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Basic Manifold Heater System Info Tests: Fusible Link 12 Valve 24 Valve Heater and relay Replacement
Intake Manifold Heater Wiring and Control
The intake manifold air heater element assembly is located in the top of the
The air heater elements are used to heat incoming air to the intake manifold. This is done to
help engine starting and improve driveability with cool or cold outside temperatures. Electrical
supply for the 2 air heater elements is controlled by the Engine Control Module (ECM)
through the 2 air heater relays.
Two heavy-duty cables connect the 2 air heater elements to the 2 air heater
of these cables will supply approximately 95 amps at 12 volts to an individual heating element
within the heater block assembly.
The ECM uses the signal from the intake manifold air temperature sensor to
determine heater cycling.
At temperatures below 60° F, the intake manifold heater will run for 10 seconds (longer at lower
temperatures) while the "Wait-to-Start" light is illuminated. When the heater has finished, the "Wait
to Start light" and heater will go off. The heater will continue cycle on and off, after the engine is
started, until the intake manifold reaches 60° or the truck reaches about 15 mph.
When the engine is running and cold, it is normal to see the voltmeter fluctuate
up and down (the
headlights will also dim) as the heaters cycle on and off. At around 15mph, the heater cycle is
disabled. If a relay sticks on while the engine is running, the voltmeter will slowly drop below 9V
and eventually the "Check Gauges" light and chime will activate. If a relay sticks when the engine
is off, the heater will quickly drain the battery, and the heater may get hot enough to start a fire.
12 Valve Ram Cummins Intake Manifold Heater
24 Valve Ram Cummins Intake Manifold Heater Parts
1. Air Heater Elements
1994 - 2002 Ram Manifold Heater Relay Parts / Location
1 - Battery & Tray - Driver's side
Manifold Heater Fuse Link Diagnosis and Testing
+12 V from the battery terminal passes through two
If the fuse links are good, both relays should have
Manifold Heater Troubleshooting 1994 - 1998 12 Valve Models
The heater relays are controlled by the PCM, with the PCM supplying ground
through the black
wires for the manifold air heater relay coils when the heaters are required. The gray wires w/
stripe receive +12V through fuse #9 (10A), in the dashboard, when the ignition key is on.
The ground path is provided by the PCM if intake manifold air temperature is
less than 60°F
(15°C) when the ignition key is turned to the ON position. When the ground is provided, the
air heater is energized to start the preheat cycle. The preheat-cycle can be tested with a
voltmeter or test light. If the intake manifold air temperature is above 15°C (59°F), the
wait-to-start warning lamp will not illuminate and the air heater will not be energized.
Note that the warning lamp will illuminate as a "bulb check" for approximately two seconds
each time the key is turned on.
The service manual says:
"CAUTION: When testing, do not energize the air heater relays more than once per 15 minutes
without running and operating the engine. Due to the high amperage needed to operate the heater,
the batteries may not have enough time to recharge. Wait 15 minutes before turning the key
back to the ON position."
In reality, you don't have to wait that long; a couple of minutes is plenty if the batteries are in
(1) With the engine not running and ambient air temperature below 16°C
(60°F), turn the
ignition key to the ON position.
(2) The wait-to-start warning lamp will come on and the air heater relays should click ON
signaling the start of the preheat cycle.
(3) Check for battery voltage at both air heater terminals. The heater will only be energized
for 10 to 20 seconds. See the Preheat Cycle Time Interval Chart below.
|Intake Temp||Heater Cycle time||Heat after Engine is Running|
|Above 59°F, 15°C||None||No|
|15°F to 59°F
-10°C to 15°C
|0°F to 15°F
-18°C to -10°C
|Below 0°F, -18°C||30 Seconds||Yes|
Intake Manifold Heater Test
(1) Disconnect both negative battery cables at both batteries.
(2) Lift the rubber shields from each of the cable connectors at the intake manifold
air heater to expose the cable terminals. Do not disconnect cable nuts.
(3) Use an ohmmeter to test the resistance between the cable terminal (not the
mounting stud) and a ground. The resistance should be zero (0). If not, inspect
for corroded or dirty cable connections. Clean or repair the connections and
retest before replacing heater. Also check both ends of the ground cable at the
rear of the heater for corroded or dirty connections. If resistance is now anything
other than zero (0), proceed to next step.
(4) Disconnect both cables from the intake manifold heater (two nuts)
(5) Measure the resistance from each of the air heater terminal threaded studs to a
ground. The resistance should be zero (0). If the ohmmeter is still reading anything
other than zero (0), replace the intake manifold air heater.
(6) After testing and repairing, connect battery cables to both batteries.
Intake Manifold Heater Relay Test
(1) Disconnect both negative battery cables at both batteries.
(2) Disconnect the four small relay trigger wires at both relays (Fig. 26). Note position
of wires before removal.
