I have a 1989 GTUs that starts fine, idles normally and warms up. Then the idle speed starts to increase and decrease and tach drops to 0. Sometimes, it will just switch to high idle only. I've replaced the plugs, wires and alternator, no help. The engine codes are 1 short and then 3 long with 4 short, which seems to indicate trailing coil and then air bypass valve. Did I miss-read the codes or does this code seem to indicate the idling and tach problem. Thanks in advance.

 

welcome to the board.

check the trailing coil. the tach involvement is telling.

 

Set tps while your at it can't hurt..

 

Tach wire runs off the leading... no?

Check each lead for spark when it starts playing up... should point you in the right direction.

Make sure you haven't mixed up leading and trailing when you did the leads.

 

if memory serves me right, the tac is the yellow lead off the trailing coils... but it seems to me that you may have a vacuum leak

 

Did you try the TPS?

 

welcome to the board.

no, but you're not totally wrong. apparently there is an OEM provision to run a tach signal from the lead coil it's just not used. as wired in the car, the actual tach signal is taken from the trailing coil.

 

Thanks Diabolical.

So correct me if I'm wrong, there is two trailing coils and one leading in this direct fire/wasted spark system. If the tach was running off the trailing (like old rx's with one trailing coil) wouldn't the tach only read half of the two pulses per rev. Therefore giving half of the correct reading.

I'm just going off mine which is an 88 S4 turbo. Mine has the yellow/green trace tach wire connected to the leading coil.

 

The Green/Yellow wire is the trigger wire for the lead coil. It is not the tach wire.

 

don't overthink this.
The Tach reads off the Trailing coil.
It can also Read off the leading coil.(IF you have a problem with the coil and you jump the wire to the leading.)That is why they have an extra test RPM connector there.

 

Thanks guys, I must have gotten mixed up and saw the leading trigger wire as yellow with green trace and not green with yellow trace... Then putting two and two together thinking it was a tach wire... as is on the old school Rx's.

I feel like a dick...

 

Apologies for the silence but I've been trying a number of things. I cleaned up all of the grounds that I could find and replaced the trailing coil, no help. The only pattern is that the engine runs fine after starting for 8-10 minutes and then the tach goes dead and the idle goes up and down. This timing is very consistent so something must be heating up and failing in about the same way every time. Have to focus on the air bypass valve now.

 

What is the voltage output of the alternator taken from the output wire when the trailing coil becomes problematic. Output should be near 14 volts. Too low and I believe the trailing coil malfunctions.

Also, if you cleared the error codes and took the error codes once again when the engine was cold do you still get the same codes?

Might check the Water Thermosensor if all else fails.

And usually, but not always, error codes are a sign of a poor connection thus make sure the connections to the trailing coil are a solid one.

And perhaps switch the lead plug wires w/the trailing wires to see if things behave in the same manner as perhaps there is too much resistance in the trailing wires when things heat up.

 

Just wanted to update on my progress, no resolution unfortunately. I cleared all of the error codes (disconnect battery), reseated all of the connections to the trailing coil and started up again. Running normally, checked the alternator voltage and it was at 14. After about 15 minutes, it started idling up and down, no tach. Water temp sendor seems to be working. Still at 14 volts on the alternator. Confirmed that nothing was going on the trailing coil with a timing light. I stopped the engine and checked the codes again, same as before, 1 and 34. I started the engine again, high idle only this time, no up and down. If is disconnect the wire on the BAC valve, the engine idles down but the tach is still dead. Going back to the shop manual on how to test the valve now.

 

The trailing coil connects to the ECU at three pins. Pin 1G is Blue/Yellow and it reads 0 volts w/key to on and .6 volts idling. Pin 1J is Brown/Yellow and reads 4.4 volts w/key to on and 2.2 volts idling. Pin 1V is Blue/Green and reads less than 2 volts w/key to on and 1.4 volts idling. If you don't get these readings make sure the connection at the ECU is a sound one.

 

I've heard of some leads causing RFI.... Just a thought.

I've also hears of the ECU dropping the trailing signal when the voltage is low to the ECU. You might have 14v at the alt and battery but do you have good voltage at the ignition switch. There is a big single wire connector (big black wire) near the battery that feeds the inside fuse box and ign switch. Mine had a massive volt drop over this connector causing it to run like crap under load. Just another thing to check.