If someone should have the misfortune to have their tachometer fail as mine did I hope my experiences of fixing mine may help.
The tacho stopped working but the car continued to run normally.
Possible problem areas / components
1. Tachometer
2. DME/ECU
3. Central Informer (rarely cause any problems)
4. Flywheel rotation/speed sensor (Car generally will not start if this is not working)
5. Broken wire between any of the connections.
6. Corrosion in any of the plugs (None found so discounted)
Tools required
Oscilloscope/Multimeter tester, wire of reasonable gauge and length, insulating tape, plastic prying kit used to remove tacho and split T16 plug, bags of patience. I bought a relatively cheap (the real ones start at £1k) combined Oscilloscope and Multimeter not ideal but it sufficed to identify the fault.
The tacho is fixed in the dash with a rubber gasket. It can be removed from the dash by first gently prying under the rubber retaining ring then teased out past the steering column shroud. I left everything in place as I think the steering wheel has to be removed to remove the top shroud. It is not easy and patience and washing up liquid plus the prying tools will enable the tacho to be removed. This is I believe only possible with the non OBC tacho as it is slimmer.
I wrongly thought that it was the tacho and sent it to be repaired/upgraded by Tore Bergvill. This was returned fully tested and cleaned etc. thus OK.
So further checks were required.
By this point I had removed the passenger seat and left the returned Tacho disconnected.
Whilst Tore does not repair DME’s he was most helpful in directing me where, how and what to test.
I first checked fuse 18 (power to tacho) which was OK.
Power check on tacho plug at Pin 7 (live) via Pin 2 (ground) gave +13v so OK.
Wiring checked, with battery disconnected for safety, from pin 6 on DME plug to pin 4 on connector designated T16 (on wiring diagram) then from Pin 4 (T16) to Pin 6 on the Tacho Plug. All the checks gave continuity so the wiring was all OK.
The car would start straight away and runs normally so the flywheel rotation/speed sensor is O.K.
Mike at Zentrum checked the central informer by substituting a known good one this had no effect on the tacho so again OK.
Which left the DME/ECU as the problem component.
To confirm it was the DME I needed to test the square wave generated between pin 6 (feed from DME) and pin 2 (ground)
The only way to test the pulse successfully is to use an oscilloscope, so my learning began. I needed to be able to check for a square wave with an amplitude of between 0v and 12volts but no less than 11volts at its peak as per sketch below. This amplitude is required to trigger the tacho anything less will not work. The wave form frequency will also increase and fall as the revs rise and fall. With the oscilloscope I could get a wave that was almost square but with only a peak amplitude of 3.03volts average. Tore said to test the circuit with the tacho attached hence the wire with a loop fashioned such that It could safely (by insulating) be attached to the tacho pins and the oscilloscope and get a reading. Still only 3.03volts.
So the only solution now was to get the DME/ECU tested and repaired.
This proved slightly more problematic than I had envisaged. I contacted quite a few companies who were either only interested in mainly current and therefore regular work or didn’t have the capability to check and repair the unit. I eventually contacted ATP electronic developments based in Cannock Staffordshire. They were initially unsure if they could diagnose then repair the unit but came back almost immediately with confidence that they could. I delivered it to them with a description of the fault and they confirmed that they could see the fault and could repair or fully refurbish the unit.
I was then faced with a choice of having the single fault repaired or the unit fully refurbished with a life time guarantee. I decided on the full refurbishment as in my experience when one component fails, more in general, usually follow. The repair took 4 days and the unit was retuned looking brand new and fully sealed.
I have now tried it briefly and all works normally including the tacho which was only connected loosely to test everything.
I have subsequently done a 10mile road trip and everything is functioning normally
So the numbers
Tacho refurbishment clean and test £200
DME Test only £65+vat
DME test and repair £320+vat
My Thanks to Tore Bergvill, Mick at Zentrum and Steve at ATP electronic developments
: www.bergvillfx.com
: www.Zentrum.Co.Uk
: www.ATPelectronics.Co.Uk
The tacho stopped working but the car continued to run normally.
