Speedo Under-reading.... a puzzle
Discussion
So after the gearbox was changed recently the speedo started to significantly under-read. An indicated 70 mph was actually 93 mph for instance.
The specialist took the car back and was very helpful in looking into the worm gear fitted in the new gearbox and to the transducer fitted. Both were right for the car (a '92 4.3). The old transducer from the original gearbox was also fitted - but still the same problem.
The air gap on the sensor is said to be fine, but will be checked again by the specialist.
This is a puzzle. The speedo was fine before the gearbox transplant, but now isn't. We are all thinking that the speedo may need to be re-calibrated (although it was overhauled in 2002 by another well known specialist). Could it be coincidence that the speedo has failed to be accurate just as the gearbox was changed - or am I missing another cause here?
Thanks ....
The specialist took the car back and was very helpful in looking into the worm gear fitted in the new gearbox and to the transducer fitted. Both were right for the car (a '92 4.3). The old transducer from the original gearbox was also fitted - but still the same problem.
The air gap on the sensor is said to be fine, but will be checked again by the specialist.
This is a puzzle. The speedo was fine before the gearbox transplant, but now isn't. We are all thinking that the speedo may need to be re-calibrated (although it was overhauled in 2002 by another well known specialist). Could it be coincidence that the speedo has failed to be accurate just as the gearbox was changed - or am I missing another cause here?
Thanks ....
Try lifting the ecu and loom out of the foot well and take it for a drive...sounds odd, but mine does this periodically.
At first I thought it was the earthing point for the dials, but i think its my wiring loom starting to decay as I can give it a shake and it starts reading correctly again instead of reading 50 when I'm doing 70mph.
At first I thought it was the earthing point for the dials, but i think its my wiring loom starting to decay as I can give it a shake and it starts reading correctly again instead of reading 50 when I'm doing 70mph.
Steve Heath has a gizmo on his website for checking the speedo sensor.
http://www.shengltd.com/catalog/
HTH
http://www.shengltd.com/catalog/
HTH
Does the pre-cat arrangement have a splitter to feed the ecu? If it does I'd just disconnect it & see if your speedo is OK with wiring from pick-up to speedo (try new lengths if necessary)?
The factory connector at the gear box end might be a suspect ~ mine was pretty yucky (before I cleaned it honest). Plus any of the routing where there might be chaffing of the insul?
It does sound odd that you are getting a consistent reading though, maybe the change of box requires a different colour drive to match it (20->24 teeth IIRC blue->red colours).
The factory connector at the gear box end might be a suspect ~ mine was pretty yucky (before I cleaned it honest). Plus any of the routing where there might be chaffing of the insul?
It does sound odd that you are getting a consistent reading though, maybe the change of box requires a different colour drive to match it (20->24 teeth IIRC blue->red colours).
Troogly said:
The old transducer from the original gearbox was also fitted - but still the same problem.
The air gap on the sensor is said to be fine, but will be checked again by the specialist.
Bit confused by this, are you saying that your '92 car has an LT77 gearbox with transducer AND a diff sensor?The air gap on the sensor is said to be fine, but will be checked again by the specialist.
Thought this arrangement was only used on 5ltr cars with LT77 box.
Hi - thanks for all of your suggestions.
I gave some duff gen regarding the diff sensor.... being an older Griff it doesn't have one (thanks AntonyJ).
The speedo needle has also picked up some spurious reading. Very occassionally, for no reason, the needle will whip around to a high reading when the car is stationary (not all the time, generally infrequently).
It seems it is picking up a spurious signal from somewhere, or a dodgy earth perhaps? I might be looking at a re-calibration job here..... just so strange it coincides with the gearbox change.
I will look into the wiring continuity next step.
thanks again
I gave some duff gen regarding the diff sensor.... being an older Griff it doesn't have one (thanks AntonyJ).
The speedo needle has also picked up some spurious reading. Very occassionally, for no reason, the needle will whip around to a high reading when the car is stationary (not all the time, generally infrequently).
It seems it is picking up a spurious signal from somewhere, or a dodgy earth perhaps? I might be looking at a re-calibration job here..... just so strange it coincides with the gearbox change.
I will look into the wiring continuity next step.
thanks again
When my SW speed packed up all together I sent it to Speedy cables, who unfortunately could'nt repair it. They said the IC had failed.
Fortunately I took it to a local industrial electronics company who sourced the IC (Speedy said they were'nt able to source it) and repaired the speedo for me.
Still have the info on the IC if it reaches that stage.
Would check the connection at the transducer as spendy suggested, its the same type of connector as the TPS.
Fortunately I took it to a local industrial electronics company who sourced the IC (Speedy said they were'nt able to source it) and repaired the speedo for me.
Still have the info on the IC if it reaches that stage.
Would check the connection at the transducer as spendy suggested, its the same type of connector as the TPS.
I dunno Ant, if its going full scale sounds more like a short somewhere to me? I'm not clear on the pre-cat feeding the speed signal to the ecu??? I'd suspect that 'splitter' or a in intermittent short in the wiring as a guess of where to start looking if it was my problem. I could understand a bad connection giving high resistance, bu cant see why that would give the max mph deflection?
Gassing Station | Griffith | Top of Page | What's New | My Stuff