Automatic Transmission Computer Details

Created: 4/13/01

Last updated: 

Author/source: Mark Hershey

Here's some pics of the automatic transmission governor computer showing
failed capacitors, replaced capacitors, and two additional .1 microfarad
bypass capacitors across pins 1 and 4 of each of the two comparator
integrated circuits.

  Fig 1

This shows one of the large power transistor replacements (a TIP 42C, common as dirt) on the upper left. The two large silver tubular capacitors, one to the left of the three black wires and the other perpendicular just behind the stryrofoam at the lower right, are replacement Tantalum units. These were replaced about 9 years ago, OK so far. The blue box is my Mobile A/T Test Fixture-- it hangs in the engine compartment of my daily driver DeLorean VIN 2790 :-)

Fig 2

This shows the same governor circuit boards before the caps were replaced. (OK, these aren't the originals-- it's a stand in picture form a David Teitelbaum unit :-). I mounted my original in the blue plastic box years ago for plug-in convenience while testing various mods. Note the two bad capacitors..the black rubber seals have been blown out of the ends. They're not supposed to do that. One is a 6 volt cap, the other is a 10 volt cap. (I've seen one board with 15 volt caps, most have two 10's) The voltage they help filter often exceeds 15 volts. Use 4.7-10 microfarad caps with as
high a voltage rating as you can least 20 volts, 50 is better. These blow "open", that is, they do not short out anything, they just become ineffective. The trans shift points change slightly, but the circuit becomes very erratic-- sometimes shifts OK, sometimes skips 1st, holds 1st, starts in 2nd, downshifts for no reason, etc. No rhyme or reason-- temp, humidity, luck, and what you had for breakfast all had equal effects.

Fig 3
A closeup of the replaced caps-- the tubular silver things.

Fig 4

Both governor boards from the back side. I've added two .1 microfarad caps across pins 1 and 4 (power and ground) on the rightmost board to eliminate the effects of spikes and transients from causing erratic shifting. Seems to have helped, not a single unexpected shift in 6 months of daily operation.

Fig 5
An extreme closeup showing which pads to solder the cap leads to if you try this at home.

Between the 15 or so units I've seen and / or talked folks through repair, and at least one unit each from Dave Santos and David Teitelbaum, (thanks for lots of help, and yes, David, eventually I'll get the schematics I owe you in a human-readable form ) all had bad caps and one had a bad multifunction switch as well. Some have had really bad solder joints, especially around the jumper wires that join the two boards. Note in Fig 1 that I replaced the jumpers with thin blue wires. 

\\ Mark Hershey

Test Box:

Here are a couple more pics to reply those who expressed interest in the 'test fixture'

The Test Box picture shows the connector for the main wiring harness (the 9 pin connector in the middle) and a 3-pin connector that goes to the governor's AC vehicle speed generator. The test points are for the transmission solenoids and one for the AC voltage from the speed generator.


These are the shift point lamps mounted in a plastic plate where the clock should be. The plastic insert is half of a Radio Shack project box; fit nicely in the clock hole with just a bit of filing on the end of the plastic. Looks dirtier in the pic that I remember....and sorry about the lack of focus!

\\ Mark Hershey
Vin 2790,now full-time automatic (so far)

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