Intermittent Speedo

[donatien]

Odd one this, after starting up and off the speedo plays up a bit.

By dodgy I mean it reads zero, so I have to go off and read the revs which are fine (though i guess not techincally legal). It's happened about 3 times in last week (I drive it regularly). Are there any obvious loose connections I can check, I really don't want to have the dash out if the issue sits behind some cable in there.

On all occasions after 5 mins or so it's all back to normal, revs match speed. Got me thinking - does the speedo get a cable feed or is it fed via the ECU?

Cheers

Dave

[onny]

Could be a loose wire but more likely to be a faulty speedo sensor. They go faulty regularly. The sensor is located on the rear diff. They cost about £75 and fairly easy to fit if you can get under the car. There is no simple way to detect if the sensor is faulty but a sympton of dodgy sensor is when you slow down at intersections, your car will start to stall.

[lone granger]

onny said:

Could be a loose wire but more likely to be a faulty speedo sensor. They go faulty regularly. The sensor is located on the rear diff. They cost about £75 and fairly easy to fit if you can get under the car. There is no simple way to detect if the sensor is faulty but a sympton of dodgy sensor is when you slow down at intersections, your car will start to stall.

Cheers - mine just stopped - instrument zeoes when ignition on but has just stopped recording - fortunately not using car, however I can look at sensor when i check for fuel leak from hoses replaced by TVR dealer...

Anyone know how to ''scope'' the sensor to check if it is that or a mouse been through sensor wire - guess i could take dash out and meter back to sensor..

[donatien]

onny said:

Could be a loose wire but more likely to be a faulty speedo sensor. They go faulty regularly. The sensor is located on the rear diff. They cost about £75 and fairly easy to fit if you can get under the car. There is no simple way to detect if the sensor is faulty but a sympton of dodgy sensor is when you slow down at intersections, your car will start to stall.

Nope car behaves 100% normal, no jerks or stalls

[Ozstyle]

If its a T5 gearbox then it could be the gap between speed sensor tip and toothed pick-up wheel is too big. If checking the wire connection worth cleaning the tip and adjusting the gap.

[rick.e]

If you really want to go to the trouble of checking the sensor, this can be done easily. The sensor outputs 12 volt pulses and you can check these by jacking up one rear wheel, remove the ECU plug (passenger footwell) and check on pin 6. Even if you don't have a scope, a voltmeter will show it up if the wheel is turned by hand.

Problem is if yours is intermittent as mine was, you may catch it in one of its working moods, so no guarrantees. What is virtually guarranteed is that it is the sensor as they do go quite regularly.

Oh (added in edit), I never experienced any jerkiness, and can't see why that would happen. I guess the theory is that if your foot is off the accelerator AND there are no pulses, then the ECU will think the car is stationary and weaken the mixture slightly to set the engine to idle (as happens when you stop at a junction). On the overrun however, there is no fuel going in anyway so it should't affect running. The only time I could see it "start to stall" is if you were running at tickover, ie foot off the accelerator but not overrunning.

Edited by rick.e on Monday 28th August 10:47

Edited by rick.e on Monday 28th August 10:50

[rick.e]

donatien said:

.

On all occasions after 5 mins or so it's all back to normal, revs match speed. Got me thinking - does the speedo get a cable feed or is it fed via the ECU?

Cheers

Dave

For what its worth, my symptoms were identical, and on several consecutive mornings the speedo burst into life about 3 miles from home within a spread of about 200 yards! I put this down to the sensor (a Hall device) warming up. Replacing the sensor cured it.

[onny]

I had the same problem, the car didn't stall at first and the speedo would jump to live after a while, but the stalling did come after a couple of months.

If its the sensor then you have to get the right one. There's 2 types, one with 4 teeth and one with 8 I think. I think my one was 4 but you need to double check. if you buy the wrong one, the speedo will either double or half the actual speed.

[rick.e]

onny said:

There's 2 types, one with 4 teeth and one with 8 I think. I think my one was 4 but you need to double check. if you buy the wrong one, the speedo will either double or half the actual speed.

What are you smoking? Are you sure your car is a Griff?

The teeth are on a ring on the driveshaft where it enters the diff, there are a hell of a lot more than 8 and the only way you could change the number of teeth would be to replace the driveshaft. There is a good pic in the bible for those who have it. (The system works on approx. 4 pulses per revolution of the rear wheel so if you multiply the final drive ratio by 4, that tells you how many teeth there are.)

This is all irrelevant as far as changing the sensor goes. The sensor itself has no moving parts and just sits on a bracket counting the teeth as the driveshaft rotates. There is no way that the wrong sensor would double or halve the speed.

Edited by rick.e on Tuesday 29th August 08:36

[shpub]

Actually there is. The LT77 boxed Griffs used an internal sensor and there are 4 and 8 and other versions which will screw up the speedo if the wrong one is used. The later T5 box cars either used a slotted wheel on the propshaft or the slotted disc on the diff.

[rick.e]

shpub said:

Actually there is. The LT77 boxed Griffs used an internal sensor and there are 4 and 8 and other versions which will screw up the speedo if the wrong one is used. The later T5 box cars either used a slotted wheel on the propshaft or the slotted disc on the diff.

Donatien's Griff is a 1999 (evident from his profile) and therefore T5 by several years!

Edited by rick.e on Tuesday 29th August 09:48