Possible problem areas / components
1. Tachometer
2. DME/ECU
3. Central Informer (rarely cause any problems)
4. Flywheel rotation/speed sensor (Car generally will not start if this is not working)
5. Broken wire between any of the connections.
6. Corrosion in any of the plugs (None found so discounted)
Tools required
Oscilloscope/Multimeter tester, wire of reasonable gauge and length, insulating tape, plastic prying kit used to remove tacho and split T16 plug, bags of patience. I bought a relatively cheap (the real ones start at £1k) combined Oscilloscope and Multimeter not ideal but it sufficed to identify the fault.
The tacho is fixed in the dash with a rubber gasket. It can be removed from the dash by first gently prying under the rubber retaining ring then teased out past the steering column shroud. I left everything in place as I think the steering wheel has to be removed to remove the top shroud. It is not easy and patience and washing up liquid plus the prying tools will enable the tacho to be removed. This is I believe only possible with the non OBC tacho as it is slimmer.
I wrongly thought that it was the tacho and sent it to be repaired/upgraded by Tore Bergvill. This was returned fully tested and cleaned etc. thus OK.
So further checks were required.
By this point I had removed the passenger seat and left the returned Tacho disconnected.
Whilst Tore does not repair DME’s he was most helpful in directing me where, how and what to test.
I first checked fuse 18 (power to tacho) which was OK.
Power check on tacho plug at Pin 7 (live) via Pin 2 (ground) gave +13v so OK.
Wiring checked, with battery disconnected for safety, from pin 6 on DME plug to pin 4 on connector designated T16 (on wiring diagram) then from Pin 4 (T16) to Pin 6 on the Tacho Plug. All the checks gave continuity so the wiring was all OK.
The car would start straight away and runs normally so the flywheel rotation/speed sensor is O.K.
Mike at Zentrum checked the central informer by substituting a known good one this had no effect on the tacho so again OK.
Which left the DME/ECU as the problem component.
To confirm it was the DME I needed to test the square wave generated between pin 6 (feed from DME) and pin 2 (ground)
The only way to test the pulse successfully is to use an oscilloscope, so my learning began. I needed to be able to check for a square wave with an amplitude of between 0v and 12volts but no less than 11volts at its peak as per sketch below. This amplitude is required to trigger the tacho anything less will not work. The wave form frequency will also increase and fall as the revs rise and fall. With the oscilloscope I could get a wave that was almost square but with only a peak amplitude of 3.03volts average. Tore said to test the circuit with the tacho attached hence the wire with a loop fashioned such that It could safely (by insulating) be attached to the tacho pins and the oscilloscope and get a reading. Still only 3.03volts.
So the only solution now was to get the DME/ECU tested and repaired.
This proved slightly more problematic than I had envisaged. I contacted quite a few companies who were either only interested in mainly current and therefore regular work or didn’t have the capability to check and repair the unit. I eventually contacted ATP electronic developments based in Cannock Staffordshire. They were initially unsure if they could diagnose then repair the unit but came back almost immediately with confidence that they could. I delivered it to them with a description of the fault and they confirmed that they could see the fault and could repair or fully refurbish the unit.
I was then faced with a choice of having the single fault repaired or the unit fully refurbished with a life time guarantee. I decided on the full refurbishment as in my experience when one component fails, more in general, usually follow. The repair took 4 days and the unit was retuned looking brand new and fully sealed.
I have now tried it briefly and all works normally including the tacho which was only connected loosely to test everything.
I have subsequently done a 10mile road trip and everything is functioning normally
So the numbers
Tacho refurbishment clean and test £200
DME Test only £65+vat
DME test and repair £320+vat
My Thanks to Tore Bergvill, Mick at Zentrum and Steve at ATP electronic developments
: www.bergvillfx.com
: www.Zentrum.Co.Uk
: www.ATPelectronics.Co.Uk
