Looks like I need to try and swap out my ECM. A while ago I was working on a high idle problem and had finally beat it. Last weekend my starter died and I replaced it, but when I disconnected the battery the ECM lost every thing and now seems to refuse to relearn. Before I disconnected the battery it was running and idling almost perfect.

I have the shop manual for my 84 so I'm not looking for a step by step procedure, but if anyone has any tips or tricks on what to be careful of that would be great. Thanks.

I have heard that the 84 has to be driven over a certain speed....like 40mph? before it re-learns.

I believe that is true, and have driver it around for four days anywhere from 20 to 85 MPH, and it hasn't reset. This problem is very frustrating because when I first start the car it runs fine and the idle will drop to where it should be. If I stay under 35, the idle drops to normal when I stop. If I hit 35, the point that the ISC resets, and then stop, the car will idle over 2,000 rpms, and if the check engine light comes on will will drop to normal. If the check engine light stays off, the idle will float between 1,800 and 2,200 RPMS, and I feel the ISC trying to drop the idle lower but it just jumps right back up.

The ISC, MAP, TPS and VSS are all new. When the check engine light comes on it sets a code 24, which the books say to ignore if the car was stopped when it sets, which is the only time it will come on in my case.

At this point I'm just guessing at the ECM because I don't know what else to try.

So does your speedometer work?

Phonedawgz

Yes, the speedo works and is within 1 MPH of my GPS. It moves smooth as speed increases You were helping me with this problem a month or so ago, and as I was working on it the car stopped the idle problem and I posted that it was fixed. It ran fine until I had to disconnect the battery to replace the starter, and then started the idle problem all over again.

Just to confuse the issue, after I did the above post, I took it for a ride. I started her up and warmed it up until the idle dropped, and drove off. Since I had gone over 35, the idle went up, the check engine light came on and then dropped to a good idle at the first light. At every stop after that she ran like she should with the idle dropping down. But I wouldn't bet any money that it won't act up tomorrow when I take it out because it had done this before.

I really don't suspect the the ECM, but I'm at a loss with this problem.

When the ECM doesn't see a VSS signal it will idle the engine higher.

It would be helpful to have a laptop hooked up to the ECM to see if the signal is coming through or not.

Shooting in the dark here as I know nothing about 84's but it sounds to be ittermitent or at least speed related. Is it possible that the VSS in the trans is flaky? Have you checked all the grounds? Added any "extra" ground cables from the engine to the chassis?

The VSS talks to the speedo at 4000 ppm, then the signal goes to a subprocessor in the dash where it's converted to 2000 ppm and sent to the ECM.

If your speedo is working smoothly, the VSS is OK.

The speedo signal goes to the ECM via C-203 H. Brown wire.

As phonedawgz suggested, you need to look at what the ECM is seeing.

The VSS is new and I also replaced the connector on the VSS with a nice clean one. We do have a lap top but I have no idea how to hook it into the cars computer. How would I do this?

I replaced the ground that runs from the front of the motor to the deck lid bolt a couple of years ago. There is also a second ground that was on the car when I got it running from the trans bolt near the distributor and the frame of the car, and I had a ground wire hanging off the drivers side deck lid bolt that I spliced a third ground wire to and bolted it to the front of the trans rear the firewall. This is a smaller wire than the other two.

Thanks for all the suggestions so far.

[This message has been edited by johnt671 (edited 08-31-2011).]

Measure pin R of C203 - A brown wire. Measure AC voltage to ground. You should get somewhere around 9v AC to ground when the car is moving. The voltage will not increase after a minimum speed. The voltage should not change but should be around 9v AC. The frequency of the signal is what changes, however when the car is stopped the frequency drops to zero and so will the voltage.

See what you get.

Used Snap-On or OTC scan tools are available on eBay. They provide data stream through the data link connector. Very handy to have in this case.

I'll take a shot at measuring the the voltage and post the results, but I'll be busy for the next few days. As soon as I can I'll do it.

I'll also look for a ODD 1 Scanner. I bid on one a while back, but the bidding went higher than what I thought it was worth. Wish I had it now.

C203 is the connector below the ECM that connects the body harness to the engine harness. The pin letters are on the clear connector but are hard to see.

The clear connector is a single piece but if you look the black connectors (body side) are actually two separate connectors.

[This message has been edited by phonedawgz (edited 09-01-2011).]

Thanks. I found it in the book after I posted the ?, but the picture is even better.

Pin R in mine is a purple/white wire. It shows all zeros under all driving conditions, sometime flashing a - sign. I tried to probe it different ways and from both sides of the connector with the same results.

I found a brown wire ( E or F) and probed that and got anywhere 29.5 to 31.6 so I guess the meter is working OK.

[This message has been edited by johnt671 (edited 09-05-2011).]

If you are getting a negative sign on the display that means your meter isn't properly set for AC voltage. You need to switch your meter to read AC voltage and then read it again.

I know very little about the 4 cyl but some years ago I did have the same problem with my 86 2.8. At a certain rpm, not mph, the motor would not come down to idle and between shifts it would not rev-down. I think 2K was the magic number and the rpm would stay there or go up to 2.5k rpm. As long as I stayed under 2K all was well.

In one "Leap of Faith" I added a 1 inch wide braided ground strap from the engine to the frame and replaced the ATS "air temp sensor" in the air cleaner and haven't had a problem since.

Also if the 4 cyl has an IAC "Idle Air Control" and it gets stuck open it will cause the idle to go high.
For a temp Grd you can use a good set of jumper cables and clamp one end to the neg bat post and the other end to the engine. Then go for a ride to see if that solves the problem.

If all else fails check to see if your floor mat is wedging against the gas pedal and if it is be sure to come back and let us all know.

Spoon

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Two yeast spent their entire life "about 2 days" discussing what the purpose of life could be and not once did they even come close to the fact that they were making champagne. Quoted by: Unknown

The rain stopped so I got to do some work again. I swapped the computer out for a used one I had on hand, and now the idle is very smooth but is about 100 RPMS or so high. Once the car warms up the idle drops nice and when I drive up the road when I stop the car it drops down to nice smooth idle but still about 100 RPMS to high. This should cause the check engine light to come on, but it doesn't. The factory tach isn't working, so I have an after market tach in place of it, so I'm wondering if the tach is off a bit.

I wanted to check the wires out so I hooked my meter up again and probed the R terminal at the C203 and and again got nothing on the meter. (I used a different ground this time.) when I probed the brown wire I go numbers around 26 V on AC. (Turning the meter on and going two clicks to the right.) When I went to DC (two clicks right) I got a reading of 6.9 to 7.1 V at 40 MPH and .5 when I stopped. I don't know if they make any sense, but I thought I'd do it anyway to learn how to use the meter. Let me know if I'm screwed up.

I still might try another ground like some of you suggested, an see what happens.

Spoon, thanks for the floor mat idea. That wasn't it but it was funny anyway. M y wife said since I've gotten old maybe I don't know what my right foot is doing. She doesn't know that pretty much all of me doesn't know what its doing.

Thanks.

My guess is the tach is off a bit.

You are reading a signal to the ECM so the speedo is putting it out.