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Hi Fritz, I've retested the pins with a new multimeter and all are OK except pin 12. It reads 1 ohm with key OFF but there is no reading at all when cranking.
I have overlooked this sentence. "No reading" means that the resistance at pin 12 to ground = 0? That would be O.K. because then ground is switched.
Do me a favor just to be sure: take a not too thick wire and insulate it 5 cm. Wrap the bare end around the connector terminal 85 on R61 and plug the relay in that way. Then with ignition on listen whether the relay switches when you tip the wire short to ground. If so leave the cable at ground and try if the car normaly starts.
If the answer to the question above is "yes" it means that the switched ground is lost somewhere on the way from pin 12 turbo switch unit to R61.
Hi Fritz, by "no reading" I mean there is no value at all on the multimeter when the engine was cranked. For clarity, pin 12 had a value of 1 ohm with the key OFF, but no value when the engine cranked. The ohms default display on the auto range multimeter did not change.
I will try wrapping the wire around the connector - you mean connector terminal 85 the relay plugs into, not 85 on the relay?
Thanks again, I really appreciate all your efforts.
Ok, if the test shows what I suspect and one must assume that all input signals are present, the only cause of the failure is actually a circuit error in the turbo switch unit.
Then I see the following alternatives:
1. A new or used turbo switch unit. Dont know whether it is possible to source one at reasonable costs.
2. Repair of the switch unit. Over here we have companies that can do this for relativly little money. The chances are better with accurate error description and reproducible errors.
3. You could connect a permanent ground to terminal 85 on the relay, e.g. by disconnecting the cable on the turbo switchgear and connecting it to ground. This has the disadvantage that the fuel pumps run when the ignition is on. This is not wanted and in case of an accident possibly dangerous.
Well Fritz, you were spot on - not that I ever doubted you. Here's a quick summary:
- battery charged overnight 13.28 volts
- terminal 30 13.24 volts key OFF - terminal 85 10.18 volts key ON
- terminal 86 12.34 volts key ON
- wired temp ground to connector 85 on relay, now at 12.55 volts key ON
- grounded wire, key ignition, engine starts immediately and idles as per normal
Is your conclusion that, although the turbo control unit tested ok, there's a fault with the relay in the unit that controls fuel pump ground? As per the fault in John McM excellent write up: https://rennlist.com/forums/964-turb...trol-unit.html?
No, not quite. You mentioned that the relay in the turbo unit was tested o.k. with a 9 V battery as we can see in the picture. I would therefore rule out the relays as the source of the fault.
We also tried to check the input signals on the turbo unit. All tested were ok.
But there is another one which should be checked, the so called electronic ground at pin 7. There is a direkt connection to the ground point XXII.
If there is continuity or 0 Ohm at pin 7 of the turbo unit plug to ground, all conditions should be met and the only remaining causes are a circuit / microchip problem or a cold solder joint.
As written in the previous post, I would look for someone who is able to check and probably repair this.
Fritz, I managed to get back to the car today and retested all pins in the turbo control unit plug with a new multimeter (the old one had a dodgy probe). Results below, hope it can be of use to others troubleshooting this issue:
- Battery = 12.90 volts
- Pin 1 (Charge Air Pressure Indicator) = less than 1 ohm with key OFF
- Pin 2 (Signal TD) = 0.95 volts cranking
- Pin 3 (Timing Ratio 50%) = 5.38 ohm key OFF
- Pin 4 (Terminal 50, starting signal) = 10 volts cranking
- Pin 6 (Terminal 30, battery direct) = 12.82 volts key OFF
- Pin 7 (Electronics Ground) = 0.1 ohm key OFF
- Pin 8 (Actuation Ignition Relay) = 12.83 volts key OFF
- Pin 9 (Output Supply Voltage) = 0 volts key OFF
- Pin 10 (Terminal 31, ground) = 0.1 ohm key OFF (this pin a little fiddly to get reading)
- Pin 11 (Terminal 15, alarm/immobilizer) = 1.1 volts key OFF
- Pin 12 (Actuation Fuel Pump) = no ohm reading key OFF & no ohm reading when cranking i.e. no value at all, the ohms default display on the auto range multimeter did not change. Dead.
Pin 7 (Electronics Ground) is reading OK at 0 ohm so the turbo control unit itself must be faulty? Do you have the details of the supplier in Germany who may be able to test and repair? Many thanks.
Cheers
Last edited by jd964t; 03-08-2023 at 12:59 AM.
Reason: omission
The measurements all look o.k. and complete so far and the test with R61 confirms that, so I think the turbo switch has an internal error.
The switch unit is new at Porsche for 735 (VAT included) €, at other suppliers a little cheaper.
However, no one can promise you that, because it happens too rarely. You have to send it with a meaningful error description and then they will check. If the repair is not possible, the companies usually charge an inspection fee of varying amounts.
You have not specified your location in your profile. With shipping there and back from the U.S. and customs clearance and possibly taxes, you have to see if that makes sense. I would look for a local supplier.
If you decide to buy a new or used unit, I would either test it with a borrowed unit first or arrange a return option.
Fritz, thanks again for this and for all your help and advice - it is very much appreciated. Thanks also to Paul and Scott for your comments. There's something comforting knowing others are thinking and working through the issue that you're going around in circles on! Next step is to get the turbo control unit repaired or replaced.
Fritz, I had a thought I wanted to run past you. If the turbo control unit is faulty and not activating the ground for the fuel pump relay, shouldn't I be able to test by putting a jumper on pin 12 and grounding? In the photo below I've removed pin 12 from the plug, added a jumper from pin 12 to ground and plugged back into the turbo control unit. But the fuel pumps still aren't running when I key ignition ON (relay 61 also plugged back in).
Should grounding pin 12 like this to bypass the possible fault in the turbo control unit, provide the necessary ground for relay 61?
Yes, the consideration is correct. We tried the same thing when we connected ground to terminal 85 of the R61 and it worked. We were there only a little earlier on the way to ground.
Now all measurements so far have shown that there is no ground at pin 12 when the starter motor is turning. If the missing ground is replaced in the way you tried, it must work the same way as the attempt with the ground directly from the screw on the strut to terminal 85.
I would now check if there is continuity from terminal 85 /R61 to your temporary eyelet on the cable of pin 12. This will be so and then it have to work.
03-02-2023, 05:18 AM
