I have installed an '87 TPI in a '90 XJS. I am having trouble with the idle and keep getting a 23 flash code (MAT Low) and it has trouble idling correctly. I have changed the MAT Sensor and am fairly confident that the old was working okay as well. One of the MAT Sensor wires is a -5 volt return line to the ECM (D2) this is shared with the TPS return (D2) and the Coolant Temp sensor (D2). I have traced them to the ECM and they are there.

My question is should the D2 circuit be actually grounded to the engine or chassis. To me, all the schematics I have seen, do not show D2 grounded seperately, some of the other EMC grounds do show being grounded to the engine, etc.

My D2 circuit shows continuity to the chassis and I'm thinking it should not. Before I go tearing everything apart looking for a short to ground in my D2 circuit, I wanted to ask.

Also, I have an OBDI (MAC Tools) diagnostic tester that I cannot get to receive data. I have A & B on the ALDL connected correctly and I have E connected to A8 which seems to be what the schematics are telling me. When I try to seek information I get something like "NO DATA SIGNAL,,, yada yada) I have tried another diagnostic unit and get the same message.

Thanks,

Rod

 

Rod,

I've been building new TPI harnesses for over two years. I am very sure that D2 does not get grounded to the engine/chassis. D2 is connected as follows:

MAT: Cavity A
CTS: Cavity A
TPS: Cavity A

Your trouble code sounds like the MAT is not plugged in. Check for continuity on the TAN wire, MAT to the ECM.

The diagnostic port must be wired as follows:
Cavity A: BLK/WHT (ground)
Cavity B: WHT/BLK (can't remember ECM pin off the top of my head)
Cavity E & M: ORANGE (A8 of the ECM)

There are a few others...

 

MAT: I have checked continuity on both MAT wires back to the ECM and have found them to be connected. I have checked resistance across the MAT and compared it to a temp chart and both sensors appear to read correct. I checked continuity from the D2 wire to the chassis and got a beep so I started wondering if I have one of the wires that go to D2 attached to a ground somewhere. Is it crucial that the ECM case not touch ground (chassis) in any way?

ALDL: So I need to have the A8 wire at both terminal E and M of the ALDL for the serial data to coimmunicate? I didn't have an ALDL connector so I had and ALDL harness made at a Fuel Injection place. They only provided me with wires for A, B & E. I guess I can run a jumper from E to M to test this approach but ultimately I would like a complete or semi-complete harness. (at least one that will communcate with my diagnostic meter.

Thanks,

Rod

 

Fix the ground problem you have with D2 and see what happens.

For the ALDL, you should have A8 going to E and M of the ALDL connector. You should also have a TAN/BLK wire going to F if you have an automatic transmission.

 

I'll try and locate the D2 short this weekend, I'll let you know how it works out.

Are you saying that I need the ALDL "F" connection in order to use the diagnostic hand-held or do I use that "F" connection for something else.

Do you make ALDL harnesses 3-4' long?
You said you make TPI Harnesses, is this something you sell or do you just make them for yourself?

Rod

 

I'm not allowed to talk about specifics on this forum because I'm not a sponsored vendor. I'll contact you privately.

 

I got a chance to check out a few things last night. The question was "Should the D2 terminal on the ECm be chassis groounded anywhere. Per most schematics it doesn't show it grounded, yet when I checked continuity from the D2 wires to chassis it was grounded. Last night I unhooked the ECM from all cables and checked continuity from D2 to the ECM case and they were connected. I then checked checked continuity from D1 (which is shown chassis grounded on schematics) and D2 and they were connected. D3, oddly, does not appear to be connected to D1, D2 or chasiis ground. Anyway I now know the reason that I had continuity from D2 to ground was that D2 and D1 are connected in the ECM (even if they are not supposed to be) and D1 is connected to the chassis.

All of this had to do with trying to diagnose why I am getting a code 23 (MAT) error. The wiring seems ot be in order and the sensor checks okay.

Still confused.