My '95 LT1/4L60e (installed in a '76) started shifting "erratically" lately...

When shifting manually from 1 thru 4, it stays in 2 and 3. (ie. when shifting from "1" to "2", tranny starts in 2nd and stays there. Then the upshift to "3" will bring it to 3rd and then it will stay in 3rd when manually shifting to "4/OD")

When in "4/OD" and letting the tranny shift, it will go down to 1st, but then seems to skip 2nd and will only make it into 3rd.

I logged the file (DataMaster) and also noted that I get a few DTC's (although my Engine Check Light does not register them); I get Datamaster's DTC of 22 (TPS sensor low) and 72 (Output Speed Loss)

I did recently install an Autometer Speedometer and per manufacturer's recommendation, tied into the yellow VSS wire to get my signal to it. It was fine for the first few days... could this be part of it? Is this the reason for DTC 72?

Also, when I got these DTC's, my "Slip RPM" went to +/- "0" and stayed there until I shut it down.

If anyone can look at my DataMaster file, please respond and I'll forward it to you.

Notes for the file:

At rec # 219 - 220, 2nd gear is skipped, and stays in 3rd thru rec# 604.

At rec # 607, it's manually put into "1", but tranny stays in 2nd.

At rec # 864, I get the DTC's of 22 and 72 and "Slip RPM" goes to 0.

At rec # 1611, again, it's manually put into "1", but stays in 2nd.

Thanks so much!

 

I think this might be the cause of your problems. Have you tried disconnecting the extra speedometer? That would be the first thing I tried. I seen several cases (not on Vettes though...) where the signal is too weak for splitting, amplifier is required. Could also be other ploblems in the VSS circuit like a bad crimp.

 

That will be the first thing I will do...just to make sure it's not a tranny problem.

At first I had it connected to the VSS Output wire (Ckt 817 @ pin# "B8"), AND SOLDERED THE CONNECTION, but that gave erratic speed readings...Autometer suggested that I tie into Ckt 400 @ Pin# "A32", so did so, but just crimped it. After I the know the ckt is the problem, I'll try soldering instead.

Thanks.

 

Crap! I bypassed the Autometer Speedo and connected ckt 400 back directly to the PCM (without going thru the speedo first) and problems have not gone away It still only goes into 2nd and 3rd when shifting manually from 1st thru 4th. When shifter is placed into 4th/OD and I let the tranny shift automatically, it skips 2nd and will not go into 4th.
What the HELL?!

I'm not getting any DTC's from the PCM, but DataMaster gives me the two codes mentioned earlier (22 - TPS sensor low, and 72 - Output Speed Loss)

Should I also get a DTC 22 from the PCM? The Service Manual states The MIL should illuminate and that there will be no 4th gear in "hot" mode...I don't get 4th in "any" mode...???

Any suggestions welcomed

 

When DTC 72 is active according to Datamaster, what is the vehicle speed (according to Datamaster)? Notice anything about the speed when the code goes active?

As far as I can tell from my 92 service manaul (things might have changed for 95...) This code will set when there is a problem in the VSS circuit. Either cable, connector or the sensor.

About your other code. This is indicating low voltage from TPS. In your log, does the TPS voltage goes below 0.3 when fault code is active? What is the voltage at idle? Does it chage when fault codes goes active?

 

Sorry, I forgot... feel free to email me your log at krps@swipnet.se

 

I don't notice anything strange about the speed when the DTC's kick in. On my latest log, the DTC's start from record #1. The TPS voltage has a minimum of .59V when in P/N or at idle in gear...this seems to be normal so not sure why this code set. The 95 Service Manual also states that if below 0.3V this code will set, but mine never came close!?!?!?!

Thanks in advance for taking a look at the file

 

I don't have much information about the 94/95 PCM so perhaps someone could fill me in... Anyway, I had a look at your log and this is what I think: There are two windows in Datamaster, one for "engine" and one for "transmission". If you look at the data (eg vehicle speed) they do not quite agree, I would guess that the PCM runs two different programs sampling their own data. If data was sent from "engine" to "transmission" via serial communication the data would be identical (unless some physical error).

The reason this is important is that the "engine" does not set the code 22 but the "trans" does. The voltage does not drop below threshold for setting the code so I would say the "engine" sets code 22 like the older ECMs and the "trans" running another program that has an added plausibility check to code 22. What I'm trying to say is that I think "trans" code 22 is a consequence of problem giving code 72 (which is not set by "engine"). But like I said, it is a guess

At about rec #831 your vehicle speed signal starts to look very noisy. At 848 is is almost constantly at zero. Later, at 864 it makes a step up to 56 mph in almost no time (TPS constant!) and code 22, 72 are set. I think you lost you speed signal at about 848 and the code validates in about 5 seconds. Do you remember your driving? Did you drive like the speed signal in the log or at more or less constant speed like the TPS voltage imply? Either way, your vehicle speed signal looks more noisy than it should.

One more thing, your voltage is only 12-ish all the time... Is your generator working? It should be around 14 volts. Don't you end up with a flat battery?

I hope this is of any help and good luck!

 

That's very interesting and I didn't notice that...can anyone confirm this?

It was pretty much stop-n-go (traffic in S. FL just plain sucks!), but don't recall "stompin" on it to get a jump in MPH like that!

It seems to be working...I get about 14.2V-ish at the "bat" terminal of the generator (cold) and stays above 13.6V (hot). I DO get a dead battery and have been trying to track down a current draw, but no luck yet. I've tried the ampmeter trick (disconnecting "neg" cable and connecting ampmeter between the cable and "neg" post), but not getting any reading!?!?! This is a '76 vehicle, so the only draw I should be getting are from the radio presets and the PCM. There is a problem somewhere though!!!!

OK, back to the original problem and another question: Autometer's instructions for the electronic speedo indicate to take the signal from the VSS (ckt 400 in my case) and direct it to the "Sig" terminal on the speedo, then from the "Out" terminal on the speedo back to the PCM. It states that this gives the PCM a "buffered" signal...do I want a "Buffered" signal?

Thanks again for looking at the file!

 

Yes I would think so.

I've seen many problems when connecting "extra" equippment directly on the vehicle speed sensor signal. The problem is the weak signal from the sensor. The "factory" solution is to run the sensor signal to an amplifier with two isolated outputs. One will go to the ECU (PCM in your case) an the other to the "extra" equippment. In some cases the ECU will have one extra isolated output where you may connect your extra equippment. In a similar way I think Autometer's output will provide an isolated output to reduce noise that otherwise might be a problem.