SC400 Gross Speedo Error

[STORMINORMAN]

1993 SC400, 95K miles, starts, idles, shifts & drives great (IMHO). Standard sized 225/55-16 tires, no CEL's (but the check engine light does come on and then turns off at start, as it is supposed to)...

Speedo consistantly 10-15 mph lower than actual speed! Dash is bright, needles all move smoothly, no indication of any display problem.

I've been advised that the OBD I system should not have any "stored codes" if there is no CEL, so the dealership's $ugge$tion that I "bring it in for a diagnostic (at $120!)" doesn't make much sense to me: I can buy an OBD 2/OBD I scanner with accessory cables for Toyota/Lexus, Ford, GM, etc. for $199, but what good is that going to do if there are no "stored codes" to begin with?

I'm advised that the VSS1 & VSS2 codes are 42 & 61 respectively...

1st posting, I'm a Newby, 1st Lexus V8 as well. Possibly my only vehicle with OBD I as well, I know for sure none of my other cars/truck are OBD 2.

Just trying to avoid a repeat of the conversation I had Friday which began with "Sir, my radar had you going 71 mph in a 55 zone..." and my speedo was reading between 55 and 60!

Thanks in advance for your kind consideration and any ideas, other than the suggestion I just buy a GPS and use it, most likely more accurate as well?

 

[4U2QUIK]

What transmission are you running? If you replaced the transmission or rear end on the car with one out of certain cars will make a 10 - 15 mph difference.

 

[STORMINORMAN]

To my knowledge...

... my 1993 SC400 is totally stock. Not the original owner but have absolutely no reason to think it is anything but..

Hooked up a Smartphone/ Speedo-GPS app and found that the normal varience between the indicated speed and actual (via the GPS) was about 10 mph LOW @ an indicated 20 and closer to 15 mph LOW @ an indicated 70 (as in 84-85 actual mph!). 2000 rpm is pretty close to 60 mph, 2500 is a little over 70, but it certainly depends upon whether I'm going up hill or down.

No codes. Odometer seems to be pretty close (it's about 180 miles between our homes) so that would kinda' rule out a different differential or tranny, would it not? Gas mileage is still mid 20's based upon odo readings and how much is local vs. freeway driving.

Another forum had a suggestion that the "speedo motor might be worn out" but not usually at what they described as "those low miles". Could this be a reference to rhe VSS1 in the dash? Is there actually a motor that drives the needle for the speedo?

I'm still stumped!

p.s. Bought a Magellan GPS so the speeding ticket exposure should drop back to normal, whatever that is!

 

[EyesofThunder]

Are the shift points normal and speedo wrong?

Do you have access to a scantool to read the ECU's #'s vs what you read on the speedo?

I have heard the GS400's have the speedo run off the front wheels' ABS sensors.

Got any ABS faults?

a dirty ABS sensor might be sending erroneous data to the ECU/Speedo.

It isn't reading KM/H when it should be reading mph is it? That is more like 30 mph = 50 KM/H, 50 = 80, 100 = 160, etc. Approx of course.

Another trick, go to 80 mph (real speed) in non OD high gear (assuming 1:1 ratio), and tach should be reading gear ratio with one decimal place off. Ex: 80 mph = 3270 rpm would be 3.27's. or 80 mph = 3920 rpm, 3.92's, etc.

I am not advocating speeding, if you need to, use 40 mph and double the #'s....

 

[STORMINORMAN]

Shift points seem very normal. Odo readings and gas mileage seem right. 1993's are OBD1, there are no codes being thrown and CEL cycles on & off at startup as it is supposed to. No way it is kph vs. mph (60 mph being 100 kph) as it is reading LOW by 10-15 mph.

Still stumped!

 

[EyesofThunder]

Is it off the same at all speeds, say 10 mph reads 15 high, 60 mph reads 15 high?

It does return to 0 when stopped? If trip odo is good and speedo is off there must be an issue in the cluster. They are usually fed off the same signal.

 

[samsdad]

This is a common problem with all Toyota electronic speedos. The odometer will still read correctly and you won't get a VSS code. You will however have speed all over the place and that's if you have speed at all. Remove and dismantle the cluster, check for and repair the burnt out tracks on the speedo PCB. I've even used data cable to do this

 

[STORMINORMAN]

Cluster removal vs. cluster....

This is my 1st Lexus, and although I was ready to take on an interior passenger door panel removing the instrument cluster (when everything except the speedo is working just fine) might be more of a bite than I'm confident I can chew...:nervous:

Unless...

Any good cluster removal links, etc.? How big a job is it?

Thanks!