E24 '88 Longstanding Transmission Gear Limp Home Mode - FIXED

[walczyk]

Hi all, this has been a troubled car we haven't been able to fix, and we stopped working on it some years ago. From a working '87 we swapped transmissions, the center console switches, and various under-the-dash computers/modules in the past. Now we're back at it, I think it's time to check voltages and grounds, including the connector that goes to the transmission itself, it apparently can cause trouble. Throttle cable was also mentioned in some older threads.

I checked the alternator voltage today and it read 13.6 volts. That's low, but is it too low? I figured I should try to get it to 14 volts before taking the seats out, jacking the car up, and taking apart the dash to get at all grounds/connectors.

Any/all advice would be much appreciated!

[bmwdirtracer]

13.6 is fine; that's not your problem at all. Throttle cable shouldn't be, either; likely that thought came from someone with a 3 series car, which used a cable downshift, unlike the "big" cars.

It's pretty difficult to put the old 4HP 22's and 24's into limp mode.....and certainly, you've gone, um, "above and beyond", replacing parts. Does the trans work correctly in manual mode?

If you'll look carefully at pages ~50 onwards of this the factory's ETM, (link below) you'll find a bunch of tests to find the issue. There are not too many external inputs to the EGS (trans computer), unlike the more modern cars. The throttle position sensor is one of them, though. You might want to check that the TPS is correctly showing something besides "idle" to the EGS. This old TPS, I'm pretty sure, will show "IDLE", WideOpenThrottle", and "everything else" (Test it by measuring resistance , unplugged.You'll have FULL, NONE, and "everything else"; do not expect a steady increase or decrease. It's position is adjustable, IF you prove that's the problem. (Otherwise, leave it alone)

http://wedophones.com/Manuals/BMW/19...g%20Manual.pdf

Other than that, , well, which of the following components are you sure have been replaced?: Trans, EGS (trans computer), torque convertor?

Is there any oil in the electrical plugs at the trans? (I've not seen this on older cars)

But, meanwhile, follow that link; the factory walks you through the diag process.....tell us what you get.

[walczyk]

Thanks! I've been looking at the diagnostics in the ETM today. The 88 is a higher compression engine with a different TPS than the 87 which was giant and had some transmission specific parts in it? I remember there being different ones but I don't remember the details. Checking the resistance of the EGS pins is my first stop on the diagnostics chart. When you say old TPS, there is IDLE, WideOpenThrottle, and EverythingElse, as in three different resistances to check for? So just check resistance for the idle and WOT positions.

>Other than that, , well, which of the following components are you sure have been replaced?: Trans, EGS (trans computer), torque convertor?

All three. To be more specific, after following the ETM chart it tells me to check C and D, the hydraulic solenoid and grounds. I'll report back later today. Thanks for all the advice.

[bmwdirtracer]

page 35 of the ETMs show the TPS as just having "IDLE" input, and "WOT" input. The ETM could be wrong; the following year, in the E34, the TPS had a sweep, but I don't think that was ever the case in an E24. So, in essence, you want to see full continuity at one extreme, and zero continuity at the other extreme....I forget which is which, sorry - really doesn't matter, if you see both readings, that will be correct.

[walczyk]

Following the troubleshooting guide I discovered the 'fuel rate gauge' signal to the AEGS control unit had low voltage, 8.6V instead of greater than 10V. I'm next supposed to check the Motronic unit's voltage on wire 10, 11, and 21, but what do these wires do?? I looked through the diagrams and found that wire 10 is from the o2 sensors, but the others I couldn't find any information at all. I went and pulled the Motronic computer under the dash and after taking the screw out of the plug I noticed I can't get at any of the pins. How are you supposed to measure the voltage when the computer is connected??? The universal connector the manual mentions... is that something I need to track down?

[bmwdirtracer]

It is not advisable to start new threads, for the same issue. This will result in the data from the previous thread being lost, so others who try to assist won't know the problems you're trying to solve.

You're not going to find a "break-out box" like the one mentioned in the ETM's. Make sure that the EGS (trans computer) is grounded properly. Make sure the engine grounds are good. (Should be one at the back of the valvecover, and one at the passenger engine mount, if I recall correctly).

