No Start Issue, 1990 S52 Swap

[e30austin]

RealOEM lists that DME coming from a 10/1996 M3 (S52 OBD2 car). So, unless the car is running on OBD2 engine management (hard to tell from the picture, but I'm leaning towards "no"), that is the incorrect DME for the car...

[zcenterb]

Well the engine is a 97 S52 so that makes sense, but I expected it to be a silver label 413... the engine has an S50 intake mani and no special key or anything (no EWS) so I can’t imagine it’s running OBD2, but the Ecu says flash on it so it could have been just removed. Like I said, the wiring is about the only thing I haven’t touched on this car...

[e30austin]

If it's an S52, why did you expect it to have a silver label 413? S52 never ever came with that, in stock form. You need to first find out if this car is running OBD1 or OBD2 engine management to start.

If it has an OBD2 DME, I would gather that it's on OBD2, but without more pics and/or info, nobody can be sure.

[zcenterb]

I was under the impression the most S50s and some early S52s had the 413. Thanks for the info.

There isn’t much at all for records from when the original swap was done, but as far as Obd is concerned, it appears to be 1. I’m pretty sure there is only the single oxygen sensor because there isn’t a cat on the custom exhaust, the secondary air has been plugged up, it has the s52 valve cover but the s50 intake mani and fuel rail. I looked at the temp sender on the head too and replaced it with a new e30 sender rethreaded for 24 valves, but there was one other plug in front of it, closer to cylinder 1. My old obd1 s50 has 3 sensors here originally so not sure. What else is needed to more accurately tell? And what pictures specifically? Thanks for the help

[pazi88]

Check if it has front or back mounted cps. Obd2 has crank sensor at the back of the block. Also the coolant temp sensor in obd2 has 4 pins and it's round.

[zcenterb]

^ Ill check tonight. On the sensor, the one I replaced was just a single wire brown top sender unit for the cluster gauge, but the other one was larger with a black plug and more wires. I didn’t unplug it to look but I’ll check that tonight too, thanks for the heads up.

[pazi88]

^ Ill check tonight. On the sensor, the one I replaced was just a single wire brown top sender unit for the cluster gauge, but the other one was larger with a black plug and more wires. I didn’t unplug it to look but I’ll check that tonight too, thanks for the heads up.

So obd2. And that ecu works fine for that. It's not for s52 but according to the text it's probably flashed with ms41.2 sw. Even if it's not, the engine will run fine. Just missing some power. And you can't even plug in obd 1 ecu to obd 2 harness even thought the connectors are the same. There is small key preventing that.

EDIT. The starter and temp gauge issues are not related to the ecu. The temp gauge and starter will work just fine even without ecu plugged in. If the starter just clicks, there are three reasons for it. 1. broken starter. 2. not good enough ground contact. 3. not good enough voltage for 12v input. Could be bad wiring or battery.

Jumping temp gauge would suggest grounding issue. Normally caused by bad engine ground strap. But it could be even just corroded negative battery terminal. But those could be just two separate problems that are not related to each other.

[zcenterb]

Thanks for the help. The battery is brand new as is most of the leads. Normally I would consider them totally separate issues but because I have replaced nearly everything in each circuit, that leads me to believe they are either related or are symptoms of the same problem somewhere. I am going to go back through all my ground connections and wiring to the starter and see if I can get anything. I mentioned it in one of my previous comments in this thread, but at one point I found a melted ground wire going to the oil sensor on the oil pan, which I replaced, so I am still leaning towards the issue being electrical in nature. Any other suggestions are appreciated.

[Metallated]

From the cars I've looked at it seems auto cars have a relay connecting to the transmission wiring so the car can only be started in park.

Check if there is a relay with a blue and brown striped wire between the yellow and black starter wire and where it goes into the harness. It's of the upper left next to the plugs:



I remove this relay and just made the start wire go straight to the starter.. not sure why this would sometimes work for you and other times not, but worth checking.

[zcenterb]

Hey, thanks for the heads up. I actually did do that part of the wiring when I swapped the engine over so I know that is good, but I will double check it, thanks for the reminder.

An update: spent an hour or so poking around this afternoon with a multimeter. It doesn't seem to be an idle power issue, power at the starter constant hot (and power block) is the same as the battery voltage, 12.35-ish. Haven't checked the keyed power yet. Battery is months old.

No resistance between the chassis and the battery ground, but I did find between .4 and 1 ohm of resistance between anywhere on the the engine (including the starter ground post) and the chassis, so across the engine ground strap. Got the same reading going from either side of the ground strap posts. Half an ohm is not much resistance, but on a 9" ground strap that seems high for just wire... going to do a more thorough run through tomorrow.

Pazi88, confirmed the 4 wire coolant sensor up front.

[atldohc]

starter ground post? i've never seen a starter with a post for ground. they will usually ground through their body as they are directly bolted to the trans or engine block. if you have batt voltage on the big lug and the signal lug when starting, the starter should be cranking over. this is assuming there is a good ground and the pos batt cable to the starter is not corroded and has sufficient gauge to handle the high amperage requirement of the starter motor. other possibilities i could think of are maybe there are some damaged starter or flywheel teeth, or a mismatch of parts? also the engine itself could be dragging for some reason, though i doubt it since you'd probably have other symptoms when driving. i would try hooking up some jumper cables from the starter body to chassis and see if it makes a difference. also you could try turning the crank by hand and trying to start again. if there are some damaged flywheel teeth just in one section it may start when turned to a different part of the flywheel.

[zcenterb]

Sorry, meant the lower most post. Was thinking ground for some reason but I believe it’s keyed hot.

Anyway, brand new bav restoration engine ground cable installed with no change. Need to spend some time with it this weekend at some point.

[zcenterb]

So interesting discovery today for anyone interested... Car was still clicking as usual, so held the key to start position for awhile and got smoke coming of the stainless clutch line, indicating the starter was trying the pull ground through the transmission rather than the engine ground... Verified this by running jumper cables from the transmission fins to the chassis and the car fired right up.

So, I installed a long ground strap from the transmission bell housing to the chassis and the car starts right up very strong and the temp gauge no longer jumps around and is pegged right at the halfway mark. The only baffling this for me is why the ground was not connecting from the transmission to the engine? When I did the clutch we used a bunch of never seize compound on the bell housing threads but I can't imagine between those and the metal gasket that there wouldn't be continuity between the trans and engine block, so I have no idea... Regardless, I am yet again considering this a solved case... Thanks everyone for the help.