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Mon Nov 18, 2013 7:58 am

I have been restoring a 1977 camper bus. The body and interior are complete. Now back to the motor. Unfortunately the previous owner thought he was an electrician. Ill spare the details. After several repairs I am left with positive voltage at both injector terminals, and no differential between them. Is this a common failure mode for the ECM? I thought about grounds last night after I had picked up. I had cleaned up the one under the manifold though. Any suggestions would be greatly appreciated.

Mon Nov 18, 2013 9:48 am

With the engine running, one side of the injector will have a voltage puslating between ~8V (?) and ~12V. and the other side will have a voltage pulsating between nearly zero volts and ~12V.

With the ignition on but the engine not running you will see 12V on both sides of the injectors.

Mon Nov 18, 2013 10:45 am

I understand what you are saying. That is true if you were to probe the wires with the injector plugged in. However I am just probing the disconnected end of the plug. And the motor will not run at this point.

Mon Nov 18, 2013 10:54 am

If you do not have access to a Bosch Injector noid light, if you were to put in a 12 volt test light into the electrical connector (polarity doesn't matter)and have someone pretend to start the engine, the test light should "flicker" while the starter is turning over.

Under the air plenum on the left side of the engine, you will see a 3 prong copper "FI Ground location" mounted to the top siode of the engine case there should be 2 crimp connections mounted there (one will have 2 brown wires going to a single crimp connection the other one brown wire to the crimp connector).

Are you getting any sort of dwell angle signal with your dwell meter from the points or the points replacement module? Is the long white wire hooked up from the "-" or "15" terminal on the coil to the ECU?

Mon Nov 18, 2013 6:49 pm

SarahsDad wrote: I understand what you are saying. That is true if you were to probe the wires with the injector plugged in. However I am just probing the disconnected end of the plug. And the motor will not run at this point.

The injectors are ganged together (I think in pairs), if you only unplug one injector you will see the 12V that is feeding back through the other injector in pair.

Mon Nov 18, 2013 9:41 pm

12 volts + comes from the switch thru the relays and then thru a resistor pack where there is 1 resistor for each injector. The resistor for each injector limits the current flow and protects the injector as it cannot handle 12V + without something limiting current. The 12V+ goes through the injector which does not touch ground electrically and come back out on the other wire. When the ECU wants to inject fuel it electronically grounds all four lines together (applies 12V negative) which allows current to flow opening all four injectors at the same time.

You are thinking conventional relay where you add voltage when you want the relay to work. The injector works the opposite. The ECU removes the ground when it DOESN'T want the injector to fire. 12V positive is ALWAYS there protected by the resistors when the key is on.

You would see the same if you hooked a light bulb up to the 12V + post on a battery and held the negative wire in your hand. There would be 12V positive on both sides of the light - until you touched the wire in your hand to ground then it would light up and the voltage would fall to near zero on the negative side. Once you lifted the ground wire it would go back to 12V positive.

Read this http://www.thesamba.com/vw/archives/manuals/afc_fi...Manual.pdf

Tue Nov 19, 2013 6:50 am

Yes I have an injector noid. It did not light or flash. The bulb analogy is true when all wires are connected. In my case the injector is unplugged. One of the wires there is a path to the ECU. I would think it would look like an open circuit normally, then a ground path when it fires the injector.
I will look into the signal wire and the ground more. Thank you for your thoughts. It can get a little frustrating at times. One thing that helps is I have the wiring schematic and the dual relay diagram blown up and laminated. I'm trying to get the buss moved to a heated barn, working outside is losing its appeal. I'm waiting for tires to arrive.
The bus is florescent green and white. I built all new cabinets. It has a stove, microwave, refrigerator, inverter, three solar panels on the roof. Dark green carpet, recovered seats. A father/daughter project now a father/father project. Ill post pictures soon.

Tue Nov 19, 2013 11:03 am

There's a power transistor inside the ECU which pulls the injector wires to ground. This allows battery voltage to push current through the series resistors and injectors, causing them to operate. When the transistor is 'off', you will measure 12 volts on both sides of the injectors.