(3) Disconnect the four large cables at each of the relay terminals (four nuts).
Note position of cables before removal.
(4) Attach an ohmmeter across two of the large studs on one of the relays.
(5) Attach a jumper wire (+ and -) to each of the small terminals on one of the relays.
Polarity is not important.
(6) Momentarily touch the jumper wires to the vehicle battery (+ and -). The relay should
click and the ohmmeter should show a closed circuit across the large terminals. If not,
replace relay and bracket assembly.
(7) Repeat the same test on the opposite relay.
(8) After testing and repairing, connect battery cables to both batteries.
Intake Manifold Air Temperature Sensor Test
(1) Disconnect the wire harness connector from the intake manifold air temperature
The sensor is located on the top of the intake manifold and to the rear of the air heater.
(2) Test the resistance of the sensor with an input impedance (digital) volt-ohmmeter. The
resistance (as measured across the sensor terminals) should be less than 1340 ohms
with the engine warm. Refer to the Sensor Resistance (OHMS)—Intake Manifold Air
Temperature resistance chart. Replace the sensor if it is not within the range of resistance
specified in the chart.
(3) Test the resistance of the wire harness. Do this between the powertrain control module
(PCM) wire harness connector terminal 21 and the sensor connector terminal. Also
check between terminal 4 to the sensor connector terminal. Repair the wire harness
as necessary if the resistance is greater than 1 ohm.
IAT Sensor Location
Manifold Heater Troubleshooting 1998 - 2000 24 Valve Models
I don't have this. They should be the same as the 2001 models. The relay wiring
should tell you whether you need to use the 1994 - 1998 version or the 2001 - 2002 version.
Manifold Heater Troubleshooting 2001 - 2002 24 Valve Models
The heater relays are controlled by the ECM, with the ECM supplying +12V through
w/ stripe and Yellow w/ stripe wires to energize the relay coils when the heaters are required.
The black coil wires are connected to chassis ground.
The factory service procedures require a DRB scan tool, connected to the diagnostic
to turn the heater relays on and off. If you do not have the scan tool, try this:
(1) If the engine temperature is cold enough, the "Wait to Start"
light should some on for 10
seconds or more when the key is turned to "Run". If this does not happen, check the Intake
Air Temperature sensor.
The IAT sensor is located in the left/rear side of the intake
1 - MAP sensor
(2) Make sure that the relay power feeds from the battery are good. You should
have +12 V
on the large terminals on the fender well side of the relays. If there is no power, the fusible
link(s) in the feed wire(s) is/are blown. Replace the feed wire fusible links.
|(3) Check the voltage from the heater feed terminals to the
negative battery terminal - both feed terminals should be 0 V.
If the voltage is not 0, the heater relay is stuck and the relay
must be replaced.
Note: the heavy feed wires from the battery and to the heaters were removed for photo below for clarity.
(4) Use an ohmmeter to check the terminal connected
to the small black wires. These terminals should be
grounded. If they are not, the ground wire has broken.
The ground starts at the battery negative terminal on
the driver's side, goes to joint connector #2 in the
power distribution center (main fuse box under the
hood), then through the wiring harness to the relay
|(5) Use an ohmmeter to check the ground terminals on the
manifold heaters. The meter should show no resistance from
the ground terminals to the battery negative terminal. If there
is more than 0.1 ohm, clean the terminals and jumper wire
connections and check again.
|(6) Use an ohmmeter to check the resistance from the heater
feed terminals to ground. The resistance should be very low
(<< 0.2 ohm). Higher resistance may be due to terminal
corrosion - clean terminals and connections, then recheck. If the
resistance is still high, the heating element should be replaced.
|(7) If the temperature is cold enough for the "Wait-to-Start"
to come on for a heat cycle, disconnect the relay power feed
from the battery.
|(8) Use a voltmeter to check for 12 V on the Orange and
Yellow wires to the relay coil. Each should read 0V. Turn
the ignition key on and, while the "Wait-to-Start" light is on,
check again for +12 V. At least one of the relays should
have power, which should then drop to 0 V when the wait
to start light goes out. If this does not happen, there is a
problem with the ECM, the ECM connector, or with the
wires going to the ECM.
|(9) If the above tests are OK, make sure that the key is off and
reconnect the heater relay battery feed to the battery terminal. Turn
the ignition key on and, while the "Wait to Start" light is on, check
for +12 V on the manifold heaters. The heaters should have power,
which should then drop to 0 V when the wait to start light goes out.
If the heaters do not receive power, the relays are bad. The
replacement unit comes as a pair of relays mounted on one bracket.
If you have the correct voltages on the manifold heaters, the heater housing
should get hot. If it does not, the heater
element has blown and must be replaced.
Last edit to this page: March 7, 2008