When you were testing voltage for that "fuel rate gauge", where did you take your ground from?

{Case in point: if this were the original thread, I'd click on the ETM link I gave you, and quickly check those pins for you.....but the link isn't here.....}

I'd suggest you return to the original thread, so other people can follow the questions at hand.

[MauiM3Mania]

Merged two same discussion threads.

[walczyk]

It is not advisable to start new threads, for the same issue. This will result in the data from the previous thread being lost, so others who try to assist won't know the problems you're trying to solve.

You're not going to find a "break-out box" like the one mentioned in the ETM's. Make sure that the EGS (trans computer) is grounded properly. Make sure the engine grounds are good. (Should be one at the back of the valvecover, and one at the passenger engine mount, if I recall correctly).

When you were testing voltage for that "fuel rate gauge", where did you take your ground from?

{Case in point: if this were the original thread, I'd click on the ETM link I gave you, and quickly check those pins for you.....but the link isn't here.....}

I'd suggest you return to the original thread, so other people can follow the questions at hand.

Thank you for the advice! I came back and the threads were thankfully merged. I don't remember my reasoning for creating a new thread, but it was faulty. Yeah I see that these boxes are worth more than gold and I won't be finding one. I will check the grounds, the ETM tells me what to measure with an ohm-meter. I've ordered a near identical ECU from a 1990 535i for $22, and swapping that in to see if the ECU is putting out the low voltage and causing the failure. I found a DIY break-out box for $89 but I would need to harvest one of the ECU's female connector and get fine solder and a fine needle point soldering iron. That way I could follow the trouble shooting guide word by word. Replacing the computer is worth a shot though!

As for where I took the ground from, I touched a bracket that was underneath the dash. The manual didn't say which ground to use, but it is probably most accurate to touch a grounded pin off the TCU plug?

[walczyk]

Update: I was able to remove electrical tape and separate the ECU connector housing to get behind the connector to test all the pins. Turns out in the 55 pin motronic 1.1 there is no wire for pin 11 and 21. There's no wire there. This clashes with the ETM manual with says to check voltages there. I'm sure the 89 would have it, as it was motronic 1.3. Does anyone have information? I will try to search the forums.

[walczyk]

Another Update: for the '88 635csi wires that were originally 11 and 21 on the 35 pin connector like in the '87, on the ECU are now 32 and 6 in the 55 pin connector. These are the the present fuel rate output and the engine speed output. Also I was able to drive the car and verify the fuel rate gauge is not working. Fingers crossed for the ECU being the culprit. This would have been after swapping transmissions and tearing apart the interior and leaving it for years. Poor '88

[bmwdirtracer]

Are we sure that the Transmission control module matches the transmission, and the year, and the rest of the items it's connected to?

Sorry, I understand this must be very frustrating. Have you considered swapping to a manual trans? Won't be easy, but the results are wonderful.

[walczyk]

No worries. Yeah the TCM is original at this point. I swapped them back once the light remained on. The fuel rate gauge is confirmed not working and that can cause the limp home mode. I think I'm tracking it down. Swapping to manual is not cost effective at all. We have a '87 M6 that was smashed in the drivers front, would be cheaper to take that to a body shop and have it straightened out lol. Thanks for the replies. A few more days til the used computer comes from (it's from a E28 but its 55-pin motronic so it should work), hopefully that solves it.

[walczyk]

Another Update: for the '88 635csi wires that were originally 11 and 21 on the 35 pin connector like in the '87, on the ECU are now 32 and 6 in the 55 pin connector. These are the the present fuel rate output and the engine speed output. Also I was able to drive the car and verify the fuel rate gauge is not working. Fingers crossed for the ECU being the culprit. This would have been after swapping transmissions and tearing apart the interior and leaving it for years. Poor '88

ITS FIXED! It was the ECU this whole time! I shouted for joy for a straight 30 seconds. Now I NEED to find those floor mats, the foam insulating ones, they've disintegrated. Would you know where to look?

[shogun]

Congrats. As for the parts, ask in the E24 forum, some members have parts E24 https://www.bimmerforums.com/forum/f...976-1989-(E24)