Hey all, had a weird random issue pop up on the G37 today if anyone has dealt with this. I dropped my son at school and had the car running. Came back and noticed the slip light was on (not the traction control or ABS light though), and then while coasting out of the parking lot the car moved itself from manual 1st gear into D and slammed into gear harder than I've ever felt (assuming when the ECM loses comm with the TCM line pressure defaults to the highest value). The car would not go back into sport mode or manual mode. Scanned codes and it pulled U0100-00. No other codes are present or pending. I shut the car off at the stop sign, and it did start back up but didn't immediately respond to the press of the start button, like it was lagging. It did start with one push, just took it like 5 seconds to realize I pushed the button.


Brake fluid level is perfect and pads have plenty of meat still. BCM is bone dry and sunroof drains were taken care of a long time ago, so I don't believe that's causing a rain related issue. The car has been in the garage all week and was dry. Battery is new and was replaced about a month ago.

Anyone else run into this or have any ideas where to start looking? I'm planning to check grounds and fuses, however the code didn't reset after clearing so I'd assume no blown fuses as that'd probably result in a no comm every time.

 

This seems to be a BCM issue as I have had the same issue. One min it's an issue and another the car fires up without drive problems. The tell tale sign is the slip light being on. However mine never shifted hard but would not go into DS and once even lost the speedometer reading until it shut off again.

Your brake fluid level has zero to do with this issue. Yes I have read the myth and someone that thinks of the brake fluid is low it is causing problems. If you believe that I also have a tale to tell you about how the G really has another 300 HP hidden and all you need to do is press the gas pedal 40 times in a min!

Hope it clears itself. As the dealer will just want to replace your BCM for thousands of dollars.

 

Yeah I only mentioned brake fluid because the G37s we rented back in the day would throw slip/VDC and ABS lights with low fluid so I just ruled that out to be safe. At least the BCM is easy to replace with a used unit to get reprogrammed if that ends up being the issue, it's just odd because the car is garaged and the sunroof drains have been fixed before they became an issue, so no water ever got to the BCM. Either way that's easier to swap than a trans if that's the case.

 

Well, the U0100 hasn't come back yet, but two days ago the car threw itself out of manual mode and into D again with the Slip light on. This time it did let me go back into manual mode immediately, but threw codes P0725 and P1721 for the engine speed input circuit and vehicle speed signal. In talking with a few locals that build and work on Z32 - Z34, and V36 platforms exclusively, they are both concluding that the valve body/TCM is starting to go on the car. As soon as the car throws those codes again, I'm going to bring it in for diagnosis as the issue is very intermittent right now (almost 500 miles since it did it last).

That being said, if it is the valve body, I have a few options and would love input on weighing them. The car is paid off, this wasn't an expense I had planned for, so while I CAN afford all of these options, cost is a factor in the decision. Ideally I want the car to last me another 5-7 years of being a daily.

I can either:
a) - Just get the valve body replaced (roughly $1700-1800 after labor and parts)
b) source a low-mile used transmission and have it installed (about the same cost if I can find a good deal on a used unit)
c) Get my current transmission rebuilt completely with upgraded clutch packs, new valve body, etc - way more expensive, as I'd want to upgrade the converter too if I'm going this far
d) find a shop that's comfortable installing a VR30 trans in the car, ordering a shorter driveshaft, etc. Unknown costs as none of the locals here have done this swap, so it will be uncharted territory for them so while they have the skillset, the labor costs are unknown for this.

What option would you personally choose for your cars?

 

e) do a VR trans swap

 

I see that you listed the VR swap as option d

I only suggested it since you were trying to upgrade the current weak one (compared to the VR trans)

 

Yeah, the biggest unknown with the VR swap is just the cost. I realistically don't want to drop $5k worth of labor when I could get a fully built trans in for that, and of course the local shops that are capable of that swap here won't give me a straight answer on price as they've never done one before. I'm thinking I might just try and find a low mile used unit to start with before I decide what to do.

 

You could swap it yourself if you really want

 

I'm trying to understand what the issue is with your current transmission

 

I haven't done the swap so I can't speak that intelligently on it, but there is a channel and the kid goes pretty in depth about how to. I will post the link below for his page. In the past few months I have seen it popping up more on more on youtube. I was going to do the level 10 for myself in the future, but it looks like the VR trans is the best bang for your buck. I am sure if you reached out to him he probably would walk you through what you need and the cost. Worth a shot to put the trans issue to bed for good.

https://www.youtube.com/user/sjaydeeem/videos

 

I vote for a used trans swap. Keep it simple. Built or VR swap is pointless without boost. nothing wrong with stock honestly. A basic tool set with at least 3ft worth of extensions worked for me. Just get her up high in the air safely or pay someone. $850+/- labor.

 

I spent a total of about 6-700 for my swap. Got a good deal on a used unit and installed it in my driveway in 2 days. You will need those basic tools, extensions, AND a transmission jack will save you a lot of swearing when you line up the tranny to bolt it in.

Any hiccups on yours? Mine sometimes doesn't want to go into 3rd when I try accelerating quickly after a stop sign. Read on the Q50 forums that it's a known issue for some.

 

Once codes pop back up again I'll be able to get freeze frame data and diag it a bit more, but we're suspecting it has a valve body going bad or the TCM, as it's had no-com issues and throwing valve body/TCM related codes. It hasn't thrown a P0720 yet, and the trans shifts fine when codes are cleared, but if the valve body is going, I'm wondering if something in the trans is causing it, and a larger failure is on the horizon. I'm not buying anything until it's been diag'd thoroughly, but since this is such an intermittent problem, it's been really hard to track down the source. I'm just trying to be prepared with the best option if a worst-case scenario ends up and this is an issue with the trans itself. i'd love for this to just be a TCM going out as I could swap that myself, but I don't have the space to do a full trans swap nor the time with 2 little kids to do the work myself on that much labor.

 

If you decide to do it, I suggest watching a bunch of youtube videos of other people taking out/putting in their 7AT's for their G37's (doesn't have to be VR trans videos). Hardest part is balancing the transmission while trying to bolt it in. Some of the bolts are high up there in the tunnel and you have to really wrap your arms around the bell housing to bolt them in.

 

The VR trans has slightly shorter gears so it helps give some torque/power under 3K rpms. It also shifts really solid compared to the one found in our cars. I'm NA but I find that it's my favorite mod on my car.