is located in the ECEC on the passenger side of the engine bay(behind traction cables).

Fuse labeled CHPU in section PF4(10amp) and should have 24v to both side.
a. Fuse blown - Replace with 15a fuse and perform continuity checks from fuse to pump.
You want to see close to 0 ohms on the wire. Also check for a short to ground in the wiring. If
tests are good, retest the heater functionality
b. No 24v to fuse- Check relay K4 in the ECEC for powers and ground. You should have 24v
to two pins and ground to one when heater active. Then the output pin to the fuse.
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i. No 24v at relay - check for 24v supply to the ECEC. If ok, replace ECEC

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ii.No Ground- This is fed by the EVCM. Check for continuity to pin B:53 at the
EVCM. If no continuity, check wiring back to the EVCM for any faults. Repair or
replace as needed. If continuity is ok, see Step 7
c. Fuse good- Check continuity from fuse to pump for open circuits. The harness comes out
of the rear of the ECEC and then goes forward to 2 round connectors. Connector
MPBCOOL1 contains the 24v(Pin 7) and ground(Pin 8) wire for the M122.

Visually inspect those connectors for damaged or pushed pins. Test for continuity back to
the fuse and from the connector to the pump.
i. No continuity to fuse- Check continuity from ECEC connector to MPBCOOL1
connector.
1. Good continuity- inspect both sides of the connection and replace
ECEC if no issues are seen.

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2. NO or Poor continuity- check harness for faults and repair/replace as
needed.

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7. No Ground to Pump - As mentioned in step 6c, the ground wire comes from the pump and goes
through the MPBCOOL1 connector pin 8.
a. Check pin 8 at MPBCOOL for any issues.
i. If OK, continue to step 7b
ii. If Not Ok, repair or replace as needed.
b. Check continuity from Pin 8 to pump.
i. If OK, continue to step 7c
ii. If Not OK, inspect the harness for any issues. Repair or replace as needed
c.Check continuity from pin 8 to B6 at the Low Voltage Grounds
i. If Ok, confirm that the grounds to the Low Voltage Grounds rail are ok
ii. If Not OK, check the harness for issues. Repair or replace as needed.

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Tags

bev

heater

cab heater

mack

volvo

Categories
Make and Model > Mack > LR Electric
Make and Model > Volvo > VNR Electric
Vehicle System > HVAC

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Related links and attachments
VNe Cabin heater update

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