Blowing the Gear Shift Interlock and Trip Computer Fuse

Hi everyone, haven’t posted for a while - car has been going great thanks to the Jag Lover archives!

A new problem cropped up this week - driving along at 70mph and the speedo went straight to 0. Nothing to do but continue to drive home with GPS to keep speed in check. Got home and put it in park. Could not get out of park again. Gear shift interlock was stuck on. Used the screwdriver trick under the cover to get moving again and got into the garage.

After checking the 94 4.0 Electrical guide, I traced the problem to blown fuse #16 on the drivers side fuse box for “Gear Shift Interlock and Trip Computer Speed Interface”. I don’t actually have a trip computer, just the old clock in the middle of the dash. Replaced the 5A fuse but didn’t get more than 100 yards the next morning before the speedo dropped back to 0 and the fuse had blown again.

Next guess was maybe a short on the sensor cable due to getting too close to the exhaust or diff casing too hot. So I disconnected it and put in another 5A fuse. This time it ran for about a mile before the fuse blew again. So now I’m thinking it isn’t the cable from the diff to the speed interface (unless running the car with speedo disconnected is itself a failure mode for the trip computer interface?) but something further down the line.

I’m think the speedometer/trip computer circuit is the culprit and the gear shift lock out is just collateral damage because of the shared fuse. A new fuse won’t trip straight away, so I’m thinking maybe a short due to vibration or bumps in the road. Car also runs perfectly, so the the ECU is getting all the info it needs.

Any tips or experience on what to look at next? Suspect a short inside the speed interface module itself?

James,

Some confusion to resolve:

  1. Can you confirm if you have AJ6 or AJ16 engine
  2. The above will drfine tranny type - either 4hp22 or 4hp24
  3. Are you using aircon all day long
  4. Is your banger convertible
  5. Can you invest your coffee money into something like that:
  6. Was a cruise control installed in you banger in the past / now removed
  7. What fuse number is it

Hi Banger,

  1. It’s AJ6 engine.
  2. It’s a ZF4HP24
  3. Aircon is permanently off, just fans blowing.
  4. Coupe.
  5. Don’t have one of those, how would I use it to diagnose this?
  6. No cruise control, never had it.
  7. Fuse 16 in the right hand fuse box under the steering wheel.

James, with specified bit, it’s easier to check the actual amp load on the fuse and at what moment the fuse goes out.
I suspect solenoid itself as with ageing it may take more amps to operate. I understand that you’ve checked the brake lights/switch already and there is no issue with these.
Another thing would be to check the load on the fuse with brake pedal depress - as far as I remember this is the solenoid reaction point. Several depresses would cause almost constant operation/increased amp drain, then you can see if it trips out. Another way to save hassle would be fitting 6amp auto fuse and see if it is still the issue. Don’t expect anything to burn with 1amp increase - there is plenty of engineering safety design there already. Similar issue with XJS relates to central locking mechanics (ageing/wear). You can check actual amps drain with simple multimeter as well, that however may be difficult during the ride…

I’ve ordered one of those fuse current testers and it’s on the way. While I wait I did some more basic testing this weekend.

  1. Opened up the gear selector cover and checked the interlock solenoid for free movement. With a fresh fuse installed, the key in but the engine not running, every time I press the brake pedal the interlock releases and I can move the lever out of park with ease.
  2. Did this some 20 times over and the fuse did not blow again.
  3. Left the cover off the fuse panel and went for a drive down the lane behind my garage.
  4. As soon as the speedo got close to 10mph the fuse went. I could clearly hear the click and the needle fell back to 0mph. No pressing on brake pedal, no shift interlock involved.

I think it’s the trip computer part of the circuit that is causing this fuse to blow and just coincidence that the the fuse serves the shift interlock as well.

When the current tester arrives I’ll do more checking. I have the 94 electrical guide and there seems to be very little between the speed interface unit in the trunk and the trip computer/binnacle. But if anyone has any ideas what wires to check for potential shorts, all help appreciated.

Jame, have you done your brake checks with engine on?

Also if you have solenoid visivle - can you disconnect it and try a road test again?

With facelift model I assume you have “watxh display” type of trip computer, it woul more likely burn out at 5 amps and never go back on again…

The inline amp meter recommended by XJSBanger arrived and I got to do some more tests this weekend.

  1. With engine running the solenoid releases the gear lever every time I press the brake pedal. No problems, no delay.
  2. The meter shows the draw at fuse 16 is 0.07.

  1. When I’m on the brake pedal the reading is 0.34, every time.

  1. I notice that the with the gear lever in R, D, 3 or 2 pressing the brake pedal has no effect on the amps, the reading stays at 0.07. So I doubt the solenoid circuit is doing anything when not in Park.

  2. With a fresh fuse in the car, I went for drive. No problems until I rolled over a speed bump - doing less than 20mph, not pressing the brake pedal - the speedometer immediately dropped to zero and the fuse was blown.

I didn’t catch what the meter said as the fuse blew, but I’m pretty sure it was the bump or vibration that caused it.

So to recap - the car is blowing fuse 16, the “Shift Interlock & Trip Computer” fuse.
I don’t have a trip computer, just have the analogue clock in the centre of the dash.
When the fuse blows, the speedometer and odometer stop working.

Based on the fuse blowing when going over a bump I’m now thinking it is a short somewhere caused by the movement/vibration?

Any thoughts welcomed.

There is a loom goes over the transmission, I would look there to start with. It sounds like the loom is shorting out when the transmission rises.

James,
Have you tried stronger fuse yet?
Also, when driving try to use lockup mode on the tranny (position 2)
If stronger fuse will go out, but not in lockip mode - Robin is probably right.

Hi,

I did some more test drives today (obviously not driving a lot with no speedo to rely on). I had some spare 7.5amp fuses so tried those same thing happens with the gearbox in 2 or D, I can’t get more than 100 yards before the fuse blows. Speed is irrelevant and no foot on the brakes, fuse seems to blow based on bumps or vibration.

@XjsBanger just out of curiosity - what different would driving in 2nd gear make? Are you suspecting something from the gearbox ECU that would be different in 2 instead of D?

@Robin_O_Connor are you referring the main gearbox harness itself? Is it accessible in side the car under the carpets or from underneath? Or are you referring to a different harness like the main rearward harness?

image

I hope to finally have time this weekend to get under the car and poke around with a torch to see what I can see.

Thanks for all suggestions so far.

The harness goes between the transmission and the tunnel so not in the cabin.
Does it only happen in 2nd or is that as far as you can get before it blows?

I don’t get very far before it blows, but I might try and get out tomorrow on a quiet stretch of flat motorway to see. At this stage it looks almost certainly like a short of some kind happening due to movement so really need to get the car up in the air and look for any obvious signs of damage to any visible wiring.

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