John Speake

As Bill says, this is not typical of a WOT switch problem.

More likely to be a self inflicted wound during R&R..... a large flat spot sounds like a bad air leak.

sharkmeister85

I was up late last night going through the WSM's looking for anything that might be relevant or give me additional guidance on what to check. Found something that sent a chill up my back. I didn't sleep well.
In Section 28 "Ignition System", pg. 28-44 "Important Car Information" point 7):

"Pull off both control unit plugs or make a firm connection between ignition coil term .4 and ground with a piece of wire before checking the compression (dangerous high voltage, insulation damage on ignition coil, high voltage distributor, ignition cables, etc.). Piece of wire must be interference suppressed with a shieldingof at least 2 k-ohms."

After I parked my car in the garage for the last time, before starting on the intake reseal, I did a compression test on the engine. I did not pull the control unit plugs! Could this be a problem?
Glenn

John Speake

If the car starts and runs then it's most unlikely you have damaged the ECUs in any way.....

sharkmeister85

Thanks, John! In the same section of the WSM they also refer to the "Final stages" as being a "control unit" and it has 2 plugs. Could this have been what they were saying to unplug? I never unplugged it either.
Glenn

heinrich

my 85 exhibited similar and it was the final stages. No idea why yours would have gone bad Glenn

sharkmeister85

Because I didn't unplug them before doing the compression test?
Glenn

Bill Ball

Right. I have found a bunch of cars where the WOT position was not triggering, all but one because the throttle cable was too loose to get to WOT. The problem is excessive slack in the cable section from the gas pedal to the throttle quadrant, so the pedal may travel several inched before threre is any movement at the quadrant. There is an adjustment at the firewall. These people had bo idea it was not working, although they all seem to believe the car is peppier after we have fixed it. I even found this problem once on my car, but I can't say the car drove any different after I snugged up the cable and the WOT test indicated it was working again. All I have ever seen with malfunction of the idle portion is a bad idle.

sharkmeister85

First, thank you to all who offered suggestions and advice, it was greatly appreciated. My friend Alex (another local 928 afficionado) came by to have a look and assist me. Despite my fear of the worst possible outcomes in the last 24 hrs, I wanted to cover off some things I couldn't do on my own. First of which was checking the TPS signals at the LH connector itself.
I had convinced myself that this would be a routine check and would lead nowhere as I had checked it previously at the terminal in the engine bay. We stumbled around looking at the WSM that has 2 different references to testing the TPS, each with different pin outs at the LH, but both printed in '84. We decided to go straight to the '85 Wiring Diagram and it was more clear. We checked the pin outs and we were getting the desired signals for idle and WOT circuits.
But something was still bugging me from when I had checked these in the engine bay before as well. The signals seemed to be coming from the reverse pins. I assumed I had just confused the oriention of the idle circuit pin and WOT pin in the WSM picture. After all, I had a brand new TPS and a brand new lead from the TPS to the LH wiring harness (this is the one that is NLA, but I had purchased it a couple of years prior). But as we thought through the possible outcome of having them reversed, it became more certain that was the issue. When you hit WOT the LH is supposed to see 0 ohms. When you close the throttle (go into the idle circuit) the LH sees 0 ohms. With them reversed, WOT now looks like the idle circuit engaged, hence NO BWAAHH! We rigged some some small leads between the LH harness and the TPS connector and reversed them to test this theory out. BWAAHH!! Take them out and hook up as before, NO BWAAHH! So the question is, "Is my new TPS wired backwards or is it the new connector to the LH that got it backwards?"
Glenn

heinrich

sweet Glenn and Alex

AO

Sounds like you got something backwards. But the most important thing is that you got BWAAAHHH again!

Bill Ball

So, if they don't work, no serious ill effects, but if they are REVERSED things get ugly. I saw one instance where the ground was swapped with the idle lead and all that did was screw up the idle. The Spanner gave us a very specific error code and message that lead us directly to the problem or we might never have found it. The Spanner would have saved you a lot of time, but you did a nice job sorting through the confusion in the end. Congrats!

sharkmeister85

Thanks, guys! As much as this has been a PITA, in hindsight it really was a good thing. It forced me to gain a better understanding of my car's systems in a practical and hands on manner. I've always had a lot of confidence in my 928, but now I have gained confidence in my own ability (with help from this Forum) to deal with "928 issues" that will inevitably crop up. I hope I am able to give back as much to others here as I have received.
Glenn

Walt L

Wow Glenn,

I've been following your dilema on the local board and here for some time but for the life of me I could not come up with one single beneficial suggestion. It almost made me want to drive around the corner to your house to see if I could help some how (Hold the light!)

Glad to see you have the problem solved!!

Walt.

V2Rocket

I think that would be more of an HAAAWB sound.

Imo000

Excellent, glad to hear that you found the problem. Wiring a new plug can be trciky, the digrams can easily be looked at the wrong way. I made the same mistake with a trailer plug just last week and it took some time to figure out the proeblem too.




Bill,

The TPS sensor has to work, I think you are confusing it with the idle or WOT switch. If the TPS is not working, the LH has no idea that the throttle palte is open or cloes. Or worse, if wired wrong it gets conflicting readings.
In any fule injected engine , the TPS is a vital part, unlike sensors that are "not as important" like an air intake temps sensor or soemthing similar.