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Thu Jul 17, 2008 5:48 am

I have an 87 Vanagon that has had some problems. One problem was resolved by borrowing a different ECU. However, presently the spark does not advance when the engine accelerates. It should advance to 35-40 BTDC @ 3000 rpm, but it stays right at 5 BTDC. The engine will accelerate with the AFM disconnected and runs well, but the spark does not advance. I bought a rebuilt ECU, but the engine will not accelerate with the AFM connected, and the engine seems to prefer the replacement ECU rather than the rebuilt one. Symptoms are the same with a new AFM. I don't know if I could get an advance with the original ECU which I believe has another problem that caused tach spikes and dying.

Why am I getting no advance?

Aloha
Tom Powell

Thu Jul 17, 2008 8:35 am

I mentioned on another thread, Tom, that you really need to ring out that whole wiring harness, especially the AFM circuits. Here's why:

You know the ECU is getting the Hall rpm pulse, because the engine runs at all. But it needs a load signal from the AFM to lookup the ignition advance. Most people think the mixture is determined by the air volume going thru the AFM, but that is not the case. The ECU can already approximate the air volume; it's the volume displaced by the engine based on rpm. But how much that volume is restricted by throttle or increases with throttle opening is expressed by the AFM's position relative to rpm. The AFM's main function is as a load signal.

The AFM also has the Intake Air Temp thermistor circuit, which tells the ECU the temp and therefore the density of the incoming air, so it can use that to correct the mass of fuel it will inject relative to the mass of the incoming air. RPM, IAT, and load are the three axes of its injection lookup table.

That's on the injection side, and it's the same for Digijet and Digifant. But the Digifant ECU also manages ignition. It needs to see the AFM signal in it's ignition section because load is one of the axes of the ignition advance lookup table. No signal, it's just going to fire spark at the base setting.

So I would say you have a problem with the load signal getting to the ECU. Your job is to find out why.[/i]

Thu Jul 17, 2008 8:43 am

To follow up on tencents post....

If you can remove the engine wiring harness you can test it with your volt ohm meter to see if there is a broken wire in there or one holding on by a single strand.

This guy builds a nice replacement wiring harness for reasonable money...

http://www.kyleautomotivespecialties.com/

Thu Jul 17, 2008 8:50 am

No need to remove it. In fact, to have the best chance of catching the fault in the act, you want to test in situ. The whole procedure is spelled out in detail in Bentley.

Sat Jul 19, 2008 9:20 am

thank you tencentlife and the others that replied to my requests for solutions

My 87 Vanagon is now operational again. It had two problems. The first was an occasional tachometer spike followed by a drop to zero as the engine died. This was solved wiith a rebuilt computer, but then there was no spark advance. Tencentlife's reply directed me to suspect the wiring between the AFM and the ECU. I replaced the harness and the engine runs with reliability.

Installing the replacement harness I was unable to determine the correct injector connections and the Bentley manual did not help me. Difficult to determine any differences on test drives.

Hopefully my last question to the forum will be how to determine the correct injector connections.

Thanks again for the help.
Aloha
Tom Powell

Sat Jul 19, 2008 10:12 am

Quote: Hopefully my last question to the forum will be how to determine the correct injector connections.

And the answer is there aren't any. It is a batch-firing system, as opposed to sequential injection. In batch-firing (shouldn't it be "batch-squirting"?) all 4 injectors spray simultaneously. Therefore there is no difference between connectors; they all receive the same signal at the same time. So there is no mixing them up. If you hooked them up, you got it right!

But hopefully you're not done with us, nor we you. Once you show up on the Samba, you're ohana. So aloha to you too.

Sat Jul 19, 2008 11:43 pm

I suspect is was the wires to the temp sensor that were failing. Removing the connector to that sensor while it is running will reset the advance. Or something like that!!
Al