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MPI ECM code questions

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OldMagicfloat

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Discussion starter · #1 ·
I got an LED and put my 502 MPI that wasn't going over 4000 RPM in service mode and counted the flashes.
Here's what I got:
Code 14: Coolant temperature sensor(CTS) Low temp
Code 21: TPS signal voltage High
Code 23: MAT Low Temp
Just to make sure that wasn't normal on a cold engine I tested the other engine with no codes.
Now what?
Do I replace sensors? or, is there an underlying problem I should look for?
Thanks,
 
#4 ·
Take the sensors one at a time and swap with the other motor. Recheck each time to see if the same code comes up with known good sensors. If so, focus on the connectorsor a possible short to ground somewhere.

The Mercruiser Stern Drive Repair manual made by SELOC and can be found at your local West Marine or Boaters World, has a good set of troubleshooting procedures for these motors and addresses each one of your codes. Most read either bad sensor, or open ground problem assuming all connectins are clean anf flush.
 
#5 ·
First,,All of these sensors run on 5v reference signal from the pcm make sure you have that...on one of the wires to each of these sensors....If you do Start with the tps and check return voltage with your voltmeter...Should be around .5 volts at closed and 4.5 to 5 volts Wide open...Then if that checks out pull it off and slowly turn with a screwdriver from .5 to 4.5 volts and check for voltage drops if your lucky your meter will pick it up some will some wont....Next check cts. if it is below .45 volts cold replace it....let us know the findings Gotta Gooooo...

Danny
 
Discussion starter · #6 ·
Ok,
I switched out the coolant sensor ( has two wires and opposite of the guage sensor on the hose neck) and the MAT sensor on the back of the manifold with the good engine.
I still got codes.
I then checked my ground wires. The two small ones coming out of the loom are grounded to the back of the block. The two larger wires along with the battery cable are ground to the block and in good condition also.
I then toned out the grounds at TPS and the CTS and got tone.
Anybody got any other ideas?
Is there a way to clear the codes without a scan tool and see if they will come back?
 
Discussion starter · #10 ·
You know;
After that last reply. My buddy remined me of something;
When we fired it up after repairing the bent valve. The MAT sensor was unplugged when we fired it up and he plugged it back in when it was running.
And we tried adjusting the idle with the allen head screw in the front. (didn't know there was no adjustment until after we tried)
Could that cause the MAT and the TPS to signal codes?
If so, where should the allen head screw be set?
 
Discussion starter · #14 · (Edited)
This time I tried switching the TPS with the other engine.
Still got codes.
Here's what I got for Volts checking the plugs with the key on, not running.
CTS: 5.05 volts
MAT: 5.05 volts
TPS:
Grey & Black: 5.06 volts
Blue & Black: .008 volts
Blue & Grey: 4.96 volts
Dan,
Was that .5 volts on which two wires?
And was that key on only?
 
#15 ·
Ok All these should be (KOEO) key on engine off, These can not be checked while unplugged...On the cts while plugged in koeo touch one lead of your voltmeter to ground and stab the other into one wire of the plug going into the cts either one....one of the wires should read around 5v This is reference voltage (Think of it as signal from the pcm) The other is return voltage to the pcm. this is the voltage that the pcm/ecm uses to determine temp....Think of it as .4 to .5 being ambient temp and 5v being around 300 degrees If the pcm sees less than around .4 to .5 volts it sees it as out of range and sets a code saying something is wrong...same with Mat sensor...(Caution it is not good to put a Voltmeter across both leads of the plug of any sensor going to the pcm...because to do so you use the signal wire going to the pcm as ground, not terrible but not good either)....The Tps is similar but not the same....Think of it as a variable resistor .45v being Koeo idle or closed plate and 5v being wot....on a scanner it would show both volts and percent open.... test this the same way plugged in check voltage by stabbing wires (not the best way but without a scanner the only Way) One wire will show reference voltage 5v one should show no voltage and the other should being signal voltage back to the pcm and should vary up and down with how far the throttle is open...Set your idle stop screw to show .45v to .5 volts at closed (Koeo)Thats a good start...
 
Discussion starter · #18 ·
OK,
I'm a little confused.
I'm trying to find out which sensor is bad if any to avoid buying all new ones.
If I've switched out the sensors that were showing codes with good sensors and I still get codes.
Wouldn't it be a problem other than the sensors?
Or, would the codes remain even after I replaced the sensors with good ones?
 
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