86 Max and Error code 21
11-18-2009, 12:46 AM
#1
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Join Date: Sep 2007
Posts: 21
86 Max and Error code 21
I gave this car to my brother and now it's stuck in my driveway and won't start. I've been looking over the wiring diagram in the chiltons manual I have, and I'm not quite sure I'm understanding the power transistor/coil circuit.
As things stand, the plugs aren't firing. At least, I put a spark tester on one and it isn't lighting up.. (Occasionally the car will start and run for a bit and the tester lights up at that point.) Also tried grabbing a coil/transistor from the yard, but it doesn't change the situation. (These don't fail that often, do they?)
I'm trying to understand the wiring for the transistor, but I'm not sure I'm grasping it. The diagram shows one leg connected to ground, one going to pin 5 of the computer, and another going to the coil/pin 3 of the computer. Does the transistor control the ground side of the circuit for the coil?
One more bit.. When I went to pull the error codes from the computer, something happened that seems odd. Normally, the two leds are flashing in tandem (one flash, two flashes, etc) but between modes two and three (I believe that's where I saw it) there was a single green blink. I don't recall having run across this in my '88. Is that normal?
Any ideas? I'd really like to get this thing going and get my brother off my back about the car.
As things stand, the plugs aren't firing. At least, I put a spark tester on one and it isn't lighting up.. (Occasionally the car will start and run for a bit and the tester lights up at that point.) Also tried grabbing a coil/transistor from the yard, but it doesn't change the situation. (These don't fail that often, do they?)
I'm trying to understand the wiring for the transistor, but I'm not sure I'm grasping it. The diagram shows one leg connected to ground, one going to pin 5 of the computer, and another going to the coil/pin 3 of the computer. Does the transistor control the ground side of the circuit for the coil?
One more bit.. When I went to pull the error codes from the computer, something happened that seems odd. Normally, the two leds are flashing in tandem (one flash, two flashes, etc) but between modes two and three (I believe that's where I saw it) there was a single green blink. I don't recall having run across this in my '88. Is that normal?
Any ideas? I'd really like to get this thing going and get my brother off my back about the car.
12-01-2009, 09:48 PM
#3
Member
Join Date: May 2002
Posts: 177
I had code 21 and it turned out to be a bad power transistor. The the car still ran, but had poor MPG and down on power. You'll probably want to make sure the PT is bad before you replace it since it is $100.
You are doing a spark jump test so that is good place to start, but sounds like your spark is intermittent. Try shaking the MAF connector and see if the spark or Code 21 changes. Another area to check is the crankshaft position sensor in the distributor. Sometimes the oring goes bad and they fill with oil or the sensor can go bad. Check all grounds at the engine and make sure the coil bracket is making a good ground to the body. Check the harness from the ECU to the PT and crank sensor.
There are errors in the Factory service manual on pages EF-EC 60 in diagram for the power
transistor shown in the box diagrams showing views A and B. The Top view of the overall circuit is correct. Nissan adds further confusion with Terminal Side and Harness Side Views of the connectors for probe points. I my opinion all connector views should be terminal side and note whether it is component or harness end.
View A of the connector in the Top view shows the Terminal side while View A in the lower diagram box incorrectly shows the harness side. The View A lower box diagram of the Power Transistor connector should be the Terminal Side (looking in the connector) on the connector pins as shown below.
4 1
3 2
For View B refer to the top schematic diagram and you will notice View B Box below incorrectly shows circuit traces for pins 3 and 4 swapped. Pin 3 should connect to the Emitter of the transistor and Pin 4 the junction of primary coil and the transistor Collector. Copy the top diagram.
The power transistor connects to the negative side of the primary coil to ground. It switches the current on/off and also increases current through the primary circuit of the coil and is controlled by the ECU pin 5 (connector pin 2) at the transistor Base of the PT. The signal is sent back on Pin 3 (connector pin 1) at the + of the primary coil for confirmation (reference) by the ECU. Once you get this set you can run through the continuity test procedure. If you have a frequency counter on your DVM you can check the output of the ECM.
To further test the PT: unplug it from the coil and connect your DVM across the leads and test for voltage. When you crank the engine you should see 12 volts. Mine read 8V, so it failed this test.
Make sure you pull your fuel pump fuse so you don't flood/hydolock the engine.
You can check the coil impedance easily according the manual, but you should also do a spark jump test since the problem may not show up unless under load. There should be strong blue spark. A weak spark can mean either a bad coil, PT or crank angle sensor, so it's best to start here and work back.
The ECU will do one flash between mode changes IIRC.
Good Luck.
You are doing a spark jump test so that is good place to start, but sounds like your spark is intermittent. Try shaking the MAF connector and see if the spark or Code 21 changes. Another area to check is the crankshaft position sensor in the distributor. Sometimes the oring goes bad and they fill with oil or the sensor can go bad. Check all grounds at the engine and make sure the coil bracket is making a good ground to the body. Check the harness from the ECU to the PT and crank sensor.
There are errors in the Factory service manual on pages EF-EC 60 in diagram for the power
transistor shown in the box diagrams showing views A and B. The Top view of the overall circuit is correct. Nissan adds further confusion with Terminal Side and Harness Side Views of the connectors for probe points. I my opinion all connector views should be terminal side and note whether it is component or harness end.
View A of the connector in the Top view shows the Terminal side while View A in the lower diagram box incorrectly shows the harness side. The View A lower box diagram of the Power Transistor connector should be the Terminal Side (looking in the connector) on the connector pins as shown below.
4 1
3 2
For View B refer to the top schematic diagram and you will notice View B Box below incorrectly shows circuit traces for pins 3 and 4 swapped. Pin 3 should connect to the Emitter of the transistor and Pin 4 the junction of primary coil and the transistor Collector. Copy the top diagram.
The power transistor connects to the negative side of the primary coil to ground. It switches the current on/off and also increases current through the primary circuit of the coil and is controlled by the ECU pin 5 (connector pin 2) at the transistor Base of the PT. The signal is sent back on Pin 3 (connector pin 1) at the + of the primary coil for confirmation (reference) by the ECU. Once you get this set you can run through the continuity test procedure. If you have a frequency counter on your DVM you can check the output of the ECM.
To further test the PT: unplug it from the coil and connect your DVM across the leads and test for voltage. When you crank the engine you should see 12 volts. Mine read 8V, so it failed this test.
Make sure you pull your fuel pump fuse so you don't flood/hydolock the engine.
You can check the coil impedance easily according the manual, but you should also do a spark jump test since the problem may not show up unless under load. There should be strong blue spark. A weak spark can mean either a bad coil, PT or crank angle sensor, so it's best to start here and work back.
The ECU will do one flash between mode changes IIRC.
Good Luck.
Last edited by Edge; 12-02-2009 at 07:41 AM.
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