CEL: Vehicle Speed Sensor

[BimmerToad]

Has anyone else gotten a CEL from the 'vehicle speed sensor'? I've been running the car for almost half a year with the light on, and I doubt it has been affecting performance (but hope it is :D). The engine is a OBD1 M52 2.8 with a 413 ecu and the chassis is a 4/90 318iS. I've looked over a few wiring diagrams, and am clueless where the DME recieves the vehicle speed signal.

I've gotta go to the CA Smog ref in order to renew my registration, and I know the car won't pass if it throws a CEL. :?

When ever I zap the light, it comes on after a few hours of highway cruising. My MPG gauge doesn't work as well, which might be tied to the vehicle speed code. (It goes from 60mpg -> 0mpg with the lightest touch on the throttle)

Experience/Suggestions would be a great help.

[Jason89i]

e36 (413dme) gets the vehicle speed signal patched to the x20 harness pin #14 (blk/wht) wire. i dont have the e30 318is wiring diagram, so i cannot comment on pin number at the c101. my 86es did not have the vehicle speed signal at the c101.....but it can be found on the harness from the cluster to the obc. however....the 88is, 89is, 89i, and 90m all had the signal at c101 pin 14 (blk/wht) but the 90m wiring was a different color.

if you scan the ecu, it will tell you whats up.

but lack of vehicle speed signal will not trip the CEL. are you sure the o2 sensor is working properly?

cheers, jason

[BimmerToad]

Thanks for the response Jason.

I've scanned the ecu before, and have gotten the error message of 'vehicle speed sensor' :? I've even had a motronic 'guru' ride shotgun in the car with his scanner hooked up to the diagnostic port, and the only message that came up was 'vehicle speed sensor.' The fact that performance isn't affected might suggest that it's not the problem throwing the CEL ...

When I get the car back, I'll disconnect the o2 and see if the idle is affected. I too have not seen many specific wiring diagrams for the 318iS, which makes it kinda difficult to know where to begin. I'll also check to see if pin 14 is hooked up.

[Jason89i]

Thanks for the response Jason.
The fact that performance isn't affected might suggest that it's not the problem throwing the CEL ....

performance will be affected. ecu needs vehicle speed signal to adjust idle (higher when car is moving). i have been told (very reputible source....roundel writer) that the vanos actuation and fuel/spark maping at w.o.t. near 6-7k will also be affected. however, i doubt it will throw a CEL.

When I get the car back, I'll disconnect the o2 and see if the idle is affected. I too have not seen many specific wiring diagrams for the 318iS, which makes it kinda difficult to know where to begin. I'll also check to see if pin 14 is hooked up.

dont just disconnect the o2 sensor. run a voltometer and check all the o2 relay wires and o2 sensor outputs. i bet the green wire at the relay isnt getting power.
'
cheers, jason

[NickP]

Alo this CEL limits your red line to 6000 or so RPM - its most likely that the speed sensor wire isn't properly hooked up.

[BimmerToad]

Unless my tach is off, the engine has no problem revving to the software limiter, 7k.

Jason: You might be right about the speed signal affecting performance at the top end. When I dyno'd the car, the engine would develop a vacuum from ~3k - redline, which suggests the engine was not under full load, even though it was under WOT. The dyno operator was also suprised that I hadn't experienced any pinging running a fairly lean AA cam chip. (15-14.5 A/F at WOT) Maybe because the ecu was backing away from a more aggressive spark/fuel maps due to a lack of a vehicle speed signal.

I'll definately check the o2 wires when I get the car back. Maybe my vehicle speed sensor gets lost in a bad instrument cluster connection.

Thanks for the ideas, now I just need my car back :?

[Ryan Stewart]

My car threw a CEL, seemed to run a little richer, and wouldnt rev all the way without this hooked up. S50B30 with 413 with turner chip.

[NickP]

My car threw a CEL, seemed to run a little richer, and wouldnt rev all the way without this hooked up. S50B30 with 413 with turner chip.

That was my experience with the same setup; then I fixed the wiring :) no more check engine light.

I've heard a couplereputable sources say that in this limp mode the motor runs rich except when staying at a high RPM when it leans - why or how I don't know.

[BimmerToad]

Update:

I've been working off of '90 325 wiring diagrams, which hopefully (*fingers crossed*) are a good representation of the '91 318iS wiring.

After ripping apart part of the dash and checking continuity of the vehicle speed sensor wires, I think I've found my problem. C101 - ECU has continuity, C101 - instrument cluster has continuity, but I believe the instrument cluster is partially faulty.



There is a burned out resistor on the speedo gauge, which I believe could be causing my vehicle speed signal to become corrupt. I believe the diff signal goes to the cluster as speed input, goes to the speedo gauge, and becomes spliced into a signal which feeds the speedometer gauge, and a vehicle speed signal which goes to the B10 pin of the cluster, which in turn goes to pin 14 of the C101 (blue/yellow). (From the C101 it goes to the ECU, then runs back to C26 of the cluster to run the MPG gauge, and splices to the OBC.)

My speedometer works, which suggests the speed input signal is reaching the cluster, so hopefully this burned out resistor is after the vehicle signal split, and has corrupted the signal on its way to the ECU.

The resistor failed by shorting itself out (X ohms -> 0 ohms) which means a signal is going to the ECU, but it is probably out of the processing range of the ECU. This might explain why the CEL would not be thrown until after almost an hour of driving.

I have two options: try to solder a new resistor in the burned one's place, or replace the speedometer gauge. If I can't replace the individual resistor ... does anyone know how to 'correct' the odometer mileage? ;) (i.e. roll back, or roll forward until mileage resets)

[Mohammed_A]

Did this sort out your problem?

[MIKe30]

however....the 88is, 89is, 89i, and 90m all had the signal at c101 pin 14 (blk/wht) but the 90m wiring was a different color.

Jason, does this mean 90 325is doesnt have it at pin 14 on c101? Hoping to deal with this potential issue before I start/run the car. Rather not find out on the dyno where my a/f ratios are going to be super important in getting my dme fine tuned...

[Jason89i]

yes, the 90is has the road speed signal in the same location. cheers, jason

[MIKe30]

yes, the 90is has the road speed signal in the same location. cheers, jason

good, so no re-wiring worries as long as my harness is correct. Thanks.