Does anyone have a schematic or a repair procedure for the guts of the speed interface unit in the trunk of 88-on XJS’? I’ve done searches here, but not found much.
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Here is a photo of the board; courtesy of someone else on the forum. The semiconductor numbers are the same between the DAC4864 and later 11399 from what I have found. I am thinking replace the caps and semiconductors on mine when I get around to it.
Pulse rate to the travel computer is 8000/mile; I assume speed interface shapes and amplifies the speed sensor signal. On my ‘88 (with DAC4864) the speedo and speed control work fine, travel computer does not log distance. Fed a 3.5v square wave signal to the travel computer of 133hz and it recognizes distance.
Thanks. More to it than I thought there’d be. You found more than I did searching. Would you provide a link to the post with that picture?
from the picture, the electrolytic capacitor (on the right side, between the IC and the blue wire) is dead / in need of a change
the wires seem to have “cooked” next to the power resistor, the insulation of the red one has already split : would de-solder them and fit some heat shrink tubing before refitting them back, to prevent shorts
From the photo, there appears to be just one electrolytic; don’t know what would generate any heat to damage the wire- probably just the PCB coating peeling from the wire. My interface module is securely glued and have not cut it open yet (prefer to have a replacement on hand first). I was a participant on the thread that the photo came from, I’ll try to find it.
Thanks, but that’s a different interface unit, unless I’m missing something.
The above thread is long, but worth reading. It contains the speed interface photos- credit for them belongs to Paul Korbes (v12 jagguy).
That’s a really good thread. I wonder what the difference is between the early and late modules, other than pcb layout? It looks like the late version might be a two layer board vs the single layer of the original.
Here’s my dilemma. I just got a 95-96 3:54 diff with the speed sensor for my outboard rear brake conversion. I’m also planning to convert my 87 to a 5 speed at some point. I’m wondering which box I need for my cluster and cruise control? Losing the trip computer wouldn’t be a huge loss if it’s not compatible. If circuit diagrams were available, I thought of making my own interface, but that’s not in the cards apparently.
Don’t know the difference between the two modules; ICs are the same. 8000 pulses per mile are common to other Lucas systems (my MGB has a ‘95 14CUX system with same rate). Might be connector differences requiring a different part number- I recall a photo indicating that. I would try it and see. Maybe count the differential carrier notches to see if the same.
I think that other IFU might be a “fuel interface unit”, something that’s supposed to convert injector pulses into fuel consumption numbers for the trip computer.
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I think he is referring to the different speed interface modules DAC4864 (‘88) and DAC11399 (later). Possibly different connector? Due to Marelli ignition/26CU?..his concern is installing a late outboard brake 3.54 differential and getting speedo to work.
Exactly. I’m going to have to do a parts manual search and figure out the sender vs interface module vs speedo vs trip computer compatibility question. I thought I had a good sender in the diff I bought, but found out it’s broken. So I can put in whatever is correct for the situation I have.
What’s also confusing is the Sportspack also had a different interface unit for some unknown reason.
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That is curious. Are the tires different on the Sportspack? Perhaps they needed to adjust the interface a little to keep the speedo accurate.
Most likely correct; as the differential mounted speed sensor triggers off of notches in the carrier, it turns at wheel speed and pulses per mile would change with tire diameter. Definitely would complicate the interface circuit to shave a small number of pulses (unless it is doing that already).
I read somewhere that it does an adjustment to the number of pulses periodically to get to 8000/mile.
Sportspack interface PN is DAC4591, FWIW. Not even sure my car is a Sportspack, but that’s what it has and the parts catalog calls it out as specific to the Sportspack.
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After doing a bunch of parts manual searching, here’s what I’ve found:
The DAC4864 or DAC4591 (Sportpack) speedo interfaces were used with all V12 USA models equiped with the DAC4296 trip computer and a diff mounted sending unit (DBC2632). That computer was used until VIN 179736.
From VIN 179737 to 188104, DAC11375 interface module was used with the DAC7362 trip computer and the same DBC2632 diff sender.
From VIN 188195 on, the DAC11399 interface unit was used with a DBC11116 diff sending unit. The parts manual says the new diff sending unit is “polarized”. The same DAC7362 trip computer was used. It appears that either the early or late diff sending unit will fit as the oring seal is the same.
For completeness, the old transmission mounted sending unit was used with a DAC2589 interface unit. The cars still used the early DAC4296 trip computer. There were also even earlier trip computers, DAC3758 and 2884, that used the same interface unit.
I haven’t been able to sort out the difference in the Sportspack and normal interface units. I suspect, as Kirby suggested, that it’s tire size, but I haven’t been able to figure out what tires the Sportspack models used. I thought there may have been a difference in Sportspack vs regular speedos, but that’s not the case, at least per the parts manuals.
Still would like an interface unit schematic, if anyone has one.
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