Power routing of Fuel Injected Volvo 1800E/ES, including Optional ECU fusing, and retrofitted Battery Polarity Safety Diode modifications.
Consideration of Power Distribution, in D-Jet equipped 1800E/ES cars.
Dec2025, R. Kwas, [Comments Added!]
Theory of Operation
Comments on Design
Function of Diode
Fuel Pump Fuse Experience
Fuel Pump Power
Why is the ECU not fused?
Link to SW-EM general page on Bosch D-Jetronic Fuel Injection.
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Routing of power in these cars is somewhat complex, due to design considerations for function and safety decisions made a long time ago.
Routing and related issues, are presented here in a who-came-first logical manner, which is intended to help with troubleshooting, Good Hunting!
Red Lines are Battery Power wiring.
Power routing of Fuel Injected Volvo 1800E/ES, including
Optional ECU fusing, and
retrofitted Battery Polarity Safety
Diode modifications.
Theory of Operation: Battery/Alternator Power is routed to Ignition Switch which then supplies Ignition Power to Fuseblock based on security of the Ignition Key. Fuse5 is the Fuel Pump Fuse, and it supplies the Fuel Pump Relay, which is under control of the D-Jet Electronic Control Unit (Pin 19). The ECU must first have power, supplied by the Fuel Injection Power Relay in order to function, to control Pin 19 (and switch it to chassis to energize the FPR), and this comes by way of a separate (GN) Battery Power Wire, and is itself enabled by Ignition Power (unfused). Example of a BPW is poor condition and unacceptable: LINK
Comments on Design: This seemingly convoluted and complicated power routing was arrived at after careful consideration by the original Volvo design team, taking into account a number of factors and conditions. [Don't second guess them...they knew what they were doing!]
Function of Diode seen in the FIPR is NOT a Coil Quenching Diode as described here. Don't be fooled! In the circuit as shown (not across the Relay Coil, but instead, in series with it), it is to prevent Relay from closing, if Battery is inadvertently installed in the wrong polarity, then enabling the Ign Sw, thus applying reversed polarity power to the ECU undoubtedly damaging it. The direction of conduction of this diode is such that it only allows the Relay to be energized if the Battery is correctly polarized. That is the diode's only function. A quenching Diode for the coil could/should be added/retrofitted also for its advantages, across the coil, but that would be a separate component and function. Ref: LINK
I call the diode in the FIPRel, the "Battery Polarity Safety Diode", and I have added a retrofitted one in green to the WD above, because this provision of protection was not present in early injected cars, it was added, internal to FIPR fitted in later cars ("Blue Dot Relays"), likely after some ECUs were turned to bricks by mechanics who weren't paying attention, and installed the Battery the wrong way, then turned ON the Ignition... The factory recognized this risk of catastrophic damage, and decided to design-in a passive Poka-Yoke mitigation to the later production (Ref: Poka-Yoke is an error proofing term originally attributed to Japanese industrial designer Shigeo Shingo in the 60s). If you are paying attention, promise to triple-check when installing your Battery, and not to install it in the wrong way, you don't need a "Blue Dot Relay" with such an internal diode...but if you wish to retrofit such a diode as a safety provision, passively in place for when someone other than careful-you might be installing a Battery, a diode can certainly be added into the circuit external to the relay, as shown, if a real "Blue Dot Relay" cannot be sourced.
