96 328i Trans Fixed!

[eurogeek]

I am new to the hut, but I thought many might appreciate some of the discoveries I have made recently while troubleshooting the "RX" automatic transmission in my 96 328i, which is showing 190K. I only bought the car last Nov, 2010; really nice, well maintained and runs super!
Last week while driving home from a 150mile round-trip, my trans went into the dreaded "limp" mode. The check engine light came on first, then the gear symbol with the exclamation point inside it came on, then the "soft feel" of the tranny. Oh crap!
I am new to the BMW world, but having owned and repaired many Porsche's for more than 10 years (mine & others), I'm not completely out of my element with the Bimmer, but still, the forum's didn't exactly cheer me up any at all when reading about the LIMP MODE! Plus, my 30+ years of automotive experience and training wouldn't let me down............I hoped! But I have learned to consider the whole package when dealing with these electrical monsters, (gremlins my eyeball)!

First, I had noticed over the last week (before the trouble started) that when starting the car, the service indicator lights (under the odometer readout) and the airbag light took about half a minute longer to go out (after starting) than they had been taking. Hmmm.....clue? I also noticed that the dash lights had a little twitch in them, ever so subtle. Hmmm........
I had had this same experience before in an old Porsche 944 that I restored some years ago.........alternator brushes about gone, I was sure of it!
So, after having the alternator tested, yep it was on the verge of "nope, no more". Replaced it with a 140 amp (was 80): WOW! But the trans light was still on, crap X2!

I wouldn't recommend the 80 to 140amp upgrade unless you can detect and repair any faults or problems that might result from this upgrade!!!!!
So I lucked up and found the BMW service electrical schematics on line, and studied! There are only two inputs that the EGS (or TCU - trans control unit) needs to be enabled for normal operation; a run signal from the engine ECM, and a temp signal from the trans valve body where there is a thermistor. The thermistor (inside the trans pan, inside the wiring sheath) returns the temp signal to the EGS which disables the limp mode circuit unless the fluid temp is out of range or too high. As long as the temp signal is present and within range, the trans should function normally. (Worn out trans is another issue!)

The temp signal is on pins 21 & 22 at the EGS (TCU) on my 96 328i 2.8L with the RX coded tranny, but you will have to determine the correct pins on yours. Might be the same if factory original like mine, but it might not!
I found the correct type of thermistor that I needed at Digikey: NTC (negative temperature coefficient) 20K ohm @25 C. Paid a whopping .80 for a replacement. Simple job to put it back in-line; crimped. New fluid and filter, started up, and what do you know, no warning lights and trans works great!

Analysis: While the alternator was dying, the electrical spikes (always present when brushes are worn out) caused an "overvoltage/overcurrent" condition that the thermistor (inside the tranny) just couldn't handle and it failed (open- no reading on meter). 14 years is a long life for any temp sensor! And while it relatively easy to change, finding it was a beeautcch! With a capital 3!

Anyway, I thought some of you here might actually be having this same problem, so I thought I would share and hopefully save you some cussing and wrench throwing! Hope it helps, sure can't hurt! Total repair cost:

Alternator, (Brand new OEM, eBay) $123.50
Thermistor (20K ohm @ 25C) with shipping 3.58
Trans Pan gasket(s) 18.50
Dextron III Fluid (4 qts. @ 3.99) 15.96
Total repair cost: $161.54

Satisfaction: PRICELESS!

eurogeek

P.S. If you are unfamiliar with the way a thermistor works just search it, lots of info about them on the net. They are very durable, and provide a very stable signal for the EGS temp loop. But like most things, they can fail, especially when there are voltage spikes present.

[bostogrun]

Wow you are really above my pay grade. Are you an electrical engineer?

For that matter, I have also replaced a lot of "unfix-able" items on my 97 328I auto. For example the little lights in the "PRNDL" display went out. For about $3.00 worth of Radio Shack paraphernalia I was able to fabricate a better than fix.

Also the large capacitor in the climate control unit tends to cause intermittent failures. Easily fixed by desoldering the capacitor and installing a new one. You spend fifty cents to save $1000. on a new climate control unit. There a number of DIYs on this out there. GOOGLE "e36 climate control fix"

Plus these cars really go through brake switches that sometimes lock up the tranny. The autos have a four pronged brake light switch vs the manuals with two.
The auto interlock side can fail but not the actual brake light itself so you wont get "BRAKE LIGHT CIRCUIT FAILURE" light on the computer display.

