97 SW with AC, where's the Engine Mgmt. Module? - Defender Source
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  #1  
Old September 19th, 2012, 02:44 PM
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97 SW with AC, where's the Engine Mgmt. Module?

Hello all,

My Defender left me stranded for the first time. I was out doing some errands and when I returned to the truck it would not turn over. I had it towed to my mechanic. He checked all the usual suspects: starter, park-neutral switch, etc. No power is getting to the starter. He also observed that fuse 7 under the hood was blown, and blew again when replaced. That fuse feeds the engine management module which houses the fuel pump relay and what he referred to as the main relay. But he can't find the module (I don't question his competence. Otherwise he is always well informed, and has been able to keep my baby alive in spite of many challenges).

He suspected it was the module piggybacked onto the ECU, but then discovered that was the immobilization module. Is the module in question the one under the passenger seat? Is there anything else that might be causing fuse 7 to blow?

Any other thoughts at all? Do I sound desperate? It's because I am.

Thanks in advance for your help.
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  #2  
Old September 19th, 2012, 03:07 PM
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Ian Gregory
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For your mechanic...

Under the hood behind the white cover on the pass side is the Engine ECU (Big Silver box with 3 connectors) and the main relay (smaller black box with 2 connectors). The main realy also includes the fuel pump relay.

Under the Pass seat is the Rough Road ECU (big silver box with 1 connector).

The Alarm ECU (Smaller green box with 2 connectors) is behind the instruments in the dash. Its mounted to the underside of the dash cowl.

From your description so far I would be taking it somewhere else but that's just me.

You have a blowing fuse so you have a short somewhere. Does he have a circuit diagram for your truck?

Edit - Circuit Diag attached in case you need it.
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File Type: pdf 97 Ignition Diags.pdf (273.2 KB, 43 views)
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Old September 19th, 2012, 04:09 PM
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Thanks for your quick reply. I appreciate it.

Yes, he and I both have a circuit diagram. The number 7 fuse has a brown/green wire that is supposed to go to the engine mgmt. relay module, but according to my mechanic, the smaller black box with two connectors that you are describing has no brown/green wire going to it. I'm going to head over there in a minute to check it out myself. I have a VW Thing that I've done a lot of electrical work on, and with age the wire colors can be deceiving. Anyway, he is under the impression that the black box with two connectors going to it next to the ECU is the immobilization module. I'll take some relevant printouts with me.

I'd be pleased to hear what other direction you'd be going.

Thanks again,

Spalding
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Old September 19th, 2012, 04:39 PM
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The black box is the master relay unit. It has both the Master relay and the fuel pump relay incorporated in it.
He might be getting confused because the Engine ECU interrupts the ground to the fuel pump relay at that box to immobilize the truck. The immobilization is controlled by the Engine ECU based on its interface with the Alarm ECU (behind the dash) though. Fuel pump is fed by a different fuse.

Based on the diag Fuse 7 is the feed for the master relay portion of the black box (not the fuel pump) and it then goes on to feed a number of items with power that are in turn grounded by the Engine ECU. From your symptoms I would be looking for a chafed or loose wire that is shorting to ground downstream of fuse 7 somewhere.

I donít think it is immobilized, I think the short that is blowing fuse 7 is stopping power getting far enough to allow the truck to do anything. That master relay controls a whole host of stuff that is going to make the truck pretty dead without it.

Since this came on suddenly I would be going over the whole engine bay and examining all the wires around the engine (especially the hot parts) looking for a bad wire or connection.
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Old September 19th, 2012, 04:57 PM
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Thanks for this post reply Ian....good stuff for my D90 tech folder!
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Old September 19th, 2012, 06:07 PM
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Quote:
Originally Posted by ipgregory View Post
. . . Since this came on suddenly I would be going over the whole engine bay and examining all the wires around the engine (especially the hot parts) looking for a bad wire or connection.
Thanks so much. A little 409 and a toothbrush revealed the wire colors and we were able to confirm that the little black box is the engine management relay module. We also were able to determine that power is not making it to the black box end of the brown/green wire coming from fuse 7. It should be a relatively simple matter to trace the harness and find the short.

Recently I installed a winch, and ran the (heavy) sheathed cables through the engine bay and under the truck. It could be that while wrangling those cables into position the harness was either abraded or compressed against a sharp edge somewhere, and the unintended results of that effort are just manifesting themselves now.

I'll post back with more questions or the resolution.

Thanks again!
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Old September 19th, 2012, 08:59 PM
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You said the fuse was blown and blew again when you replaced it. If the fuse is still blown then you won't have power at the master relay unit.

If you checked it for continuity instead of power then disregard my last sentence...
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Old September 27th, 2012, 08:49 PM
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Well, It's all good now.

It turns out the problem was a fried ignition switch. I know that seems rather obvious, but there is a huge mitigating circumstance.

What made it so confusing was this issue with fuse 7 under the hood. After no small amount of head scratching my mechanic discovered that the fuse box is wired back to front. Yes, that's right. So the the numer 7 fuse that we thought should be feeding the Engine Relay Mgmt Module was actually wired as the number 1 fuse, feeding the rear interior light. There is a short in the rear light circuit that accounts for the fuse blowing.

Hopefully that's the problem solved and all done, but who knows? I'll be off-roading this weekend in the San Bernardino mountains. Anybody I can call if I get stuck?
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