Flashing “Auto Trans” light and ECM (Engine Control Module)

[DesmondD]

On my 1993 Bighorn (3.2L petrol [added]), I have for some time had an intermittent flashing of my “Auto Trans” light, and it jumping into, or staying in, limp-mode 3rd gear.
It stays off or “a while” after I turn off the ignition and restart, but then it comes back unpredictably. For example, two months ago, it happened 10 times between Wanganui and Stratford, but not at all from Stratford to New Plymouth to Lower Hutt. Hutt Automatics could find no stored DTCs (Diagnostic Trouble Codes), nor any evidence of burnt transmission oil.

Then this last week it happened once between Lower Hutt and Rotorua, then daily within 20 metres of morning starts in Rotorua (so not a temperature problem), and then about 20 times between Rotorua and Bulls, but after my “fix” (described below) not at all between Bulls and Lower Hutt! My question is, was my fix really a fix?

In Bulls, I noticed that the engine would not idle, and would only run at driving speeds. So I started to think. I figured that there may be an ECM problem, so I stopped, disconnected and reconnected the battery to re-initialise the ECM and possibly the TCM. After this, it idles normally, and there is no more flashing of the “Auto Trans” light (at least not so far – touch wood!). But did I really fix anything?

And if this sort of thing happens again from time to time, should I install a battery master switch so that (eg if it’s raining) I would have fast way of re-initialising the ECM and possibly the TCM? And if I do this, should I install it on the positive or the negative battery terminal? And what current rating switch should I buy (100 amps or 200 amps)?

Many thanks for any comments or suggestions.

[Crash bandicoot]

firstly is it petrol or diesel ?

[DesmondD]

Sorry, I forgot to say it is petrol 3.2L.

[pmfg]

Im lead to beleive its the speed sensor that plugs into the gearbox.

Mine had the same problem. However mines diesel.

[DesmondD]

Thank you for your interest. I believe the problem is now solved.

(1) I finally managed, by shorting pins 1 & 3 of the green 3-pin socket, to get the code out of the transmission control computer as 49.

(2) After extensive google searching, I found the transmission diagnostic codes at two places:

(a) The 2005 Technical Seminar ATRA manual at
http://oskin.ru/pub/chrysler-dodge/manu ... Manual.pdf
says that for the 4L30-E transmission this means"High battery voltage (above 16v)"

(b) but better is the article Transmission Therapy: the 4L30E files declassified" in Gears (Aug 2003) at
http://www.atraonline.com/gears/2003/20 ... 3_8_26.pdf
that says this means "High Battery Voltage (above 16v at TCM pin 37)" and that pin 37 is "Ignition" which should be at battery positive voltage.

(3) So next time the Check Trans light flashed and it shifted to 3rd gear I knew to look at the voltmeter and it was over at maximum, 16v or more. It returned to 14v before I could check the battery with a digital voltmeter. I cleared the diagnostic code (disconnecting battery for 5 mins – 10 secs might be enough) and it happened again, and again the code was set to 49. So I knew I had tracked down the problem to the alternator. Whatever the high voltage was, it also blew a headlight bulb. These "spikes" seem to last from 30 secs to about 2 mins.

(4) Today I got a new alternator installed, and the mechanic said the old one was putting out over 40v. He also said it was full of oil and dirt. So I guess we Bighorn/Trooper owners should try to clean our alternators (with compressed air??) from time to time.

I did all the diagnosis myself, that my local automatic transmission shop couldn’t do. My local garage agreed with this diagnosis, and changed the alternator. I am now hoping my problem is solved (touch wood!).