Also be careful of the brake light bulb holder on the passenger side.

[eurogeek]

No, I'm the guy EE's go to when they are...........stuck!

Many years of PLC programming and motor controller installs and repairs. Comes in handy with the electrical problems on automobiles.

Appreciate the 411 on the brake & climate controls, always good to know before a failure!

After thinking about what I wrote last night here, suffice to say it is really easy (now that I know) to just test the thermistor leads at the TCU to determine if there is an open circuit. If it is open, you can bet that the thermistor shorted out, for whatever reason. But maybe this repair can get someone some more milage out of their tranny before it must be changed.

Learning and understanding are different realms!
JC Wright

[bostogrun]

Here is info on low vacume



Vacuum Gauge
Though little understood, the vacuum gauge is probably the best single indicator of your engine's health you can get.

A Little Background
When your car is idling-whether it's fuel injected or carbureted-the throttle plate or plates are restricting the amount of air the engine can breathe in. The pistons are attempting to "suck" the mixture past the throttle. (Of course, in reality, it is atmospheric pressure that is attempting to "push" air into the engine as the pistons travel downward on their intake strokes.) When throttle is closed, vacuum is high in the intake manifold, from the throttle plate(s) to the combustion chambers. By contrast, at wide open throttle there is relatively little restriction to outside air entering the intake manifold, so vacuum in the manifold is very low.

A vacuum gauge reads pressure differences from atmospheric pressure, so the reading is zero in our "normal" sea of air. By convention, vacuum gauges in the US read "inches of Mercury."

Reading the Gauge
Unlike a fuel gauge, the vacuum gauge will keep you entertained with its instantaneous, wide-ranging movements.

When you floor the accelerator pedal, you can watch manifold pressure (another word for vacuum) swing from strongly negative to nearly zero (atmospheric pressure). When your engine is "on the overrun," like using engine braking down a steep hill at high RPM, you'll see really high vacuum readings. Naturally, turbocharged and supercharged will show very different results, with readings swinging into the positive at high speed. IAP's vacuum gauge is not designed for turbo or supercharged vehicles. Your vacuum gauge is also a sort of "poor man's" fuel mileage indicator; when vacuum is low, you are burning more fuel.

Absolute readings are not as useful as changes over time. That is, if you establish baseline readings under a variety of circumstances, you will know what to look for if your engine begins to deviate. Everything else aside, a high vacuum reading tends to indicate a healthy engine.

Having said this, we can make generalities about the readings you can expect. Note that engines with performance camshafts tend to read lower vacuum. Readings are also lower at higher altitudes; the rule of thumb is approximately 1 inch of mercury for every 1,000 feet of altitude gain.

The following readings will not apply to turbocharged engines, or cars with a separate venturi for each cylinder (like Weber DCOE or Dellorto carbs). All readings are inches of mercury (in. Hg.).

ENGINE STATE VACUUM GAUGE READING INDICATION
Steady idle (800-1200rpm) Gauge steady, 17-22 Normal & healthy
Steady idle (800-1200 rpm) Intermittently drops several needle divisions Sticking valve or broken valve spring
Steady idle (800-1200 rpm) Steady, low reading, 8-14 Small vacuum leak or valve timing off; could have low compression/worn rings (verify with a compression or leakdown test).
Steady idle (800-1200 rpm) Steady, low reading, under 8 Vacuum leak (check brake booster, vacuum lines, etc.)
Idle (800-1200 rpm) Needle drops sharply on a regular rhythm Burnt valve, or a valve with clearance too tight
Idle (800-1200 rpm) Needle drifts up & down, along with rpm drift Mixture off or small vacuum leak
Idle (800-1200rpm) Vacuum gradually drops Excessive exhaust back pressure (plugged muffler or catalytic converter)
Idle (800-1200rpm) Intermittent fluctuation Ignition miss; sticking valve
Idle (800-1200rpm) Steady, above 22 Ignition timing may be too advanced
Open & close throttle quickly Drops to about 2, jumps to about 25 Healthy engine
Open & close throttle quickly Drops to 0, jumps to about 20 May confirm worn rings (especially if idle shows only about 15-20) Verify with a compression or leakdown test.

- Mark Lee


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