The question is, why isn't the transistor operating? The ECU takes the points signal, processes and modifies it, and uses it to turn on the power transistor.

If you have a points signal at the ECU, and the ECU is properly powered and grounded (check all with a voltmeter), then you have an ECU problem. The power transistor hates being accidentally tied directly to hard +12 volts (e.g. messed up / shorted harness or injector / series resistor). As it can't pull the battery to ground, it gives up and lets out its purple smoke...

Tue Nov 19, 2013 5:01 pm

ALL FOUR injector harnesses have to be off. The four ground sides go into the ECU so if any one of the four are on you may still get 12V+ when it isn't running. It gets the trigger signal from the coil so the coil has to be hooked up and working with the trigger wire also properly attached.

You really need to read the book all the way thru and study it. Nothing back there will tolerate rogue voltages. The equipment built in the 1970s did not have circuits that operated to protect things at the speed of light.

Wed Nov 20, 2013 6:58 am

:idea: :idea: :idea: :idea: :idea:
I see now.
The diagram does not show that the injector grounds are tied together. I see how the voltage on a plugged in injector causes me to see voltage on the one I unplugged and probed. I will also check the signal end at the coil. I have not worked on her yet, perhaps this afternoon. Thanks Thanks boB

Thu Nov 21, 2013 1:40 pm

Well I'm just about tested out. The last line of one manual said: replace ECU. I checked continuity on the coil to ECU wire. Measured resistance block. The injector resistance through the ECU plug, I do have one injector that measures 16k ohms. It needs to be replaced. Not the problem though. I jumpered the air valve, unplugged all injectors, plugged the noid in, cranked the motor. No signal. Grounds are good, voltage from double relay is present. Points are good. Voltage at coil. Now when we bought this bus we knew the motor had been replaced. However I drove it home from North Carolina to New York and it seemed fine. The motor is a 1977. I'm pretty sure that the ECU Is not original ether. I found the lower retainer bolt a quarter inch loose and the upper one missing. I pulled out the box. The number on it instead of ending in a "S" ended in "AA". The California model. I don't think this is a California motor. I think from what I'm reading the ECU could care less. So I opened up the box for inspection. Two board model. No bad solder. No evidence of high current. It looks pristine. Is there anyone out there that tests/repairs them? I see an "S" model for sale $80. Might be worth a gamble. Any thoughts would be appreciated.

Thu Nov 21, 2013 2:00 pm

I posted a few pictures under SarahsBus/herDad. Enjoy

Thu Nov 21, 2013 3:45 pm

While you were probing that ECU plug did you happen to notice if the contacts were bent or loose in any way?

Thu Nov 21, 2013 5:01 pm

I just went through the same thing. Though possible it is probably not the ECU. It is really easy to test at the ECM connector but I will need to get home for the details.

At the end of testing I connected my spare distributor to ground and the coil to test the injectors electrical operation. With the key on and the coil high voltage lead grounded you can turn the distributor by hand and it will trigger the injectors every other coil trigger. (4 stroke only need fuel every 720 degrees of crank revolution per cylinder) Without anything else running you will easily hear the injectors click.

Thu Nov 21, 2013 5:14 pm

Chris_914 wrote: it will trigger the injectors every other coil trigger. (4 stroke only need fuel every 720 degrees of crank revolution per cylinder) .
How does it tell which cycle?, AFAIK L-jet shoots twice per cycle (as in every single igniton spark), each shot is 1/2 what the cylinder needs and the first one waits on the warm intake valve until it opens.

Thu Nov 21, 2013 6:06 pm

busdaddy wrote: Chris_914 wrote: it will trigger the injectors every other coil trigger. (4 stroke only need fuel every 720 degrees of crank revolution per cylinder) .
How does it tell which cycle?, AFAIK L-jet shoots twice per cycle (as in every single igniton spark), each shot is 1/2 what the cylinder needs and the first one waits on the warm intake valve until it opens.