I do recommend a more robust 1N5403 (3A rated) Diode for this duty, than the modest 1N400X (1A rated) series used for Coil Snubbing duty, the reason being that in coil snubbing duty, total energy in the spike is minimal, so current in the diode is also minimal and only flows for the short spike duration time, while in polarity protection duty, this current is still low, and probably not over 100mA (est,) but this full coil energization current flows, and continuously, through the diode, so I like a generous safety margin for this component. An owner with a failed "Blue Dot Relay" is invited to donate it for a post mortem design investigation (and possibly repair, it means just replacing a failed diode). SW-EM can supply both of these as Retrofit kits. LINK
Fuel Pump Fuse Experience: Fuse5 supplies Ignition Power to the Fuel Pump Relay, which is under control of the ECU, with its safety timer circuit, controlling the Relay. (Ref: LINK) Field experience has shown that with the non-conductive film which can develop on the fuse-end contacts over time, this can lead to loss of power to the FuPu, and owners suspecting the FuPu itself and beginning their troubleshooting at the Fuel Pump, when the first thing they should check is Fuse5! ...and here is a perfect example of why ALL fuses should have Gas-Tight-Joints at their contact ends! I give the reader one guess as to how I would recommend assuring this...Ref: ACZP on Fuse-Ends
Fuel Pump Power:
Some troubleshooters work better from a list! In order for
the Puel Pump to be powered, here are conditions to be met in a logical
troubleshooting order:
Preconditions: A well-charged Battery must be installed with good clean
and snug connections...conditions described here are for an engine OFF
condition, but are also applicable for an engine ON condition (with the
exception that FPR and FP will remain energized since ECU gets
continuous engine run pulses from Distributor Contact Assembly
(see No. 3).
1. Ignition must be ON (it applies power to Fuseblock, including Fuse5),
and also Fuel Injection Power Relay.
2. Fuse5 and associated connections and contacts must have continuity
(just because FPR is energized, does NOT mean FP is...it means ECU
is calling for/enabling FPR, but power which FPR switches through,
is NOT common to its coil circuit, and is supplied by Fuse5...don't be fooled!).
3. Fuel Pump Relay must be energized...by power
routed through FIPR , and enabling ("Low") chassis connection from Pin 19
of ECU, which in-turn must be powered by FIPR, AND which must see pulses
from DCA. If it does not see these pulses (engine is not running,
enabling signal from ECU will time out in approximately one second, dropping
FPR, and power to FP. Ref:
LINK
Tip: When troubleshooting, don't let this somewhat complex routing a
combination of conditions distract you! It is best to follow the power
from source, Battery, through contacts and connections, finally arriving at
FP. That's how an experienced troubleshooter would proceed.
Sorry for the complexity, but again, Volvo designers wouldn't have made it such
without reason.
Why is the ECU not fused? This is a question of design philosophy, the decision having been arrived at after consideration by engineers (and accountants). Typically in the era these cars were designed, taking into account factors such as additional cost and complexity of components, as well as reliability [for example, the Fuel Pump Fuse Experience above...!] results of the decision was made not to fuse certain circuits, preferring instead to incur a risk of damage, but keep the car running to make it home. It was therefore common practice to not fuse "mission critical" loads like the ignition, headlights, and in the case of D-Jet cars, the ECU. Some individuals have questioned the later practice. I agree that any fault resulting in an overcurrent high enough to blow an ECU fuse would likely render the car inoperative anyway (and needing to come home or to a shop, not under its own power anyway), so I have added to the Wiring Diagram, an Optional Modification to retrofit the previously unallocated Fuse7 into the (Grn) Battery Power Supply Wire, for this purpose (see above!). I highly recommend to use best wiring practices (including protective PCV sleaving for the run to the FuseBlock), and the fuse value should the 10A [this fuse rating needs to be verified against current consumption test data, and I strongly recommend it be installed with ACZP!]
Reference Information:
Read about retrofitting Coil Quenching Diodes here (it was proven that adding these mitigated power buss upsets which caused momentary drop-outs of the D-Jet System: LINK [Note: The Fuel Injection Power, and Fuel Pump Relays are not contributors of momentary power buss upsets per se, since they are powered the entire engine running session, but the Ignition Switch contact applying power to them would definitely benefit from retrofitting these! See LINK!.]
Deutsche Industie Norm (DIN 72552) Specification for numbered electrical terminals (German).
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External material sources are attributed. Otherwise, this article is Copyright © 2026. Ronald Kwas. The terms Volvo, and names of other suppliers shown here are used for reference only. I have no affiliation with any of these companies other than to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here. The information presented comes from my own experience and carefully considered opinion (and fruitful little gray cells), and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion. As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future!
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