Once every other trigger is twice per cycle. :wink:

Thu Nov 21, 2013 6:51 pm

busdaddy wrote: Chris_914 wrote: it will trigger the injectors every other coil trigger. (4 stroke only need fuel every 720 degrees of crank revolution per cylinder) .
How does it tell which cycle?, AFAIK L-jet shoots twice per cycle (as in every single igniton spark), each shot is 1/2 what the cylinder needs and the first one waits on the warm intake valve until it opens.

It doesn't know which cycle, it's just a divider circuit that halves the input. So every other input from the coil to the ECU creates a pulse for the injectors. In addition it takes input from the AFM and TS2 to further shape the duration of the pulse.

On to the ECM plug testing...
There are 35 connectors, 1-18 on the right and 19-35 on the left (lower numbers on the bottom with 1 being the lowest 18 the highest) Hook side is the top and clip or harness wire exit side is the bottom.

1 - Coil input, disconnect and check ohms end to end should be near 0
2 - unused
3 - WOT switch, check ohms with 18 and throttle wide open should be near 0
4 - 86 on double relay, 12V+ with start on. This is unused in 75 but could be a starting enrichment on later models. D-Jet uses this for sure.
5 - ground, ohms to ground should be near 0 - this is the ground for the AFM
6 - AFM 6
7 - AFM 7
8 - AFM 8
9 - AFM 9
10 - 88b on double relay, 12V+ when key is on
11 - unused
12 - unused
13 - TS2, around 2300+ ohms cold
14 - injector 1, 12V+ with key on
15 - injector 4, 12V+ with key on
16 - gound, ohms to ground should be near 0 - this is a ECU ground
17 - ground, ohms to ground should be near 0 - this is a ECU ground
18 - WOT switch, check ohms with 3 and throttle wide open should be near 0

19 - unused
20 - 86b on double relay (not connected to ECM circuit for my 75)
21 - unused
22 - unused
23 - unused
24 - unused
25 - unused
26 - unused
27 - 27 AFM (76-79) TS1 (unused 75)
28 - unused
29 - unused
30 - unused
31 - unused
32 - injector 2, 12V+ with key on
33 - injector 3, 12V+ with key on
34 - ground for AAR, ties to 16-17 ground on ECM board
35 - unused

I doesn't get any better than this for the double relay
http://www.thesamba.com/vw/forum/viewtopic.php?t=513607&highlight=double+relay

L-jet info I found useful
http://manuals.type4.org/ljet/

https://bowlsby.net/914/Classic/zTN_Man08.pdf

http://foxed.ca/rx7manual/manuals/BOSCH%20L-Jetronic%20Injection%20Manual.pdf

http://www.vaglinks.com/Docs/VW/Misc/VW_L-Jetronic_Fuel_Injection_Workshop_Manual.pdf

Thu Nov 21, 2013 7:04 pm

Here is a wonderful break down of the ECM in a 914 D-jet. Many things don't apply but it's great for operation of things like the TS2 circuit.

http://members.rennlist.com/pbanders/ecu.htm

Thu Nov 21, 2013 7:27 pm

Uh oh, brain overload after a long busy day :shock: I'll trust you but I'm sure I'll wake up in the middle of the night contemplating the 1/2 speed cam driving the distributor with 4 different combustion cycles occuring simutaniously, arrrrghhhhhh :D

And for some payback contemplate in your head a 9 cylinder radial engines firing order (usually 135792468) and how it's all done with a single 4 lobe cam.........sleep well :D

Thu Nov 21, 2013 11:01 pm

the usual issues we've had with other bus owners when problems arise in this area are:

coil wire to wrong post.
ground under plenum fallen off
bent pin on ECU or connector
double relay problem
ground wire at double relay not mounted to screw*

* - see this article for good explanation of double relay http://www.thesamba.com/vw/forum/viewtopic.php?t=334340&highlight=double+relay+ground+screw